ADC0801/ADC0802/ADC0803/ADC0804/ADC0805
8-Bit µP Compatible A/D Converters
General Description
The ADC0801, ADC0802, ADC0803, ADC0804 and
ADC0805 are CMOS 8-bit successive approximation A/D
converters that use a differential potentiometric
laddersimilar to the 256R products. These converters are
designed to allow operation with the NSC800 and INS8080A
derivative control bus with TRI-STATE output latches directly
driving the data bus. These A/Ds appear like memory loca-
tions or I/O ports to the microprocessor and no interfacing
logic is needed.
Differential analog voltage inputs allow increasing the
common-mode rejection and offsetting the analog zero input
voltage value. In addition, the voltage reference input can be
adjusted to allow encoding any smaller analog voltage span
to the full 8 bits of resolution.
Features
nCompatible with 8080 µP derivativesno interfacing
logic needed - access time - 135 ns
nEasy interface to all microprocessors, or operates “stand
alone”
nDifferential analog voltage inputs
nLogic inputs and outputs meet both MOS and TTL
voltage level specifications
nWorks with 2.5V (LM336) voltage reference
nOn-chip clock generator
n0V to 5V analog input voltage range with single 5V
supply
nNo zero adjust required
n0.3" standard width 20-pin DIP package
n20-pin molded chip carrier or small outline package
nOperates ratiometrically or with 5 V
DC
, 2.5 V
DC
,or
analog span adjusted voltage reference
Key Specifications
nResolution 8 bits
nTotal error ±
1
4
LSB, ±
1
2
LSB and ±1 LSB
nConversion time 100 µs
Connection Diagram
Ordering Information
TEMP RANGE 0˚C TO 70˚C 0˚C TO 70˚C −40˚C TO +85˚C
±
1
4
Bit Adjusted ADC0801LCN
ERROR ±
1
2
Bit Unadjusted ADC0802LCWM ADC0802LCN
±
1
2
Bit Adjusted ADC0803LCN
±1Bit Unadjusted ADC0804LCWM ADC0804LCN ADC0805LCN/ADC0804LCJ
PACKAGE OUTLINE M20BSmall
Outline N20AMolded DIP
Z-80®is a registered trademark of Zilog Corp.
ADC080X
Dual-In-Line and Small Outline (SO) Packages
DS005671-30
See Ordering Information
November 1999
ADC0801/ADC0802/ADC0803/ADC0804/ADC0805 8-Bit µP Compatible A/D Converters
© 2001 National Semiconductor Corporation DS005671 www.national.com
Typical Applications
Error Specification (Includes Full-Scale,
Zero Error, and Non-Linearity)
Part Full- V
REF
/2=2.500 V
DC
V
REF
/2=No Connection
Number Scale (No Adjustments) (No Adjustments)
Adjusted
ADC0801 ±
1
4
LSB
ADC0802 ±
1
2
LSB
ADC0803 ±
1
2
LSB
ADC0804 ±1 LSB
ADC0805 ±1 LSB
DS005671-1
8080 Interface
DS005671-31
ADC0801/ADC0802/ADC0803/ADC0804/ADC0805
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Absolute Maximum Ratings (Notes 1, 2)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage (V
CC
) (Note 3) 6.5V
Voltage
Logic Control Inputs −0.3V to +18V
At Other Input and Outputs −0.3V to (V
CC
+0.3V)
Lead Temp. (Soldering, 10 seconds)
Dual-In-Line Package (plastic) 260˚C
Dual-In-Line Package (ceramic) 300˚C
Surface Mount Package
Vapor Phase (60 seconds) 215˚C
Infrared (15 seconds) 220˚C
Storage Temperature Range −65˚C to +150˚C
Package Dissipation at T
A
=25˚C 875 mW
ESD Susceptibility (Note 10) 800V
Operating Ratings (Notes 1, 2)
Temperature Range T
MIN
T
A
T
MAX
ADC0804LCJ −40˚CT
A
+85˚C
ADC0801/02/03/05LCN −40˚CT
A
+85˚C
ADC0804LCN 0˚CT
A
+70˚C
ADC0802/04LCWM 0˚CT
A
+70˚C
Range of V
CC
4.5 V
DC
to 6.3 V
DC
Electrical Characteristics
The following specifications apply for V
CC
=5 V
DC
,T
MIN
T
A
T
MAX
and f
CLK
=640 kHz unless otherwise specified.
Parameter Conditions Min Typ Max Units
ADC0801: Total Adjusted Error (Note 8) With Full-Scale Adj. ±
1
4
LSB
(See Section 2.5.2)
ADC0802: Total Unadjusted Error (Note 8) V
REF
/2=2.500 V
DC
±
1
2
LSB
ADC0803: Total Adjusted Error (Note 8) With Full-Scale Adj. ±
1
2
LSB
(See Section 2.5.2)
ADC0804: Total Unadjusted Error (Note 8) V
REF
/2=2.500 V
DC
±1 LSB
ADC0805: Total Unadjusted Error (Note 8) V
REF
/2-No Connection ±1 LSB
V
REF
/2 Input Resistance (Pin 9) ADC0801/02/03/05 2.5 8.0 k
ADC0804 (Note 9) 0.75 1.1 k
Analog Input Voltage Range (Note 4) V(+) or V(−) Gnd–0.05 V
CC
+0.05 V
DC
DC Common-Mode Error Over Analog Input Voltage ±1/16 ±
1
8
LSB
Range
Power Supply Sensitivity V
CC
=5 V
DC
±10% Over ±1/16 ±
1
8
LSB
Allowed V
IN
(+) and V
IN
(−)
Voltage Range (Note 4)
AC Electrical Characteristics
The following specifications apply for V
CC
=5 V
DC
and T
MIN
T
A
T
MAX
unless otherwise specified.
Symbol Parameter Conditions Min Typ Max Units
T
C
Conversion Time f
CLK
=640 kHz (Note 6) 103 114 µs
T
C
Conversion Time (Notes 5, 6) 66 73 1/f
CLK
f
CLK
Clock Frequency V
CC
=5V, (Note 5) 100 640 1460 kHz
Clock Duty Cycle 40 60 %
CR Conversion Rate in Free-Running INTR tied to WR with 8770 9708 conv/s
Mode CS =0 V
DC
,f
CLK
=640 kHz
t
W(WR)L
Width of WR Input (Start Pulse Width) CS =0 V
DC
(Note 7) 100 ns
t
ACC
Access Time (Delay from Falling C
L
=100 pF 135 200 ns
Edge of RD to Output Data Valid)
t
1H
,t
0H
TRI-STATE Control (Delay C
L
=10 pF, R
L
=10k 125 200 ns
from Rising Edge of RD to (See TRI-STATE Test
Hi-Z State) Circuits)
t
WI
,t
RI
Delay from Falling Edge 300 450 ns
of WR or RD to Reset of INTR
C
IN
Input Capacitance of Logic 5 7.5 pF
Control Inputs
ADC0801/ADC0802/ADC0803/ADC0804/ADC0805
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AC Electrical Characteristics (Continued)
The following specifications apply for V
CC
=5 V
DC
and T
MIN
T
A
T
MAX
unless otherwise specified.
Symbol Parameter Conditions Min Typ Max Units
C
OUT
TRI-STATE Output 5 7.5 pF
Capacitance (Data Buffers)
CONTROL INPUTS [Note: CLK IN (Pin 4) is the input of a Schmitt trigger circuit and is therefore specified separately]
V
IN
(1) Logical “1” Input Voltage V
CC
=5.25 V
DC
2.0 15 V
DC
(Except Pin 4 CLK IN)
V
IN
(0) Logical “0” Input Voltage V
CC
=4.75 V
DC
0.8 V
DC
(Except Pin 4 CLK IN)
I
IN
(1) Logical “1” Input Current V
IN
=5 V
DC
0.005 1 µA
DC
(All Inputs)
I
IN
(0) Logical “0” Input Current V
IN
=0 V
DC
−1 −0.005 µA
DC
(All Inputs)
CLOCK IN AND CLOCK R
V
T
+ CLK IN (Pin 4) Positive Going 2.7 3.1 3.5 V
DC
Threshold Voltage
V
T
CLK IN (Pin 4) Negative 1.5 1.8 2.1 V
DC
Going Threshold Voltage
V
H
CLK IN (Pin 4) Hysteresis 0.6 1.3 2.0 V
DC
(V
T
+)−(V
T
−)
V
OUT
(0) Logical “0” CLK R Output I
O
=360 µA 0.4 V
DC
Voltage V
CC
=4.75 V
DC
V
OUT
(1) Logical “1” CLK R Output I
O
=−360 µA 2.4 V
DC
Voltage V
CC
=4.75 V
DC
DATA OUTPUTS AND INTR
V
OUT
(0) Logical “0” Output Voltage
Data Outputs I
OUT
=1.6 mA, V
CC
=4.75 V
DC
0.4 V
DC
INTR Output I
OUT
=1.0 mA, V
CC
=4.75 V
DC
0.4 V
DC
V
OUT
(1) Logical “1” Output Voltage I
O
=−360 µA, V
CC
=4.75 V
DC
2.4 V
DC
V
OUT
(1) Logical “1” Output Voltage I
O
=−10 µA, V
CC
=4.75 V
DC
4.5 V
DC
I
OUT
TRI-STATE Disabled Output V
OUT
=0 V
DC
−3 µA
DC
Leakage (All Data Buffers) V
OUT
=5 V
DC
A
DC
I
SOURCE
V
OUT
Short to Gnd, T
A
=25˚C 4.5 6 mA
DC
I
SINK
V
OUT
Short to V
CC
,T
A
=25˚C 9.0 16 mA
DC
POWER SUPPLY
I
CC
Supply Current (Includes f
CLK
=640 kHz,
Ladder Current) V
REF
/2=NC, T
A
=25˚C
and CS =5V
ADC0801/02/03/04LCJ/05 1.1 1.8 mA
ADC0804LCN/LCWM 1.9 2.5 mA
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. DC and AC electrical specifications do not apply when operating
the device beyond its specified operating conditions.
Note 2: All voltages are measured with respect to Gnd, unless otherwise specified. The separate A Gnd point should always be wired to the D Gnd.
Note 3: A zener diode exists, internally, from VCC to Gnd and has a typical breakdown voltage of 7 VDC.
Note 4: For VIN(−)VIN(+) the digital output code will be 0000 0000.Two on-chip diodes are tied to each analog input (see block diagram) which will forward conduct
for analog input voltages one diode drop below ground or one diode drop greater than the VCC supply. Be careful, during testing at low VCC levels (4.5V), as high
level analog inputs (5V) can cause this input diode to conduct–especially at elevated temperatures, and cause errors for analog inputs near full-scale. The spec
allows 50 mV forward bias of either diode. This means that as long as the analog VIN does not exceed the supply voltage by more than 50 mV, the output code will
be correct. To achieve an absolute 0 VDC to5V
DC input voltage range will therefore require a minimum supply voltage of 4.950 VDC over temperature variations,
initial tolerance and loading.
Note 5: Accuracy is guaranteed at fCLK = 640 kHz. At higher clock frequencies accuracy can degrade. For lower clock frequencies, the duty cycle limits can be
extended so long as the minimum clock high time interval or minimum clock low time interval is no less than 275 ns.
Note 6: With an asynchronous start pulse, up to 8 clock periods may be required before the internal clock phases are proper to start the conversion process. The
start request is internally latched, see
Figure 4
and section 2.0.
ADC0801/ADC0802/ADC0803/ADC0804/ADC0805
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AC Electrical Characteristics (Continued)
Note 7: The CS input is assumed to bracket the WR strobe input and therefore timing is dependent on the WR pulse width.An arbitrarily wide pulse width will hold
the converter in a reset mode and the start of conversion is initiated by the low to high transition of the WR pulse (see timing diagrams).
Note 8: None of these A/Ds requires a zero adjust (see section 2.5.1). To obtain zero code at other analog input voltages see section 2.5 and
Figure 7
.
Note 9: The VREF/2 pin is the center point of a two-resistor divider connected from VCC to ground. In all versions of the ADC0801, ADC0802, ADC0803, and
ADC0805, and in the ADC0804LCJ, each resistor is typically 16 k. In all versions of the ADC0804 except the ADC0804LCJ, each resistor is typically 2.2 k.
Note 10: Human body model, 100 pF discharged through a 1.5 kresistor.
Typical Performance Characteristics
Logic Input Threshold Voltage
vs. Supply Voltage
DS005671-38
Delay From Falling Edge of
RD to Output Data Valid
vs. Load Capacitance
DS005671-39
CLK IN Schmitt Trip Levels
vs. Supply Voltage
DS005671-40
f
CLK
vs. Clock Capacitor
DS005671-41
Full-Scale Error vs
Conversion Time
DS005671-42
Effect of Unadjusted Offset Error
vs. V
REF
/2 Voltage
DS005671-43
Output Current vs
Temperature
DS005671-44
Power Supply Current
vs Temperature (Note 9)
DS005671-45
Linearity Error at Low
V
REF
/2 Voltages
DS005671-46
ADC0801/ADC0802/ADC0803/ADC0804/ADC0805
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TRI-STATE Test Circuits and Waveforms
Timing Diagrams (All timing is measured from the 50% voltage points)
t
1H
DS005671-47
t
1H
,C
L
=10 pF
DS005671-48
tr=20 ns
t
0H
DS005671-49
t
0H
,C
L
=10 pF
DS005671-50
tr=20 ns
DS005671-51
ADC0801/ADC0802/ADC0803/ADC0804/ADC0805
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Timing Diagrams (All timing is measured from the 50% voltage points) (Continued)
Typical Applications
Output Enable and Reset with INTR
DS005671-52
Note: Read strobe must occur 8 clock periods (8/fCLK) after assertion of interrupt to guarantee reset of INTR .
6800 Interface
DS005671-53
Ratiometeric with Full-Scale Adjust
DS005671-54
Note: before using caps at VIN or VREF/2,
see section 2.3.2 Input Bypass Capacitors.
ADC0801/ADC0802/ADC0803/ADC0804/ADC0805
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Typical Applications (Continued)
Absolute with a 2.500V Reference
DS005671-55
*For low power, see also LM385–2.5
Absolute with a 5V Reference
DS005671-56
Zero-Shift and Span Adjust: 2V V
IN
5V
DS005671-57
Span Adjust: 0V V
IN
3V
DS005671-58
ADC0801/ADC0802/ADC0803/ADC0804/ADC0805
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Typical Applications (Continued)
Directly Converting a Low-Level Signal
DS005671-59
VREF/2=256 mV
A µP Interfaced Comparator
DS005671-60
For:
VIN(+)>VIN(−)
Output=FFHEX
For:
VIN(+)<VIN(−)
Output=00HEX
1 mV Resolution with µP Controlled Range
DS005671-61
VREF/2=128 mV
1 LSB=1 mV
VDACVIN(VDAC+256 mV)
0VDAC <2.5V
ADC0801/ADC0802/ADC0803/ADC0804/ADC0805
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Typical Applications (Continued)
Digitizing a Current Flow
DS005671-62
Self-Clocking Multiple A/Ds
DS005671-63
* Use a large R value
to reduce loading
at CLK R output.
External Clocking
DS005671-64
100 kHzfCLK1460 kHz
ADC0801/ADC0802/ADC0803/ADC0804/ADC0805
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Typical Applications (Continued)
Self-Clocking in Free-Running Mode
DS005671-65
*After power-up, a momentary grounding of the WR input is needed to
guarantee operation.
µP Interface for Free-Running A/D
DS005671-66
Operating with “Automotive” Ratiometric Transducers
DS005671-67
*VIN(−)=0.15 VCC
15% of VCCVXDR85% of VCC
Ratiometric with V
REF
/2 Forced
DS005671-68
µP Compatible Differential-Input Comparator with Pre-Set V
OS
(with or without Hysteresis)
DS005671-69
*See
Figure 5
to select R value
DB7=“1” for VIN(+)>VIN(−)+(VREF/2)
Omit circuitry within the dotted area if
hysteresis is not needed
ADC0801/ADC0802/ADC0803/ADC0804/ADC0805
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Typical Applications (Continued)
Handling ±10V Analog Inputs
DS005671-70
*Beckman Instruments #694-3-R10K resistor array
Low-Cost, µP Interfaced, Temperature-to-Digital
Converter
DS005671-71
µP Interfaced Temperature-to-Digital Converter
DS005671-72
*Circuit values shown are for 0˚CTA+128˚C
***Can calibrate each sensor to allow easy replacement, then A/D can be calibrated with a pre-set input voltage.
ADC0801/ADC0802/ADC0803/ADC0804/ADC0805
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Typical Applications (Continued)
Handling ±5V Analog Inputs
DS005671-33
*Beckman Instruments #694-3-R10K resistor array
Read-Only Interface
DS005671-34
µP Interfaced Comparator with Hysteresis
DS005671-35
Protecting the Input
DS005671-9
Diodes are 1N914
ADC0801/ADC0802/ADC0803/ADC0804/ADC0805
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Typical Applications (Continued)
Analog Self-Test for a System
DS005671-36
A Low-Cost, 3-Decade Logarithmic Converter
DS005671-37
*LM389 transistors
A, B, C, D = LM324A quad op amp
ADC0801/ADC0802/ADC0803/ADC0804/ADC0805
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Typical Applications (Continued)
3-Decade Logarithmic A/D Converter
DS005671-73
Noise Filtering the Analog Input
DS005671-74
fC=20 Hz
Uses Chebyshev implementation for steeper roll-off unity-gain, 2nd order,
low-pass filter
Adding a separate filter for each channel increases system response time
if an analog multiplexer is used
Multiplexing Differential Inputs
DS005671-75
Output Buffers with A/D Data Enabled
DS005671-76
*A/D output data is updated 1 CLK period prior to assertion of INTR
Increasing Bus Drive and/or Reducing Time on Bus
DS005671-77
*Allows output data to set-up at falling edge of CS
ADC0801/ADC0802/ADC0803/ADC0804/ADC0805
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Typical Applications (Continued)
Functional Description
1.0 UNDERSTANDING A/D ERROR SPECS
A perfect A/D transfer characteristic (staircase waveform) is
shown in
Figure 1
. The horizontal scale is analog input
voltage and the particular points labeled are in steps of 1
LSB (19.53 mV with 2.5V tied to the V
REF
/2 pin). The digital
output codes that correspond to these inputs are shown as
D−1, D, and D+1. For the perfect A/D, not only will
center-value (A−1, A, A+1, ....)analog inputs produce
the correct output digital codes, but also each riser (the
transitions between adjacent output codes) will be located
±
1
2
LSB away from each center-value. As shown, the risers
are ideal and have no width. Correct digital output codes will
be provided for a range of analog input voltages that extend
Sampling an AC Input Signal
DS005671-78
Note 11: Oversample whenever possible [keep fs >2f(−60)] to eliminate input frequency folding (aliasing) and to allow for the skirt response of the filter.
Note 12: Consider the amplitude errors which are introduced within the passband of the filter.
70% Power Savings by Clock Gating
DS005671-79
(Complete shutdown takes 30 seconds.)
Power Savings by A/D and V
REF
Shutdown
DS005671-80
*Use ADC0801, 02, 03 or 05 for lowest power consumption.
Note: Logic inputs can be driven to VCC with A/D supply at zero volts.
Buffer prevents data bus from overdriving output of A/D when in shutdown mode.
ADC0801/ADC0802/ADC0803/ADC0804/ADC0805
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Functional Description (Continued)
±
1
2
LSB from the ideal center-values. Each tread (the range
of analog input voltage that provides the same digital output
code) is therefore 1 LSB wide.
Figure 2
shows a worst case error plot for the ADC0801. All
center-valued inputs are guaranteed to produce the correct
output codes and the adjacent risers are guaranteed to be
no closer to the center-value points than ±
1
4
LSB. In other
words, if we apply an analog input equal to the center-value
±
1
4
LSB,
we guarantee
that the A/D will produce the correct
digital code. The maximum range of the position of the code
transition is indicated by the horizontal arrow and it is guar-
anteed to be no more than
1
2
LSB.
The error curve of
Figure 3
shows a worst case error plot for
the ADC0802. Here we guarantee that if we apply an analog
input equal to the LSB analog voltage center-value the A/D
will produce the correct digital code.
Next to each transfer function is shown the corresponding
error plot. Many people may be more familiar with error plots
than transfer functions. The analog input voltage to the A/D
is provided by either a linear ramp or by the discrete output
steps of a high resolution DAC. Notice that the error is
continuously displayed and includes the quantization uncer-
tainty of the A/D. For example the error at point 1 of
Figure 1
is +
1
2
LSB because the digital code appeared
1
2
LSB in
advance of the center-value of the tread. The error plots
always have a constant negative slope and the abrupt up-
side steps are always 1 LSB in magnitude.
Transfer Function
DS005671-81
Error Plot
DS005671-82
FIGURE 1. Clarifying the Error Specs of an A/D Converter
Accuracy=±0 LSB: A Perfect A/D
Transfer Function
DS005671-83
Error Plot
DS005671-84
FIGURE 2. Clarifying the Error Specs of an A/D Converter
Accuracy=±
1
4
LSB
ADC0801/ADC0802/ADC0803/ADC0804/ADC0805
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Functional Description (Continued)
2.0 FUNCTIONAL DESCRIPTION
The ADC0801 series contains a circuit equivalent of the
256R network. Analog switches are sequenced by succes-
sive approximation logic to match the analog difference input
voltage [V
IN
(+)−V
IN
(−)] to a corresponding tap on the R
network. The most significant bit is tested first and after 8
comparisons (64 clock cycles) a digital 8-bit binary code
(1111 1111 = full-scale) is transferred to an output latch and
then an interrupt is asserted (INTR makes a high-to-low
transition). A conversion in process can be interrupted by
issuing a second start command. The device may be oper-
ated in the free-running mode by connecting INTR to the WR
input with CS =0. To ensure start-up under all possible
conditions, an external WR pulse is required during the first
power-up cycle.
On the high-to-low transition of the WR input the internal
SAR latches and the shift register stages are reset. As long
as the CS input and WR input remain low, theA/D will remain
in a reset state.
Conversion will start from 1 to 8 clock
periods after at least one of these inputs makes a low-to-high
transition
.
A functional diagram of the A/D converter is shown in
Figure
4
. All of the package pinouts are shown and the major logic
control paths are drawn in heavier weight lines.
The converter is started by having CS and WR simulta-
neously low. This sets the start flip-flop (F/F) and the result-
ing “1” level resets the 8-bit shift register, resets the Interrupt
(INTR) F/F and inputs a “1” to the D flop, F/F1, which is at the
input end of the 8-bit shift register. Internal clock signals then
transfer this “1” to the Q output of F/F1. The AND gate, G1,
combines this “1” output with a clock signal to provide a reset
signal to the start F/F. If the set signal is no longer present
(either WR or CS is a “1”) the start F/F is reset and the 8-bit
shift register then can have the “1” clocked in, which starts
the conversion process. If the set signal were to still be
present, this reset pulse would have no effect (both outputs
of the start F/F would momentarily be at a “1” level) and the
8-bit shift register would continue to be held in the reset
mode. This logic therefore allows for wide CS and WR
signals and the converter will start after at least one of these
signals returns high and the internal clocks again provide a
reset signal for the start F/F.
Transfer Function
DS005671-85
Error Plot
DS005671-86
FIGURE 3. Clarifying the Error Specs of an A/D Converter
Accuracy=±
1
2
LSB
ADC0801/ADC0802/ADC0803/ADC0804/ADC0805
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Functional Description (Continued)
After the “1” is clocked through the 8-bit shift register (which
completes the SAR search) it appears as the input to the
D-type latch, LATCH 1. As soon as this “1” is output from the
shift register, theAND gate, G2, causes the new digital word
to transfer to the TRI-STATE output latches. When LATCH 1
is subsequently enabled, the Q output makes a high-to-low
transition which causes the INTR F/F to set. An inverting
buffer then supplies the INTR input signal.
Note that this SET control of the INTR F/F remains low for 8
of the external clock periods (as the internal clocks run at
1
8
of the frequency of the external clock). If the data output is
continuously enabled (CS and RD both held low), the INTR
output will still signal the end of conversion (by a high-to-low
transition), because the SET input can control the Q output
of the INTR F/F even though the RESET input is constantly
at a “1” level in this operating mode. This INTR output will
therefore stay low for the duration of the SET signal, which is
8 periods of the external clock frequency (assuming the A/D
is not started during this interval).
When operating in the free-running or continuous conversion
mode (INTR pin tied to WR and CS wired lowsee also
section 2.8), the START F/F is SET by the high-to-low tran-
sition of the INTR signal. This resets the SHIFT REGISTER
which causes the input to the D-type latch, LATCH 1, to go
low.As the latch enable input is still present, the Q output will
go high, which then allows the INTR F/F to be RESET. This
reduces the width of the resulting INTR output pulse to only
a few propagation delays (approximately 300 ns).
When data is to be read, the combination of both CS and RD
being low will cause the INTR F/F to be reset and the
TRI-STATE output latches will be enabled to provide the 8-bit
digital outputs.
2.1 Digital Control Inputs
The digital control inputs (CS, RD, and WR) meet standard
T
2
L logic voltage levels. These signals have been renamed
when compared to the standardA/D Start and Output Enable
labels. In addition, these inputs are active low to allow an
easy interface to microprocessor control busses. For
non-microprocessor based applications, the CS input (pin 1)
can be grounded and the standard A/D Start function is
obtained by an active low pulse applied at the WR input (pin
3) and the Output Enable function is caused by an active low
pulse at the RD input (pin 2).
DS005671-13
Note 13: CS shown twice for clarity.
Note 14: SAR = Successive Approximation Register.
FIGURE 4. Block Diagram
ADC0801/ADC0802/ADC0803/ADC0804/ADC0805
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Functional Description (Continued)
2.2 Analog Differential Voltage Inputs and
Common-Mode Rejection
This A/D has additional applications flexibility due to the
analog differential voltage input. The V
IN
(−) input (pin 7) can
be used to automatically subtract a fixed voltage value from
the input reading (tare correction). This is also useful in 4
mA–20 mA current loop conversion. In addition,
common-mode noise can be reduced by use of the differen-
tial input.
The time interval between sampling V
IN
(+) and V
IN
(−) is 4-
1
2
clock periods. The maximum error voltage due to this slight
time difference between the input voltage samples is given
by:
where:
V
e
is the error voltage due to sampling delay
V
P
is the peak value of the common-mode voltage
f
cm
is the common-mode frequency
As an example, to keep this error to
1
4
LSB (5 mV) when
operating with a 60 Hz common-mode frequency, f
cm
, and
using a 640 kHzA/D clock, f
CLK
, would allow a peak value of
the common-mode voltage, V
P
, which is given by:
or
which gives
V
P