Differential Input, 1 MSPS
10-Bit and 12-Bit ADCs in an 8-Lead SOT-23
Data Sheet AD7440/AD7450A
Rev. D Document Feedback
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FEATURES
Fast throughput rate: 1 MSPS
Specified for VDD of 3 V and 5 V
Low power at max throughput rate
4 mW max at 1 MSPS with 3 V supplies
9.25 mW max at 1 MSPS with 5 V supplies
Fully differential analog input
Wide input bandwidth
70 dB SINAD at 100 kHz input frequency
Flexible power/serial clock speed management
No pipeline delays
High speed serial interface
SPI/QSPI™/MICROWIRE™/DSP compatible
Power-down mode: 1 μA max
8-lead SOT-23 and MSOP packages
APPLICATIONS
Transducer interface
Battery-powered systems
Data acquisition systems
Portable instrumentation
Motor control
GENERAL DESCRIPTION
The AD7440/AD7450A1 are 10-bit and 12-bit high speed, low
power, successive approximation (SAR) analog-to-digital
converters with a fully differential analog input. These parts
operate from a single 3 V or 5 V power supply and use
advanced design techniques to achieve very low power
dissipation at throughput rates up to 1 MSPS. The SAR
architecture of these parts ensures that there are no pipeline
delays.
The parts contain a low noise, wide bandwidth, differential
track-and-hold amplifier (T/H) that can handle input
frequencies up to 3.5 MHz. The reference voltage is applied
externally to the VREF pin and can be varied from 100 mV to
3.5 V depending on the power supply and what suits the
application. The value of the reference voltage determines the
common-mode voltage range of the part. With this truly
differential input structure and variable reference input, the
user can select a variety of input ranges and bias points.
The conversion process and data acquisition are controlled
using CS and the serial clock, allowing the device to interface
with microprocessors or DSPs. The input signals are sampled
1 Protected by U.S. Patent Number 6,681,332.
FUNCTIONAL BLOCK DIAGRAM
Figure 1.
on the falling edge of CS; the conversion is also initiated at this
point. The SAR architecture of these parts ensures that there are
no pipeline delays. The AD7440 and the AD7450A use advanced
design techniques to achieve very low power dissipation at high
throughput rates.
PRODUCT HIGHLIGHTS
1. Operation with either 3 V or 5 V power supplies.
2. High throughput with low power consumption.
With a 3 V supply, the AD7440/AD7450A offer 4 mW
max power consumption for 1 MSPS throughput.
3. Fully differential analog input.
4. Flexible power/serial clock speed management.
The conversion rate is determined by the serial clock,
allowing the power to be reduced as the conversion time
is reduced through the serial clock speed increase. These
parts also feature a shutdown mode to maximize power
efficiency at lower throughput rates.
5. Variable voltage reference input.
6. No pipeline delay.
7. Accurate control of the sampling instant via a CS input and
once-off conversion control.
8. ENOB > eight bits typically with 100 mV reference.
03051-A-001
V
REF
T/H
CONTROL LOGIC
12-BIT
SUCCESSIVE
APPROXIMATION
ADC
GND
SCLK
SDATA
CS
V
DD
AD7440/AD7450A
V
IN+
V
IN–
AD7440/AD7450A Data Sheet
Rev. D | Page 2 of 27
TABLE OF CONTENTS
Features .............................................................................................. 1
Applications ....................................................................................... 1
General Description ......................................................................... 1
Functional Block Diagram .............................................................. 1
Product Highlights ........................................................................... 1
Revision History ............................................................................... 2
AD7440–Specifications .................................................................... 3
AD7450A–Specifications ................................................................. 5
Timing Specifications ....................................................................... 7
Absolute Maximum Ratings ............................................................ 8
ESD Caution .................................................................................. 8
Pin Configurations and Function Descriptions ........................... 9
Terminology .................................................................................... 10
AD7440/AD7450A–Typical Performance Characteristics ....... 12
Circuit Information ........................................................................ 15
Converter Operation .................................................................. 15
ADC Transfer Function ............................................................. 15
Typical Connection Diagram ................................................... 16
Analog Input ............................................................................... 16
Driving Differential Inputs ....................................................... 18
Digital Inputs .............................................................................. 19
Reference ..................................................................................... 19
Single-Ended Operation ............................................................ 20
Serial Interface ............................................................................ 21
Modes of Operation ....................................................................... 23
Normal Mode .............................................................................. 23
Power-Down Mode .................................................................... 23
Power-Up Time .......................................................................... 24
Power vs. Throughput Rate ....................................................... 24
Grounding and Layout Hints.................................................... 25
Evaluating the AD7440/AD7450A Performance ................... 25
Outline Dimensions ....................................................................... 26
Ordering Guide .......................................................................... 27
REVISION HISTORY
7/15—Rev. C to Rev. D
Changed FSCLK to fSCLK .................................................... Throughout
Changes to Figure 34 ...................................................................... 18
Changes to Power vs. Throughput Rate Section ........................ 24
Deleted Microprocessor and DSP Interfacing Section,
AD7440/AD7450A to ADSP-21xx Section, Table 7, and Figure
45; Renumbered Sequentially ....................................................... 25
Deleted AD7440/AD7450A to TMS320C5x/C54x Section,
Figure 46, AD7440/AD7450A to DSP56xxx Section, and
Figure 47 .......................................................................................... 26
Updated Outline Dimensions ....................................................... 27
Changes to Ordering Guide .......................................................... 28
9/05—Rev. B to Rev. C
Changes to Ordering Guide .......................................................... 28
2/04—Rev. A to Rev. B
Added Patent Note ............................................................................ 1
1/04—Rev. 0 to Rev. A
Updated Format .................................................................. Universal
Changes to General Description ..................................................... 1
Changes to Table 1 Footnotes .......................................................... 3
Changes to Table 2 Footnotes .......................................................... 5
Changes to Table 3 Footnotes .......................................................... 7
8/03—Revision 0: Initial Version
Data Sheet AD7440/AD7450A
Rev. D | Page 3 of 27
AD7440–SPECIFICATIONS
Table 1. VDD = 2.7 V to 3.6 V, fSCLK = 18 MHz, fS = 1 MSPS, VREF = 2.0 V; VDD = 4.75 V to 5.25 V, fSCLK = 18 MHz, fS = 1 MSPS,
VREF = 2.5 V; VCM1 = VREF; TA = TMIN to TMAX, unless otherwise noted. Temperature range for B Version: –40°C to +85°C.
Parameter Test Conditions/Comments B Version Unit
DYNAMIC PERFORMANCE fIN = 100 kHz
Signal-to-(Noise + Distortion) (SINAD)2 61 dB min
Total Harmonic Distortion (THD)2 –82 dB typ –74 dB max
Peak Harmonic or Spurious Noise2 –82 dB typ –76 dB max
Intermodulation Distortion (IMD)2 fa = 90 kHz, fb = 110 kHz
Second-Order Terms –83 dB typ
Third-Order Terms –83 dB typ
Aperture Delay2 5 ns typ
Aperture Jitter2 50 ps typ
Full Power Bandwidth2, 3 @ –3 dB 20 MHz typ
@ –0.1 dB 2.5 MHz typ
DC ACCURACY
Resolution 10 Bits
Integral Nonlinearity (INL)2 ±0.5 LSB max
Differential Nonlinearity (DNL)2 Guaranteed no missed codes to 10 bits ±0.5 LSB max
Zero-Code Error2 ±2.5 LSB max
Positive Gain Error2 ±1 LSB max
Negative Gain Error2 ±1 LSB max
ANALOG INPUT
Full-Scale Input Span 2 × VREF4 VIN+ – VIN– V
Absolute Input Voltage
VIN+ VCM = VREF VCM ± VREF/2 V
VIN– VCM = VREF VCM ± VREF/2 V
DC Leakage Current ±1 μA max
Input Capacitance When in track-and-hold 30/10 pF typ
REFERENCE INPUT
VREF Input Voltage VDD = 4.75 V to 5.25 V (±1% tolerance for
specified performance)
2.55 V
VDD = 2.7 V to 3.6 V (±1% tolerance for specified
performance)
2.06 V
DC Leakage Current ± 1 μA max
VREF Input Capacitance When in track-and-hold 10/30 pF typ
LOGIC INPUTS
Input High Voltage, VINH 2.4 V min
Input Low Voltage, VINL 0.8 V max
Input Current, IIN Typically 10 nA, VIN = 0 V or VDD ±1 μA max
Input Capacitance, CIN7 10 pF max
LOGIC OUTPUTS
Output High Voltage, VOH VDD = 4.75 V to 5.25 V; ISOURCE = 200 μA 2.8 V min
V
DD = 2.7 V to 3.6 V; ISOURCE = 200 μA 2.4 V min
Output Low Voltage, VOL ISINK = 200 μA 0.4 V max
Floating-State Leakage Current ±1 μA max
Floating-State Output Capacitance7 10 pF max
Output Coding Twos complement
AD7440/AD7450A Data Sheet
Rev. D | Page 4 of 27
Parameter Test Conditions/Comments B Version Unit
CONVERSION RATE
Conversion Time 888 ns with an 18 MHz SCLK 16 SCLK cycles
Track-and-Hold Acquisition Time2 Sine wave input 200 ns max
Step input 290 ns max
Throughput Rate
1
MSPS max
POWER REQUIREMENTS
V
DD
Range: 3 V + 20%/–10%; 5 V ± 5% 2.7/5.25 V min/V max
I
DD8
Normal Mode (Static) SCLK on or off 0.5 mA typ
Normal Mode (Operational) V
DD
= 4.75 V to 5.25 V 1.95 mA max
V
DD
= 2.7 V to 3.6 V 1.45 mA max
Full Power-Down Mode SCLK on or off 1 µA max
Power Dissipation
Normal Mode (Operational)
VDD = 5 V, 1.55 mW typ for 100 kSPS9
9.25
mW max
V
DD
= 3 V, 0.6 mW typ for 100 kSPS
9
4 mW max
Full Power-Down Mode V
DD
= 5 V, SCLK on or off 5 µW max
V
DD
= 3 V, SCLK on or off 3 µW max
1 Common-mode voltage. The input signal can be centered on a dc common-mode voltage in the range specified in Figure 28 and Figure 29.
2 See the Terminology section.
3 Analog inputs with slew rates exceeding 27 V/µs (full-scale input sine wave > 3.5 MHz) within the acquisition time can cause the converter to return an
incorrect result.
4 Because the input spans of VIN+ and VIN– are both VREF and are 180° out of phase, the differential voltage is 2 × VREF.
5 The AD7440 is functional with a reference input from 100 mV and for VDD = 5 V; the reference can range up to 3.5 V.
6 The AD7440 is functional with a reference input from 100 mV and for VDD = 3 V; the reference can range up to 2.2 V.
7 Guaranteed by characterization.
8 Measured with a midscale dc input.
9 See the Power vs. Throughput section.
Data Sheet AD7440/AD7450A
Rev. D | Page 5 of 27
AD7450A–SPECIFICATIONS
Table 2. VDD = 2.7 V to 3.6 V, fSCLK = 18 MHz, fS = 1 MSPS, VREF = 2.0 V; VDD = 4.75 V to 5.25 V, fSCLK = 18 MHz, fS = 1 MSPS,
VREF = 2.5 V; VCM
1 = VREF; TA = TMIN to TMAX, unless otherwise noted. Temperature range for B Version: –40°C to +85°C.
Parameter Test Conditions/Comments B Version Unit
DYNAMIC PERFORMANCE f
IN
= 100 kHz
Signal-to-(Noise + Distortion) (SINAD)
2
70 dB min
Total Harmonic Distortion (THD)
2
V
DD
= 4.75 V to 5.25 V, –86 dB typ –76 dB max
V
DD
= 2.7 V to 3.6 V, –84 dB typ –74 dB max
Peak Harmonic or Spurious Noise2 V
DD
= 4.75 V to 5.25 V, –86 dB typ –76 dB max
V
DD
= 2.7 V to 3.6 V, –84 dB typ –74 dB max
Intermodulation Distortion (IMD)2
fa = 90 kHz, fb = 110 kHz
Second-Order Terms –89 dB typ
Third-Order Terms –89 dB typ
Aperture Delay2 5 ns typ
Aperture Jitter2 50 ps typ
Full Power Bandwidth2, 3 @ –3 dB 20 MHz typ
@ –0.1 dB 2.5 MHz typ
DC ACCURACY
Resolution 12 Bits
Integral Nonlinearity (INL)
2
±1 LSB max
Differential Nonlinearity (DNL)
2
Guaranteed no missed codes to 12 bits ±0.95 LSB max
Zero-Code Error
2
±6 LSB max
Positive Gain Error2 ±2 LSB max
Negative Gain Error2 ±2 LSB max
ANALOG INPUT
Full-Scale Input Span 2 × V
REF
4 V
IN+
– V
IN–
V
Absolute Input Voltage
V
IN+
V
CM
= V
REF
V
CM
± V
REF
/2 V
V
IN–
V
CM
= V
REF
V
CM
± V
REF
/2 V
DC Leakage Current ±1 µA max
Input Capacitance When in track-and-hold 30/10 pF typ
REFERENCE INPUT
VREF Input Voltage VDD = 4.75 V to 5.25 V
(±1% tolerance for specified performance)
2.5
5
V
VDD = 2.7 V to 3.6 V
(±1% tolerance for specified performance)
2.06 V
DC Leakage Current ±1 µA max
V
REF
Input Capacitance When in track-and-hold 10/30 pF typ
LOGIC INPUTS
Input High Voltage, V
INH
2.4 V min
Input Low Voltage, V
INL
0.8 V max
Input Current, I
IN
Typically 10 nA, V
IN
= 0 V or V
DD
±1 µA max
Input Capacitance, C
IN
7 10 pF max
LOGIC OUTPUTS
Output High Voltage, V
OH
V
DD
= 4.75 V to 5.25 V; I
SOURCE
= 200 µA 2.8 V min
V
DD
= 2.7 V to 3.6 V; I
SOURCE
= 200 µA 2.4 V min
Output Low Voltage, V
OL
I
SINK
= 200 µA 0.4 V max
Floating-State Leakage Current ±1 µA max
Floating-State Output Capacitance
7
10 pF max
Output Coding Twos complement
AD7440/AD7450A Data Sheet
Rev. D | Page 6 of 27
Parameter Test Conditions/Comments B Version Unit
CONVERSION RATE
Conversion Time 888 ns with an 18 MHz SCLK 16 SCLK cycles
Track-and-Hold Acquisition Time2 Sine wave input 200 ns max
Step input 290 ns max
Throughput Rate
1
MSPS max
POWER REQUIREMENTS
V
DD
Range: 3 V + 20%/–10%; 5 V ± 5% 2.7/5.25 V min/V max
I
DD8
Normal Mode (Static) SCLK on or off 0.5 mA typ
Normal Mode (Operational) V
DD
= 4.75 V to 5.25 V 1.95 mA max
V
DD
= 2.7 V to 3.6 V 1.45 mA max
Full Power-Down Mode SCLK on or off 1 µA max
Power Dissipation
Normal Mode (Operational)
VDD = 5 V, 1.55 mW typ for 100 kSPS9
9.25
mW max
V
DD
= 3 V, 0.6 mW typ for 100 kSPS
9
4 mW max
Full Power-Down V
DD
= 5 V, SCLK on or off 5 µW max
V
DD
= 3 V, SCLK on or off 3 µW max
1 Common-mode voltage. The input signal can be centered on a dc common-mode voltage in the range specified in Figure 28 and Figure 29.
2 See the Terminology section.
3 Analog inputs with slew rates exceeding 27 V/µs (full-scale input sine wave > 3.5 MHz) within the acquisition time can cause the converter to return an
incorrect result.
4 Because the input spans of VIN+ and VIN– are both VREF and are 180° out of phase, the differential voltage is 2 × VREF.
5 The AD7450A is functional with a reference input from 100 mV and for VDD = 5 V; the reference can range up to 3.5 V.
6 The AD7450A is functional with a reference input from 100 mV and for VDD = 3 V; the reference can range up to 2.2 V.
7 Guaranteed by characterization.
8 Measured with a midscale dc input.
9 See the Power vs. Throughput section.
Data Sheet AD7440/AD7450A
Rev. D | Page 7 of 27
TIMING SPECIFICATIONS
Guaranteed by characterization. All input signals are specified with tr = tf = 5 ns (10% to 90% of VDD) and timed from a voltage level of
1.6 V. See Figure 2, Figure 3, and the Serial Interface section.
Table 3. VDD = 2.7 V to 3.6 V, fSCLK = 18 MHz, fS = 1 MSPS, VREF = 2.0 V; VDD = 4.75 V to 5.25 V, fSCLK = 18 MHz, fS = 1 MSPS,
VREF = 2.5 V; VCM1 = VREF; TA = TMIN to TMAX, unless otherwise noted.
Parameter Limit at TMIN, TMAX Unit Description
fSCLK2 10 kHz min
18 MHz max
tCONVERT 16 × tSCLK tSCLK = 1/fSCLK
888 ns max
tQUIET 60 ns min Minimum quiet time between the end of a serial read and the next falling edge of CS
t1 10 ns min Minimum CS pulse width
t2 10 ns min CS falling edge to SCLK falling edge setup time
t33 20 ns max Delay from CS falling edge until SDATA three-state disabled
t43 40 ns max Data access time after SCLK falling edge
t5 0.4 tSCLK ns min SCLK high pulse width
t6 0.4 tSCLK ns min SCLK low pulse width
t7 10 ns min SCLK edge to data valid hold time
t84 10 ns min SCLK falling edge to SDATA three-state enabled
35 ns max SCLK falling edge to SDATA three-state enabled
tPOWER-UP5 1 μs max Power-up time from full power-down
1 Common-mode voltage.
2 Mark/space ratio for the SCLK input is 40/60 to 60/40.
3 Measured with the load circuit of Figure 4 and defined as the time required for the output to cross 0.8 V or 2.4 V with VDD = 5 V or 0.4 V or 2.0 V for VDD = 3 V.
4 t8 is derived from the measured time taken by the data outputs to change 0.5 V when loaded with the circuit of Figure 4. The measured number is then extrapolated
back to remove the effects of charging or discharging the 25 pF capacitor. This means that the time, t8, quoted in the Timing Specifications is the true bus relinquish
time of the part and is independent of the bus loading.
5 See Power-Up Time section.
Figure 2. AD7450A Serial Interface Timing Diagram
Figure 3. AD7440 Serial Interface Timing Diagram
t
3
t
2
t
4
t
7
t
8
t
6
t
1
t
5
t
QUIET
t
CONVERT
CS
SCLK
SDATA
4 LEADING ZEROS THREE-STATE
12345 13141516
0 0 0 0 DB11 DB10 DB2 DB1 DB0
B
03051-A-002
t
3
t
2
t
4
t
7
t
8
t
6
t
1
t
5
t
QUIET
t
CONVERT
CS
SCLK
SDATA
4 LEADING ZEROS 2 TRAILING ZEROS THREE-STATE
12345 13141516
0 0 0 0 DB9 DB8 DB0 0 0
B
03051-A-003
AD7440/AD7450A Data Sheet
Rev. D | Page 8 of 27
ABSOLUTE MAXIMUM RATINGS
TA = 25°C, unless otherwise noted.
Table 4.
Parameter Rating
V
to GND –0.3 V to +7 V
V
to GND –0.3 V to V
DD
+ 0.3 V
V
to GND –0.3 V to V
DD
+ 0.3 V
Digital Input Voltage to GND –0.3 V to +7 V
Digital Output Voltage to GND –0.3 V to V
DD
+ 0.3 V
V
to GND –0.3 V to V
DD
+ 0.3 V
±10 mA
Operating Temperature Range
Commercial (B Version) –40°C to +85°C
Storage Temperature Range –65°C to +150°C
Junction Temperature 150°C
θ
Thermal Impedance
MSOP 205.9°C/W
SOT-23 211.5°C/W
θ
Thermal Impedance
MSOP 43.74°C/W
SOT-23 91.99°C/W
Lead Temperature, Soldering
Vapor Phase (60 sec) 215°C
Infrared (15 sec) 220°C
ESD 1 kV
1 Transient currents of up to 100 mA do not cause SCR latch up.
Stresses at or above those listed under Absolute Maximum
Ratings may cause permanent damage to the product. This is a
stress rating only; functional operation of the product at these
or any other conditions above those indicated in the operational
section of this specification is not implied. Operation beyond
the maximum operating conditions for extended periods may
affect product reliability.
Figure 4. Load Circuit for Digital Output Timing Specifications
ESD CAUTION
03051-A-004
1.6mA I
OL
200µA I
OH
1.6V
TO OUTPUT
PIN C
L
25pF
Data Sheet AD7440/AD7450A
Rev. D | Page 9 of 27
PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS
Figure 5. Pin Configuration for 8-Lead SOT-23
Figure 6. Pin Configuration for 8-Lead MSOP
Table 5. Pin Function Descriptions
Mnemonic Function
VREF Reference Input for the AD7440/AD7450A. An external reference must be applied to this input. For a 5 V power supply, the
reference is 2.5 V (±1%) for specified performance. For a 3 V power supply, the reference is 2 V (±1%) for specified
performance. This pin should be decoupled to GND with a capacitor of at least 0.1 µF. See the Reference section for more
details.
V
IN+
Positive Terminal for Differential Analog Input.
V
IN–
Negative Terminal for Differential Analog Input.
GND Analog Ground. Ground reference point for all circuitry on the AD7440/AD7450A. All analog input signals and any external
reference signal should be referred to this GND voltage.
CS Chip Select. Active low logic input. This input provides the dual function of initiating a conversion on the AD7440/AD7450A
and framing the serial data transfer.
SDATA Serial Data. Logic output. The conversion result from the AD7440/AD7450A is provided on this output as a serial data stream.
The bits are clocked out on the falling edge of the SCLK input. The data stream of the AD7450A consists of four leading zeros
followed by the 12 bits of conversion data, which are provided MSB first; the data stream of the AD7440 consists of four
leading zeros, followed by the 10 bits of conversion data, followed by two trailing zeros. In both cases, the output coding is
twos complement.
SCLK Serial Clock. Logic input. SCLK provides the serial clock for accessing data from the part. This clock input is also used as the
clock source for the conversion process.
VDD Power Supply Input. VDD is 3 V (+20%/–10%) or 5 V (±5%). This supply should be decoupled to GND with a 0.1 µF capacitor and
a 10 µF tantalum capacitor in parallel.
03051-A-005
V
REF
V
IN+
V
IN–
GND
8
7
6
5
V
DD 1
SCLK
2
SDATA
3
CS
4
AD7440/
AD7450A
TOP VIEW
(Not to Scale)
03051-A-006
VDD
SCLK
SDATA
CS
8
7
6
5
VREF 1
VIN+ 2
VIN– 3
GND 4
AD7440/
AD7450A
TOP VIEW
(Not to Scale)
AD7440/AD7450A Data Sheet
Rev. D | Page 10 of 27
TERMINOLOGY
Signal-to-(Noise + Distortion) Ratio
This is the measured ratio of signal to (noise + distortion) at the
output of the ADC. The signal is the rms amplitude of the
fundamental. Noise is the sum of all nonfundamental signals
up to half the sampling frequency (fS/2), excluding dc. The
ratio is dependent on the number of quantization levels in the
digitization process; the more levels, the smaller the quanti-
zation noise. The theoretical signal-to-(noise + distortion) ratio
for an ideal N-bit converter with a sine wave input is given by
the following:
Signal-to-(Noise + Distortion) = (6.02N + 1.76) dB
Thus for a 12-bit converter, this is 74 dB; and for a 10-bit
converter, this is 62 dB.
Total Harmonic Distortion (THD)
THD is the ratio of the rms sum of harmonics to the
fundamental. For the AD7440/AD7450A, it is defined as
1
2
6
2
5
2
4
2
3
2
2
V
VVV
VV
THD +++
+
=log20)dB(
where V1 is the rms amplitude of the fundamental and V2, V3,
V4, V5, and V6 are the rms amplitudes of the second to the sixth
harmonics.
Peak Harmonic or Spurious Noise
Peak harmonic (spurious noise) is the ratio of the rms value of
the next largest component in the ADC output spectrum (up to
fS/2 and excluding dc) to the rms value of the fundamental.
Normally, the value of this specification is determined by the
largest harmonic in the spectrum, but for ADCs where the
harmonics are buried in the noise floor, it is a noise peak.
Intermodulation Distortion
With inputs consisting of sine waves at two frequencies,
fa and fb, any active device with nonlinearities creates distortion
products at the sum and difference frequencies of mfa ± nfb
where m, n = 0, 1, 2, 3, and so on. Intermodulation distortion
terms are those for which neither m nor n is equal to 0. For
example, the second-order terms include (fa + fb) and (fa – fb),
while the third-order terms include (2fa + fb), (2fa – fb),
(fa + 2fb), and (fa – 2fb).
The AD7440/AD7450A is tested using the CCIF standard of two
input frequencies near the top end of the input bandwidth. In this
case, the second-order terms are distanced in frequency from the
original sine waves, while the third-order terms are at a frequency
close to the input frequencies. As a result, the second- and third-
order terms are specified separately. The calculation of the
intermodulation distortion is as per the THD specification, where it
is the ratio of the rms sum of the individual distortion products to
the rms amplitude of the sum of the fundamentals, expressed in dB.
Aperture Delay
This is the amount of time from the leading edge of the
sampling clock until the ADC actually takes the sample.
Aperture Jitter
This is the sample-to-sample variation in the effective point in
time at which the actual sample is taken.
Full Power Bandwidth
The full power bandwidth of an ADC is the input frequency at
which the amplitude of the reconstructed fundamental is
reduced by 0.1 dB or 3 dB for a full-scale input.
Common-Mode Rejection Ratio (CMRR)
The common-mode rejection ratio is the ratio of the power
in the ADC output at full-scale frequency, f, to the power of a
100 mV p-p sine wave applied to the common-mode voltage of
VIN+ and VIN– of frequency fS as follows:
CMRR (dB) = 10 log (Pf/Pfs)
Pf is the power at the frequency f in the ADC output; Pfs is the
power at frequency fS in the ADC output.
Integral Nonlinearity (INL)
This is the maximum deviation from a straight line passing
through the endpoints of the ADC transfer function.
Differential Nonlinearity (DNL)
This is the difference between the measured and the ideal
1 LSB change between any two adjacent codes in the ADC.
Zero-Code Error
This is the deviation of the midscale code transition
(111...111 to 000...000) from the ideal VIN+ − VIN– (that is,
0 LSB).
Positive Gain Error
This is the deviation of the last code transition (011...110 to
011...111) from the ideal VIN+ – VIN– (that is, +VREF − 1 LSB),
after the zero code error has been adjusted out.
Negative Gain Error
This is the deviation of the first code transition (100...000 to
100...001) from the ideal VIN+ − VIN– (that is, –VREF + 1
LSB), after the zero code error has been adjusted out.
Track-and-Hold Acquisition Time
The track-and-hold acquisition time is the minimum time
required for the track-and-hold amplifier to remain in track
mode for its output to reach and settle to within 0.5 LSB of the
applied input signal.
Data Sheet AD7440/AD7450A
Rev. D | Page 11 of 27
Power Supply Rejection Ratio (PSRR)
The power supply rejection ratio is the ratio of the power in
the ADC output at full-scale frequency, f, to the power of a
100 mV p-p sine wave applied to the ADC VDD supply of
frequency fS. The frequency of this input varies from 1 kHz to
1 MHz.
PSRR (dB) = 10log(Pf/PfS)
Pf is the power at frequency f in the ADC output; Pfs is the
power at frequency fS in the ADC output.
AD7440/AD7450A Data Sheet
Rev. D | Page 12 of 27
AD7440/AD7450A–TYPICAL PERFORMANCE CHARACTERISTICS
TA = 25°C, fS = 1 MSPS, fSCLK = 18 MHz, unless otherwise noted.
Figure 7. AD7450A SINAD vs. Analog Input Frequency for Various
Supply Voltages
Figure 8. CMRR vs. Frequency for VDD = 5 V and 3 V
Figure 9. PSRR vs. Supply Ripple Frequency Without Supply Decoupling
Figure 10. AD7450A Dynamic Performance with VDD = 5 V
Figure 11. Typical DNL for the AD7450A for VDD = 5 V
Figure 12. Typical INL for the AD7450A for VDD = 5 V
75
70
65
60
55
10 100 1000
03051-A-007
FREQUENCY (kHz)
SINAD (dB)
V
DD
= 5.25V V
DD
= 4.75V
V
DD
= 3.6V
V
DD
= 2.7V
0
–100
–90
–80
–70
–60
–50
–40
–30
–20
–10
10 1000100 10000
03051-A-008
FREQUENCY (kHz)
CMRR (dB)
V
DD
= 3V
V
DD
= 5V
0
–120
–20
–40
–60
–80
–100
0 100 200 300 400 500 600 700 800 900 1000
03051-A-009
SUPPLY RIPPLE FREQUENCY (kHz)
PSRR (dB)
100mV p-p SINEWAVE ON V
DD
NO DECOUPLING ON V
DD
V
DD
= 3V
V
DD
= 5V
0
–140
–120
–100
–80
–60
–40
–20
0 100 200 300 400 500
03051-A-010
FREQUENCY (kHz)
SNR (dB)
8192 POINT FFT
f
SAMPLE
= 1MSPS
f
IN
= 100kSPS
SINAD = +71.7dB
THD = –82dB
SFDR = –83dB
1.0
–1.0
–0.8
–0.6
–0.4
–0.2
0
0.2
0.4
0.6
0.8
0 1024 2048 3072 4096
03051-A-011
CODE
DNL ERROR (LSB)
1.0
–1.0
–0.8
–0.6
–0.4
–0.2
0
0.2
0.4
0.6
0.8
0 1024 2048 3072 4096
03051-A-012
CODE
INL ERROR (LSB)
Data Sheet AD7440/AD7450A
Rev. D | Page 13 of 27
Figure 13. Change in DNL vs. VREF for the AD7450A for VDD = 5 V
Figure 14. Change in DNL vs. VREF for the AD7450A for VDD = 3 V
Figure 15. Change in INL vs. VREF for the AD7450A for VDD = 5 V
Figure 16. Change in INL vs. VREF for the AD7450A for VDD = 3 V
Figure 17. Change in Zero-Code Error vs. Reference Voltage for
VDD = 5 V and 3 V for the AD7450A
Figure 18. Change in ENOB vs. Reference Voltage for VDD = 5 V and 3 V
for the AD7450A
3.0
–1.0
–0.5
0
0.5
1.0
1.5
2.0
2.5
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
03051-A-013
V
REF
(V)
CHANGE IN DNL (LSB)
POSITIVE DNL
NEGATIVE DNL
2.5
–1.0
–0.5
0
0.5
1.0
1.5
2.0
0 0.5 1.0 1.5 2.0 2.2 2.5
03051-A-014
V
REF
(V)
CHANGE IN DNL (LSB)
POSITIVE DNL
NEGATIVE DNL
5
–5
–4
–3
–2
–1
0
1
2
3
4
0 0.5 1.0 1.5 2.52.0 3.0 3.5
03051-A-015
V
REF
(V)
CHANGE IN INL (LSB)
POSITIVE INL
NEGATIVE INL
2.5
–2.0
–1.5
–1.0
–0.5
0
0.5
1.0
1.5
2.0
0 0.5 1.0 1.5 2.0 2.2 2.5
03051-A-016
V
REF
(V)
CHANGE IN INL (LSB)
POSITIVE INL
NEGATIVE INL
8
0
1
2
3
4
5
6
7
00.5 1.0 1.5 2.0 2.5 3.0 3.5
03051-A-017
V
REF
(V)
ZERO-CODE ERROR (LSB)
V
DD
= 5V
V
DD
= 3V
12.0
7.0
7.5
8.0
8.5
9.0
9.5
10.0
10.5
11.0
11.5
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
03051-A-018
V
REF
(V)
EFFECTIVE NUMBER OF BITS
V
DD
= 5V
V
DD
= 3V
AD7440/AD7450A Data Sheet
Rev. D | Page 14 of 27
Figure 19. Histogram of 10,000 Conversions of a DC Input for the
AD7450A with VDD = 5 V
Figure 20. AD7440 Dynamic Performance with VDD = 5 V
Figure 21. Typical DNL for the AD7440 for VDD = 5 V
Figure 22. Typical INL for the AD7440 for VDD = 5 V
10,000
0
1,000
2,000
3,000
4,000
5,000
6,000
7,000
8,000
9,000
2044 2045 2046 2047 2048 2049
03051-A-019
CODE
10,000
CODES
V
IN+
= V
IN–
10,000 CONVERSIONS
f
S
= 1MSPS
0
–140
–120
–100
–80
–60
–40
–20
0 100 200 300 400 500
03051-A-020
FREQUENCY (kHz)
SNR (dB)
8192 POINT FFT
f
SAMPLE
= 1MSPS
f
IN
= 100kHz
SINAD = +61.6dB
THD = –81.7dB
SFDR = –83.1dB
0.5
–0.5
–0.4
–0.3
–0.2
–0.1
0
0.1
0.2
0.3
0.4
0 256 512 768 1024
03051-A-021
CODE
DNL ERROR (LSB)
0.5
–0.5
–0.4
–0.3
–0.2
–0.1
0
0.1
0.2
0.3
0.4
0 256 512 768 1024
03051-A-022
CODE
INL ERROR (LSB)
Data Sheet AD7440/AD7450A
Rev. D | Page 15 of 27
CIRCUIT INFORMATION
The AD7440/AD7450A are 10-bit and 12-bit fast, low power,
single-supply, successive approximation analog-to-digital
converters (ADCs). They can operate with a 5 V or 3 V power
supply and are capable of throughput rates up to 1 MSPS when
supplied with an 18 MHz SCLK. They require an external
reference to be applied to the VREF pin, with the value of the
reference chosen depending on the power supply and what suits
the application.
When they are operated with a 5 V supply, the maximum
reference that can be applied is 3.5 V. When they are operated
with a 3 V supply, the maximum reference that can be applied is
2.2 V (see the Reference section).
The AD7440/AD7450A have an on-chip differential track-and-
hold amplifier, a successive approximation (SAR) ADC, and a
serial interface housed in either an 8-lead SOT-23 or an MSOP
package. The serial clock input accesses data from the part and
provides the clock source for the successive approximation
ADC. The AD7440/AD7450A feature a power-down option for
reduced power consumption between conversions. The power-
down feature is implemented across the standard serial interface
as described in the Modes of Operation section.
CONVERTER OPERATION
The AD7440/AD7450A are successive approximation ADCs
based around two capacitive DACs. Figure 23 and Figure 24
show simplified schematics of the ADC in acquisition and
conversion phase, respectively. The ADC is comprised of
control logic, an SAR, and two capacitive DACs. In Figure 23
(acquisition phase), SW3 is closed, SW1 and SW2 are in
Position A, the comparator is held in a balanced condition,
and the sampling capacitor arrays acquire the differential
signal on the input.
Figure 23. ADC Acquisition Phase
When the ADC starts a conversion (Figure 24), SW3 opens and
SW1 and SW2 move to Position B, causing the comparator to
become unbalanced. Both inputs are disconnected once the
conversion begins. The control logic and the charge redistri-
bution DACs are used to add and subtract fixed amounts of
charge from the sampling capacitor arrays to bring the compar-
ator back into a balanced condition. When the comparator is
rebalanced, the conversion is complete. The control logic
generates the ADC’s output code. The output impedances of the
sources driving the VIN+ and the VIN– pins must be matched;
otherwise, the two inputs have different settling times, resulting
in errors.
Figure 24. ADC Conversion Phase
ADC TRANSFER FUNCTION
The output coding for the AD7440/AD7450A is twos comple-
ment. The designed code transitions occur at successive LSB
values (1 LSB, 2 LSBs, and so on). The LSB size of the AD7450A
is 2 × VREF/4096, and the LSB size of the AD7440 is 2 × VREF/1024.
The ideal transfer characteristic of the AD7440/AD7450A is
shown in Figure 25.
Figure 25. AD7440/AD7450A Ideal Transfer Characteristic
03051-A-023
V
IN+
V
IN–
A
B
SW1 SW3
COMPARATOR
CONTROL
LOGIC
CAPACITIVE
DAC
CAPACITIVE
DAC
C
S
C
S
V
REF
SW2
B
A
03051-A-024
V
IN+
V
IN–
A
B
SW1 SW3
COMPARATOR
CONTROL
LOGIC
CAPACITIVE
DAC
CAPACITIVE
DAC
C
S
C
S
V
REF
SW2
B
A
03051-A-025
100...000
ANALOG INPUT
(V
IN+
– V
IN–
)
011...111
100...001
100...010
011...110
000...001
111...111
1 LSB
1LSB = 2
×
V
REF
/4096 AD7450A
1LSB = 2
×
V
REF
/1024 AD7440
+V
REF
– 1 LSB
–V
REF
0 LSB
000...000
ADC CODE
AD7440/AD7450A Data Sheet
Rev. D | Page 16 of 27
TYPICAL CONNECTION DIAGRAM
Figure 26 shows a typical connection diagram for the
AD7440/AD7450A for both 5 V and 3 V supplies. In this setup,
the GND pin is connected to the analog ground plane of the
system. The VREF pin is connected to either a 2.5 V or a 2 V
decoupled reference source, depending on the power supply, to
set up the analog input range. The common-mode voltage has
to be set up externally and is the value on which the two inputs
are centered. The conversion result is output in a 16-bit word
with four leading zeros followed by the MSB of the 12-bit or
10-bit result. The 10-bit result of the AD7440 is followed by two
trailing zeros. For more details on driving the differential inputs
and setting up the common mode, refer to the Driving
Differential Inputs section.
Figure 26. Typical Connection Diagram
ANALOG INPUT
The analog input of the AD7440/AD7450A is fully differential.
Differential signals have a number of benefits over single-
ended signals, including noise immunity based on the device’s
common-mode rejection, improvements in distortion perfor-
mance, doubling of the device’s available dynamic range, and
flexibility in input ranges and bias points. Figure 27 defines the
fully differential analog input of the AD7440/AD7450A.
Figure 27. Differential Input Definitions
The amplitude of the differential signal is the difference
between the signals applied to the VIN+ and VIN– pins
(that is, VIN+ – VIN–). VIN+ and VIN– are simultaneously driven by
two signals each of amplitude VREF that are 180° out of phase.
The amplitude of the differential signal is therefore –VREF to
+VREF peak-to-peak (2 × VREF). This is true regardless of the
common mode (CM).
The common mode is the average of the two signals, that is,
(VIN+ + VIN–)/2 and is therefore the voltage that the two inputs
are centered on. This results in the span of each input being
CM ± VREF/2. This voltage has to be set up externally, and its
range varies with VREF. As the value of VREF increases, the
common-mode range decreases. When driving the inputs with
an amplifier, the actual common-mode range is determined by
the amplifier’s output voltage swing.
Figure 28 and Figure 29 show how the common-mode range
typically varies with VREF for both a 5 V and a 3 V power supply.
The common mode must be in this range to guarantee the
functionality of the AD7440/AD7450A.
For ease of use, the common mode can be set up to equal VREF,
resulting in the differential signal being ±VREF centered on VREF.
When a conversion takes place, the common mode is rejected,
resulting in a virtually noise-free signal of amplitude –VREF to
+VREF, corresponding to the digital codes of 0 to 4096 in the
case of the AD7450A and 0 to 1024 in the AD7440.
Figure 28. Input Common-Mode Range vs. VREF
(VDD = 5 V and VREF (Max) = 3.5 V)
Figure 29. Input Common-Mode Range vs. VREF
(VDD = 3 V and VREF (Max) =2V)
03051-A-026
AD7440/
AD7450A
0.1µF
0.1µF
10µF
VREF
VDD
VIN+ SCLK
3V/5V
SUPPLY
SERIAL
INTERFACE
µC/µP
SDATA
CS
GND
VIN–
2V/2.5V
VREF
*CM IS THE COMMON-MODE VOLTAGE.
CM*
VREF
p-p
CM*
VREF
p-p
03051-A-027
V
REF
p-p V
IN+
V
IN–
V
REF
p-p
AD7440/
AD7450A
COMMON-MODE
VOLTAGE
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
00 0.5 1.0 1.5 2.0 2.5 3.0 3.5
03051-A-028
V
REF
(V)
COMMON-MODE VOLTAGE (V)
1.75V
3.25V
COMMON-MODE RANGE
2.5
0.5
1.0
1.5
2.0
00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00
03051-A-029
V
REF
(V)
COMMON-MODE VOLTAGE (V)
1V
2V
COMMON-MODE RANGE
Data Sheet AD7440/AD7450A
Rev. D | Page 17 of 27
Figure 30 shows examples of the inputs to VIN+ and VIN– for
different values of VREF for VDD = 5 V. It also gives the maximum
and minimum common-mode voltages for each reference value
according to Figure 28.
Figure 30. Examples of the Analog Inputs to VIN+ and VIN– for
Different Values of VREF for VDD = 5 V
Analog Input Structure
Figure 31 shows the equivalent circuit of the analog input
structure of the AD7440/AD7450A. The four diodes provide
ESD protection for the analog inputs. Care must be taken to
ensure that the analog input signals never exceed the supply
rails by more than 300 mV. This causes these diodes to become
forward biased and start conducting into the substrate. These
diodes can conduct up to 10 mA without causing irreversible
damage to the part. The capacitors, C1 in Figure 31, are
typically 4 pF and can primarily be attributed to pin
capacitance. The resistors are lumped components made up of
the on resistance of the switches. The value of these resistors is
typically about 100 Ω. The capacitors, C2, are the ADC’s
sampling capacitors and have a capacitance of 16 pF typically.
Figure 31. Equivalent Analog Input Circuit
Conversion Phase–Switches Open; Track Phase–Switches Closed
For ac applications, removing high frequency components from
the analog input signal through the use of an RC low-pass filter
on the relevant analog input pins is recommended. In applica-
tions where harmonic distortion and signal-to-noise ratio are
critical, the analog input should be driven from a low impe-
dance source. Large source impedances significantly affect the
ac performance of the ADC. This may necessitate the use of an
input buffer amplifier. The choice of op amp is a function of the
particular application.
When no amplifier is used to drive the analog input, the source
impedance should be limited to low values. The maximum
source impedance depends on the amount of total harmonic
distortion (THD) that can be tolerated. The THD increases as
the source impedance increases, and performance degrades.
Figure 32 shows a graph of THD vs. the analog input signal
frequency for different source impedances for VDD = 5 V.
Figure 32. THD vs. Analog Input Frequency for Various Source Impedances
for VDD = 5 V
Figure 33 shows a graph of the THD vs. the analog input
frequency for VDD of 5 V ± 5% and 3 V + 20%/–10%, while
sampling at 1 MSPS with an SCLK of 18 MHz. In this case, the
source impedance is 10 Ω.
Figure 33. THD vs. Analog Input Frequency for 3 V and 5 V Supply Voltages
03051-A-030
COMMON-MODE (CM)
CM
MIN
= 1V
CM
MAX
= 4V
REFERENCE = 2V
V
IN–
V
IN+
2V p-p
COMMON-MODE (CM)
CM
MIN
= 1.25V
CM
MAX
= 3.75V
REFERENCE = 2.5V
V
IN–
V
IN+
2.5V p-p
03051-A-031
C1
C2
R1
D
D
C1
C2
R1
D
D
V
DD
V
DD
V
IN+
V
IN–
0
–100
–80
–60
–40
–20
10 100 1000
03051-A-032
INPUT FREQUENCY (kHz)
THD (dB)
T
A
= 25°C
V
DD
= 5V
R
IN
= 1kΩ
R
IN
= 510Ω
R
IN
= 10ΩR
IN
= 300Ω
–50
–90
–85
–80
–75
–70
–65
–60
–55
10 100 1000
03051-A-033
INPUT FREQUENCY (kHz)
THD (dB)
T
A
= 25°C
V
DD
= 2.7V
V
DD
= 3.6V
V
DD
= 5.25V
V
DD
= 4.75V
AD7440/AD7450A Data Sheet
Rev. D | Page 18 of 27
DRIVING DIFFERENTIAL INPUTS
Differential operation requires VIN+ and VIN– to be driven
simultaneously with two equal signals that are 180° out of
phase. The common mode must be set up externally and has a
range determined by VREF, the power supply, and the particular
amplifier used to drive the analog inputs (see Figure 28 and
Figure 29). Differential modes of operation with either an ac or
dc input provide the best THD performance over a wide
frequency range. Because not all applications have a signal
preconditioned for differential operation, there is often a need
to perform single-ended-to-differential conversion.
Differential Amplifier
An ideal method of applying differential drive to the
AD7440/AD7450A is to use a differential amplifier such as the
AD8138. This part can be used as a single-ended-to-differential
amplifier or as a differential-to-differential amplifier. In both
cases, the analog input needs to be bipolar. It also provides
common-mode level shifting and buffering of the bipolar input
signal. Figure 34 shows how the AD8138 can be used as a
single-ended-to-differential amplifier. The positive and negative
outputs of the AD8138 are connected to the respective inputs
on the ADC via a pair of series resistors to minimize the effects
of switched capacitance on the front end of the ADCs. The RC
low-pass filter on each analog input is recommended in ac
applications to remove high frequency components of the
analog input. The architecture of the AD8138 results in outputs
that are very highly balanced over a wide frequency range
without requiring tightly matched external components.
If the analog input source being used has zero impedance, all
four resistors (RG1, RG2, RF1, and RF2) should be the same. If
the source has a 50 Ω impedance and a 50 Ω termination, for
example, the value of RG2 should be increased by 25 Ω to
balance this parallel impedance on the input and thus ensure
that both the positive and negative analog inputs have the same
gain (see Figure 34). The outputs of the amplifier are perfectly
matched, balanced differential outputs of identical amplitude
and are exactly 180° out of phase.
The AD8138 is specified with +3 V, +5 V, and ±5 V power
supplies, but the best results are obtained with a ±5 V supply.
The AD8132 is a lower cost device that could also be used in
this configuration with slight differences in characteristics to
the AD8138 but with similar performance and operation.
Figure 34. Using the AD8138 as a Single-Ended-to-Differential Amplifier
03051-A-034
+2.5V
GND
–2.5V
AD8138
R
G
1R
S
*
C*
C*
R
S
*
R
G
2
R
F
2
V
OCM
R
F
1
3.75V
V
IN–
V
IN+
V
REF
2.5V
1.25V
3.75V
2.5V
1.25V
AD7440/
AD7450A
*MO UNT AS CL OSE TO THE AD7440/AD7450A AS POSS IBLE
AND ENSURE HIG H P RE CISI ON R
S
AND C
S
ARE USED.
R
S
= 50Ω; C = 1nF
R
G
1 = R
F
1 = R
F
2 = 499Ω; R
G
2 = 523Ω
EXTERNAL
V
REF
(2. 5V )
Data Sheet AD7440/AD7450A
Rev. D | Page 19 of 27
Op Amp Pair
An op amp pair can be used to directly couple a differential
signal to the AD7440/AD7450A. The circuit configurations
shown in Figure 35 and Figure 36 show how a dual op amp can
be used to convert a single-ended signal into a differential
signal for both a bipolar and unipolar input signal, respectively.
The voltage applied to Point A sets up the common-mode
voltage. In both diagrams, it is connected in some way to the
reference, but any value in the common-mode range can be
input here to set up the common mode. The AD8022 is a
suitable dual op amp that could be used in this configuration
to provide differential drive to the AD7440/AD7450A.
Take care when choosing the op amp; the selection depends on
the required power supply and system performance objectives.
The driver circuits in Figure 35 and Figure 36 are optimized for
dc coupling applications requiring best distortion performance.
The circuit configuration shown in Figure 35 converts a
unipolar, single-ended signal into a differential signal.
The differential op amp driver circuit in Figure 36 is configured
to convert and level shift a single-ended, ground-referenced
(bipolar) signal to a differential signal centered at the VREF level
of the ADC.
Figure 35. Dual Op Amp Circuit to Convert a Single-Ended Unipolar Signal
into a Differential Signal
Figure 36. Dual Op Amp Circuit to Convert a Single-Ended Bipolar Signal into
a Differential Signal
RF Transformer
An RF transformer with a center tap offers a good solution for
generating differential inputs in systems that do not need to
be dc-coupled. Figure 37 shows how a transformer is used for
single-ended-to-differential conversion. It provides the benefits
of operating the ADC in the differential mode without contri-
buting additional noise and distortion. An RF transformer also
has the benefit of providing electrical isolation between the
signal source and the ADC. A transformer can be used for most
ac applications. The center tap is used to shift the differential
signal to the common-mode level required; in this case, it is
connected to the reference so the common-mode level is the
value of the reference.
Figure 37. Using an RF Transformer to Generate Differential Inputs
DIGITAL INPUTS
The digital inputs applied to the device are not limited by the
maximum ratings, which limit the analog limits. Instead, the
digital inputs applied, CS and SCLK, can go to 7 V and are not
restricted by the VDD + 0.3 V limits as on the analog input.
The main advantage of the inputs not being restricted to the
VDD + 0.3 V limit is that power supply sequencing issues are
avoided. If CS and SCLK are applied before VDD, there is no risk
of latch-up as there would be on the analog inputs if a signal
greater than 0.3 V was applied prior to VDD.
REFERENCE
An external reference source is required to supply the reference
to the device. This reference input can range from 100 mV to
3.5 V. With a 5 V power supply, the specified reference is 2.5 V
and the maximum reference is 3.5 V. Wi t h a 3 V power supply,
the specified reference is 2 V and the maximum reference is
2.2 V. In both cases, the reference is functional from 100 m V.
Ensure that, when choosing the reference value for a particular
application, the maximum analog input range (VIN max) is
never greater than VDD + 0.3 V to comply with the maximum
ratings of the device. The following two examples calculate the
maximum VREF input that can be used when operating the
AD7440/AD7450A at a VDD of 5 V and 3 V, respectively.
03051-A-036
VDD
2
×
VREF p-p
VREF
GND
390Ω
220Ω
220Ω
220Ω
10kΩ
27Ω
27Ω
0.1µF
V+
V–
V+
V–
A
VIN+
VIN– VREF
AD7440/
AD7450A
EXTERNAL
VREF
03051-A-035
GND V+
V–
V+
V–
A
VIN+
VDD
VIN– VREF
AD7440/
AD7450A
2
×
VREF p-p
390Ω
220Ω
220Ω
220Ω
220Ω
10kΩ
20kΩ
27Ω
27Ω
0.1µF
EXTERNAL
VREF
03051-A-037
R
RC
3.75V
2.5V
1.25V
3.75V
2.5V
1.25V
RV
IN+
V
IN–
V
REF
AD7440/
AD7450A
EXTERNAL
V
REF
AD7440/AD7450A Data Sheet
Rev. D | Page 20 of 27
Example 1
VIN max = VDD + 0.3
VIN max = VREF + VREF/2
If VDD = 5 V, t h e n VIN max = 5.3 V.
Therefore
3 × VREF/2 = 5.3 V
VREF max = 3.5 V
Thus, when operating at VDD = 5 V, the value of VREF can range
from 100 mV to a maximum value of 3.5 V. When VDD = 4.75 V,
VREF max = 3.17 V.
Example 2
VIN max = VDD + 0.3
VIN max = VREF + VREF/2
If VDD = 3 V, t h e n VIN max = 3.3 V.
Therefore,
3 × VREF/2 = 3.3 V
VREF max = 2.2 V
Thus, when operating at VDD = 3 V, the value of VREF can range
from 100 mV to a maximum value of 2.2 V. When VDD = 2.7 V,
VREF max = 2 V.
These examples show that the maximum reference applied to
the AD7440/AD7450A is directly dependent on the value
applied to VDD.
The value of the reference sets the analog input span and the
common-mode voltage range. Errors in the reference source
result in gain errors in the AD7440/AD7450A transfer function
and add to specified full-scale errors on the part. A 0.1 µF
capacitor should be used to decouple the VREF pin to GND.
Figure 38 shows a typical connection diagram for the VREF pin.
Table 6 lists examples of suitable voltage references.
Figure 38. Typical VREF Connection Diagram for VDD = 5 V
Table 6. Examples of Suitable Voltage References
Reference
Output
Voltage (V)
Initial
Accuracy (%)
Operating
Current (µA)
AD780 2.5/3 0.04 1000
ADR421 2.5 0.04 500
ADR420 2.048 0.05 500
SINGLE-ENDED OPERATION
When supplied with a 5 V power supply, the AD7440/AD7450A
can handle a single-ended input. The design of these devices is
optimized for differential operation, so with a single-ended
input, performance degrades. Linearity degrades by typically
0.2 LSB, the full-scale errors degrade typically by 1 LSB, and ac
performance is not guaranteed.
To operate the AD7440/AD7450A in single-ended mode, the
VIN+ input is coupled to the signal source, while the VIN– input is
biased to the appropriate voltage corresponding to the midscale
code transition. This voltage is the common mode, which is a
fixed dc voltage (usually the reference). The VIN+ input swings
around this value and should have a voltage span of 2 × VREF to
make use of the full dynamic range of the part. The input signal
therefore has peak-to-peak values of common mode ±VREF. If
the analog input is unipolar, an op amp in a noninverting unity
gain configuration can be used to drive the VIN+ pin. The ADC
operates from a single supply, so it is necessary to level shift
ground-based bipolar signals to comply with the input
requirements. An op amp can be configured to rescale and level
shift the ground-based bipolar signal, so it is compatible with
the selected input range of the AD7440/AD7450A (Figure 39).
Figure 39. Applying a Bipolar Single-Ended Input to the AD7440/AD7450A
03051-A-038
1
AD780
NC
8
2
V
IN
NC
7
3
GND
6
4
TEMP
5
OPSEL
TRIM
V
OUT
AD7440/
AD7450A*
V
REF
2.5V
NC
V
DD
NC
V
DD
NC = NO CONNECT
10nF 0.1µF0.1µF
0.1µF
*ADDITIONAL PINS OMITTED FOR CLARITY
03051-A-039
R
5V
2.5V
0V
+2.5V
0V
–2.5V
R
R
0.1µF
R
AD7440/
AD7450A
VREF
VIN+
VIN–
V
IN
EXTERNAL
V
REF
(2.5V)
Data Sheet AD7440/AD7450A
Rev. D | Page 21 of 27
SERIAL INTERFACE
Figure 2 and Figure 3 show detailed timing diagrams for the
serial interface of the AD7450A and the AD7440, respectively.
The serial clock provides the conversion clock and also controls
the transfer of data from the devices during conversion. CS
initiates the conversion process and frames the data transfer.
The falling edge of CS puts the track-and-hold into hold mode
and takes the bus out of three-state. The analog input is sampled
and the conversion is initiated at this point. The conversion
requires 16 SCLK cycles to complete.
Once 13 SCLK falling edges have occurred, the track-and-hold
goes back into track on the next SCLK rising edge, as shown at
Point B in Figure 2 and Figure 3. On the 16th SCLK falling
edge, the SDATA line goes back into three-state. If the rising
edge of CS occurs before 16 SCLKs have elapsed, the conversion
terminates and the SDATA line goes back into three-state.
The conversion result from the AD7440/AD7450A is provided
on the SDATA output as a serial data stream. The bits are
clocked out on the falling edge of the SCLK input. The data
stream of the AD7450A consists of four leading zeros followed
by 12 bits of conversion data provided MSB first; the data
stream of the AD7440 consists of four leading zeros, followed
by the 10 bits of conversion data followed by two trailing zeros,
which is also provided MSB first. In both cases, the output
coding is twos complement.
Sixteen serial clock cycles are required to perform a conversion
and access data from the AD7440/AD7450A. CS going low
provides the first leading zero to be read in by the DSP or
microcontroller. The remaining data is then clocked out on the
subsequent SCLK falling edges beginning with the second
leading zero. Thus, the first falling clock edge on the serial clock
provides the second leading zero. The final bit in the data
transfer is valid on the 16th falling edge, having been clocked
out on the previous (15th) falling edge. Once the conversion is
complete and the data has been accessed after the 16 clock
cycles, it is important to ensure that before the next conversion
is initiated, enough time is left to meet the acquisition and
quiet time specifications (see Timing Example 1 and Timing
Example 2). To achieve 1 MSPS with an 18 MHz clock for VDD =
3 V and 5 V, an 18-clock burst performs the conversion and
leaves enough time before the next conversion for the acquisition
and quiet time.
In applications with a slower SCLK, it may be possible to read in
data on each SCLK rising edge; that is, the first rising edge of
SCLK after the CS falling edge would have the leading zero
provided and the 15th SCLK edge would have DB0 provided.
Figure 40. Serial Interface Timing Example
03051-A-040
t
2
t
8
t
6
t
5
t
CONVERT
CS
SCLK 12345 13141516
12.5(1/
f
SCLK
)
t
ACQUISITION
1/THROUGHPUT
t
QUIET
10ns B C
AD7440/AD7450A Data Sheet
Rev. D | Page 22 of 27
Timing Example 1
Having fSCLK = 18 MHz and a throughput rate of 1 MSPS gives a
cycle time of
1/Throughput = 1/1,000,000 = 1 µs
A cycle consists of
t2 + 12.5(1/fSCLK) + tACQ = 1 µs
Therefore, if t2 = 10 ns
10 ns + 12.5(1/18 MHz) + tACQ = 1 µs
tACQ = 296 ns
This 296 ns satisfies the requirement of 290 ns for tACQ.
From Figure 40, tACQ comprises
2.5(1/fSCLK) + t8 + tQUIET
where t8 = 35 ns. This allows a value of 122 ns for tQUIET,
satisfying the minimum requirement of 60 ns.
Timing Example 2
Having fSCLK = 5 MHz and a throughput rate of 315 kSPS gives a
cycle time of
1/Throughput = 1/315,000 = 3.174 µs
A cycle consists of
t2 + 12.5(1/fSCLK) + tACQ = 3.174 µs
Therefore, if t2 is 10 ns
10 ns + 12.5(1/5 MHz) + tACQ = 3.174 µs
tACQ = 664 ns
This 664 ns satisfies the requirement of 290 ns for tACQ.
From Figure 40, tACQ comprises
2.5(1/fSCLK) + t8 + tQUIET
where t8 = 35 ns. This allows a value of 129 ns for tQUIET,
satisfying the minimum requirement of 60 ns.
As in this example and with other slower clock values, the signal
may already be acquired before the conversion is complete, but
it is still necessary to leave 60 ns minimum tQUIET between
conversions. In Timing Example 2, the signal should be fully
acquired at approximately Point C in Figure 40.
Data Sheet AD7440/AD7450A
Rev. D | Page 23 of 27
MODES OF OPERATION
The operational mode of the AD7440/AD7450A is selected by
controlling the logic state of the CS signal during a conversion.
There are two possible modes of operation, normal and power-
down. The point at which CS is pulled high after the conversion
has been initiated determines whether or not the device enters
power-down mode. Similarly, if already in power-down, CS
controls whether the devices return to normal operation or
remain in power-down. These modes of operation are designed
to provide flexible power management options. These options
can be chosen to optimize the power dissipation/throughput
rate ratio for differing application requirements.
NORMAL MODE
This mode is intended for fastest throughput rate performance.
The user does not have to worry about any power-up times with
the AD7440/AD7450A remaining fully powered up all the time.
Figure 41 shows the general diagram of the operation of the
AD7440/AD7450A in this mode. The conversion is initiated on
the falling edge of CS, as described in the Serial Interface
section. To ensure the part remains fully powered up, CS must
remain low until at least 10 SCLK falling edges have elapsed
after the falling edge of CS.
Figure 41. Normal Mode Operation
If CS is brought high any time after the 10th SCLK falling edge,
but before the 16th SCLK falling edge, the part remains
powered up but the conversion terminates and SDATA goes
back into three-state. Sixteen serial clock cycles are required to
complete the conversion and access the complete conversion
result. CS may idle high until the next conversion or may idle
low until sometime prior to the next conversion. Once a data
transfer is complete, when SDATA has returned to three-state,
another conversion can be initiated after the quiet time, tQUIET,
has elapsed by again bringing CS low.
POWER-DOWN MODE
This mode is intended for use in applications where slower
throughput rates are required; either the ADC is powered down
between each conversion, or a series of conversions may be
performed at a high throughput rate and the ADC is then
powered down for a relatively long duration between these
bursts of conversions. When the AD7440/AD7450A are in the
power-down mode, all analog circuitry is powered down. To
enter power-down mode, the conversion process must be
interrupted by bringing CS high anywhere after the second
falling edge of SCLK and before the 10th falling edge of SCLK,
as shown in Figure 42.
Figure 42. Entering Power-Down Mode
Once CS has been brought high in this window of SCLKs, the
part enters power-down, the conversion that was initiated by
the falling edge of CS is terminated, and SDATA goes back into
three-state. The time from the rising edge of CS to SDATA
three-state enabled is never greater than t8 (refer to the Timing
Specifications). If CS is brought high before the second SCLK
falling edge, the part remains in normal mode and does not
power down. This avoids accidental power-down due to glitches
on the CS line.
In order to exit this mode of operation and power up the
AD7440/AD7450A again, a dummy conversion is performed.
On the falling edge of CS, the device begins to power up and
continues to power up as long as CS is held low until after the
falling edge of the 10th SCLK. The device is fully powered up
after 1 μs has elapsed and, as shown in Figure 43, valid data
results from the next conversion.
Figure 43. Exiting Power-Down Mode
03051-A-041
110
CS
SCLK
SDATA
16
4 LEADING ZEROS + CONVERSION RESULT
03051-A-042
110
SCLK
SDATA THREE-STATE
2
CS
03051-A-043
CS
SCLK
S
DAT
A
110 16 1 10 16
A
THIS PART IF FULLY POWERED
UP WITH V
IN
FULLY ACQUIRED
PART BEGINS
TO POWER UP
INVALID DATA VALID DATA
t
POWER-UP
AD7440/AD7450A Data Sheet
Rev. D | Page 24 of 27
If CS is brought high before the 10th falling edge of SCLK, the
AD7440/AD7450A again goes back into power-down. This
avoids accidental power-up due to glitches on the CS line or an
inadvertent burst of eight SCLK cycles while CS is low. So
although the device may begin to power up on the falling edge
of CS, it again powers down on the rising edge of CS as long as
it occurs before the 10th SCLK falling edge.
POWER-UP TIME
The power-up time of the AD7440/AD7450A is typically 1 µs,
which means that with any frequency of SCLK up to 18 MHz,
one dummy cycle is always sufficient to allow the device to
power up. Once the dummy cycle is complete, the ADC is fully
powered up and the input signal is acquired properly. The quiet
time, tQUIET, must still be allowed from the point at which the
bus goes back into three-state after the dummy conversion to
the next falling edge of CS.
When running at the maximum throughput rate of 1 MSPS, the
AD7440/AD7450A power up and acquire a signal within
±0.5 LSB in one dummy cycle, 1 µs. When powering up from
the power-down mode with a dummy cycle, as in Figure 43, the
track-and-hold, which was in hold mode while the part was
powered down, returns to track mode after the first SCLK edge
the part receives after the falling edge of CS. This is shown as
Point A in Figure 43.
Although at any SCLK frequency one dummy cycle is sufficient
to power up the device and acquire VIN, it does not mean that a
full dummy cycle of 16 SCLKs must always elapse to power up
the device and acquire VIN fully; 1 µs is sufficient to power up
the device and acquire the input signal.
For example, if a 5 MHz SCLK frequency was applied to the
ADC, the cycle time would be 3.2 µs (1/(5 MHz) × 16). In one
dummy cycle, 3.2 µs, the part would be powered up and VIN
acquired fully. However, after 1 µs with a 5 MHz SCLK, only
five SCLK cycles would have elapsed. At this stage, the ADC
would be fully powered up and the signal acquired. Therefore,
in this case, the CS can be brought high after the 10th SCLK
falling edge and brought low again after a time, tQUIET, to initiate
the conversion.
When power supplies are first applied to the device, the ADC
may power up in either power-down mode or normal mode.
Because of this, it is best to allow a dummy cycle to elapse to
ensure the part is fully powered up before attempting a valid
conversion. Likewise, if the user wants the part to power up in
power-down mode, the dummy cycle may be used to ensure the
device is in power-down by executing a cycle such as the one
shown in Figure 42.
Once supplies are applied to the AD7440/AD7450A, the power-
up time is the same as that when powering up from power-
down mode. It takes about 1 µs to power up fully if the part
powers up in normal mode. It is not necessary to wait 1 µs
before executing a dummy cycle to ensure the desired mode of
operation. Instead, the dummy cycle can occur directly after
power is supplied to the ADC. If the first valid conversion is
then performed directly after the dummy conversion, ensure
that adequate acquisition time has been allowed.
As mentioned earlier, when powering up from the power-down
mode, the part returns to track mode upon the first SCLK edge
applied after the falling edge of CS. However, when the ADC
powers up initially after supplies are applied, the track-and-hold
is already in track mode. Assuming the user has the facility to
monitor the ADC supply current, this means the ADC powers
up in the desired mode of operation, and thus a dummy cycle is
not required to change mode. A dummy cycle is therefore not
required to place the track-and-hold into track mode.
POWER vs. THROUGHPUT RATE
By using the power-down mode on the AD7440/AD7450A
when not converting, the average power consumption of the
ADC decreases at lower throughput rates. Figure 44 shows how,
as the throughput rate is reduced, the device remains in its
power-down state longer and the average power consumption is
reduced accordingly for both 5 V and 3 V power supplies.
For example, if the AD7440/AD7450A are operated in
continuous sampling mode with a throughput rate of 100 kSPS
and an SCLK of 18 MHz, and the device is placed in power-
down mode between conversions, the power consumption is
calculated as follows:
Power Dissipation during Normal Operation = 9.25 mW
max (for VDD = 5 V)
If the power-up time is one dummy cycle (1 µs), and the
remaining conversion time is another cycle (1 µs), the
AD7440/AD7450A can be said to dissipate 9.25 mW for 2 µs
during each conversion cycle. This 2 µs figure assumes a very
short time to enter power-down mode. This increases as the
burst of clocks used to enter this mode is increased.
If the throughput rate = 100 kSPS, the cycle time = 10 µs and
the average power dissipated during each cycle is
(2/10) × 9.25 mW = 1.85 mW.
For the same scenario, if VDD = 3 V, the power dissipation
during normal operation is 4 mW max.
The AD7440/AD7450A can now be said to dissipate 4 mW for
2 µs during each conversion cycle. This 2 µs figure assumes a
very short time to enter power-down mode. This increases as
the burst of clocks used to enter this mode is increased.
Data Sheet AD7440/AD7450A
Rev. D | Page 25 of 27
Thus, the average power dissipated during each cycle with a
throughput rate of 100 kSPS is (2/10) × 4 mW = 0.8 m W.
This is how the power numbers in Figure 44 are calculated.
For throughput rates above 320 kSPS, it is recommended to
reduce the serial clock frequency for best power performance.
Figure 44. Power vs. Throughput Rate for Power-Down Mode
GROUNDING AND LAYOUT HINTS
The printed circuit board that houses the AD7440/AD7450A
should be designed so that the analog and digital sections are
separated and confined to certain areas of the board. This
facilitates the use of ground planes that can be easily separated.
A minimum etch technique is generally best for ground planes
as it gives the best shielding. Digital and analog ground planes
should be joined in only one place, a star ground point
established as close to the GND pin on the AD7440/AD7450A
as possible. Avoid running digital lines under the devices
because this couples noise onto the die. The analog ground
plane should be allowed to run under the AD7440/AD7450A to
avoid noise coupling. The power supply lines to the AD7440/
AD7450A should use as large a trace as possible to provide low
impedance paths and reduce the effects of glitches on the power
supply line.
Fast switching signals like clocks should be shielded with digital
ground to avoid radiating noise to other sections of the board,
and clock signals should never run near the analog inputs.
Avoid crossover of digital and analog signals. Traces on
opposite sides of the board should run at right angles to each
other. This reduces the effects of feedthrough through the
board. A microstrip technique is by far the best but is not
always possible with a double-sided board.
In this technique, the component side of the board is dedicated
to ground planes while signals are placed on the solder side.
Good decoupling is also important. All analog supplies should
be decoupled with 10 µF tantalum capacitors in parallel with
0.1 µF capacitors to GND. To achieve the best from these
decoupling components, they must be placed as close as
possible to the device.
EVALUATING THE AD7440/AD7450A
PERFORMANCE
The evaluation board package includes a fully assembled and
tested evaluation board, documentation, and software for
controlling the board from a PC via the evaluation board
controller. The evaluation board controller can be used in
conjunction with the AD7440/AD7450A evaluation board, as
well as many other Analog Devices evaluation boards ending
with the CB designator, to demonstrate and evaluate the ac and
dc performance of the AD7440/AD7450A.
The software allows the user to perform ac (fast Fourier
transform) and dc (histogram of codes) tests on the device. See
the AD7440/AD7450A application note that accompanies the
evaluation kit for more information.
03051-A-044
THROUGHPUT (kSPS)
100
0 350
POWER (mW)
0.01 50 100 150 200 250 300
0.1
1
10 VDD = 5V
VDD = 3V
AD7440/AD7450A Data Sheet
Rev. D | Page 26 of 27
OUTLINE DIMENSIONS
Figure 45. 8-Lead Small Outline Transistor Package [SOT-23]
(RJ-8)
Dimensions shown in millimeters
Figure 46. 8-Lead Mini Small Outline Package [MSOP]
(RM-8)
Dimensions shown in millimeters
COMPLIANT TO JEDEC STANDARDS MO-178-BA
8°
4°
0°
SEATING
PLANE
1.95
BSC
0.65 BSC
0.60
BSC
7 6
1 2 3 4
5
3.00
2.90
2.80
3.00
2.80
2.60
1.70
1.60
1.50
1.30
1.15
0.90
0.15 MAX
0.05 MIN
1.45 MAX
0.95 MIN
0.22 MAX
0.08 MIN
0.38 MAX
0.22 MIN
0.60
0.45
0.30
PIN 1
INDICATOR
8
12-16-2008-A
COM P LIANT T O JEDEC S TANDARDS M O-187-AA
6°
0°
0.80
0.55
0.40
4
8
1
5
0.65 BSC
0.40
0.25
1.10 M AX
3.20
3.00
2.80
COPLANARITY
0.10
0.23
0.09
3.20
3.00
2.80
5.15
4.90
4.65
PIN 1
IDENTIFIER
15° M AX
0.95
0.85
0.75
0.15
0.05
10-07-2009-B
Data Sheet AD7440/AD7450A
Rev. D | Page 27 of 27
ORDERING GUIDE
Model1 Temperature Range Linearity Error (LSB)2
Package
Description Package Option Branding
AD7440BRTZ-REEL7 –40°C to +85°C ±0.5 8-lead SOT-23 RJ-8 C3J
AD7440BRTZ-R2 –40°C to +85°C ±0.5 8-lead SOT-23 RJ-8 C3J
AD7440BRMZ –40°C to +85°C ±0.5 8-lead MSOP RM-8 C3J
AD7450ABRTZ-REEL7 –40°C to +85°C ±1 8-lead SOT-23 RJ-8 C4N
AD7450ABRMZ –40°C to +85°C ±1 8-lead MSOP RM-8 C4N
EVAL-AD7450ASDZ Evaluation Board
EVAL-SDP-CB1Z Controller Board
1 Z = RoHS Compliant Part.
2 Linearity error refers to integral nonlinearity error.
©2003–2015 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
D03051-0-7/15(D)