FEATURES
●4-WIRE TOUCH SCREEN INTERFACE
●RATIOMETRIC CONVERSION
●SINGLE SUPPLY: 2.7V to 5V
●UP TO 125kHz CONVERSION RATE
●SERIAL INTERFACE
●PROGRAMMABLE 8- OR 12-BIT RESOLUTION
●2 AUXILIARY ANALOG INPUTS
●FULL POWER-DOWN CONTROL
DESCRIPTION
The ADS7843 is a 12-bit sampling Analog-to-Digital Con-
verter (ADC) with a synchronous serial interface and low on-
resistance switches for driving touch screens. Typical power
dissipation is 750µW at a 125kHz throughput rate and a
+2.7V supply. The reference voltage (VREF) can be varied
between 1V and +VCC, providing a corresponding input
voltage range of 0V to VREF. The device includes a shutdown
mode which reduces typical power dissipation to under
0.5µW. The ADS7843 is specified down to 2.7V operation.
Low power, high speed, and onboard switches make the
ADS7843 ideal for battery-operated systems such as per-
sonal digital assistants with resistive touch screens and other
portable equipment. The ADS7843 is available in an
SSOP-16 package and is specified over the –40°C to +85°C
temperature range.
APPLICATIONS
●PERSONAL DIGITAL ASSISTANTS
●PORTABLE INSTRUMENTS
●POINT-OF-SALES TERMINALS
●PAGERS
●TOUCH SCREEN MONITORS
TOUCH SCREEN CONTROLLER
CDAC
SAR
Comparator
Four
Channel
Multiplexer Serial
Interface
and
Control
CS
DIN
DOUT
BUSY
DCLK
X+
PENIRQ
X–
Y+
Y–
IN3
IN4
VREF
+VCC
ADS7843
SBAS090B – SEPTEMBER 2000 – REVISED MAY 2002
www.ti.com
PRODUCTION DATA information is current as of publication date.
Products conform to specifications per the terms of Texas Instruments
standard warranty. Production processing does not necessarily include
testing of all parameters.
Copyright © 2001, Texas Instruments Incorporated
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of
Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
US Patent No. 6246394
ADS7843
2SBAS090B
www.ti.com
PIN NAME DESCRIPTION
1+V
CC Power Supply, 2.7V to 5V.
2 X+ X+ Position Input. ADC input Channel 1.
3 Y+ Y+ Position Input. ADC input Channel 2.
4X–X– Position Input
5Y–Y– Position Input
6 GND Ground
7 IN3 Auxiliary Input 1. ADC input Channel 3.
8 IN4 Auxiliary Input 2. ADC input Channel 4.
9V
REF Voltage Reference Input
10 +VCC Power Supply, 2.7V to 5V.
11 PENIRQ Pen Interrupt. Open anode output (requires 10kΩ
to 100kΩ pull-up resistor externally).
12 DOUT Serial Data Output. Data is shifted on the falling
edge of DCLK. This output is high impedance
when CS is HIGH.
13 BUSY Busy Output. This output is high impedance when
CS is HIGH.
14 DIN Serial Data Input. If CS is LOW, data is latched on
rising edge of DCLK.
15 CS Chip Select Input. Controls conversion timing and
enables the serial input/output register.
16 DCLK External Clock Input. This clock runs the SAR con-
version process and synchronizes serial data I/O.
ABSOLUTE MAXIMUM RATINGS(1)
+VCC to GND ........................................................................ –0.3V to +6V
Analog Inputs to GND ............................................ –0.3V to +VCC + 0.3V
Digital Inputs to GND ............................................. –0.3V to +VCC + 0.3V
Power Dissipation .......................................................................... 250mW
Maximum Junction Temperature................................................... +150°C
Operating Temperature Range ........................................–40°C to +85°C
Storage Temperature Range .........................................–65°C to +150°C
Lead Temperature (soldering, 10s)............................................... +300°C
NOTE: (1) Stresses above those listed under “Absolute Maximum Ratings”
may cause permanent damage to the device. Exposure to absolute maximum
conditions for extended periods may affect device reliability.
ELECTROSTATIC
DISCHARGE SENSITIVITY
This integrated circuit can be damaged by ESD. Texas Instru-
ments recommends that all integrated circuits be handled with
appropriate precautions. Failure to observe proper handling
and installation procedures can cause damage.
ESD damage can range from subtle performance degradation
to complete device failure. Precision integrated circuits may be
more susceptible to damage because very small parametric
changes could cause the device not to meet its published
specifications.
1
2
3
4
5
6
7
8
+V
CC
X+
Y+
X–
Y–
GND
IN3
IN4
DCLK
CS
DIN
BUSY
DOUT
PENIRQ
+V
CC
V
REF
16
15
14
13
12
11
10
9
ADS7843
PIN CONFIGURATION
Top View SSOP
PIN DESCRIPTION
MAXIMUM
INTEGRAL
SPECIFIED
LINEARITY PACKAGE TEMPERATURE PACKAGE ORDERING TRANSPORT
PRODUCT ERROR (LSB) PACKAGE-LEAD DESIGNATOR(1) RANGE MARKING NUMBER MEDIA, QUANTITY
ADS7843E ±2 SSOP-16 DBQ –40°C to +85°C ADS7843E ADS7843E Rails, 100
"" " " "ADS7843E ADS7843E/2K5 Tape and Reel, 2500
NOTES: (1) For the most current specifications and package information, refer to our web site at www.ti.com.
PACKAGE/ORDERING INFORMATION
ADS7843 3
SBAS090B www.ti.com
PARAMETER CONDITIONS MIN TYP MAX UNITS
ANALOG INPUT
Full-Scale Input Span Positive Input – Negative Input 0 VREF V
Absolute Input Range Positive Input –0.2 +VCC +0.2 V
Negative Input –0.2 +0.2 V
Capacitance 25 pF
Leakage Current 0.1 µA
SYSTEM PERFORMANCE
Resolution 12 Bits
No Missing Codes 11 Bits
Integral Linearity Error ±2 LSB(1)
Offset Error ±6LSB
Offset Error Match 0.1 1.0 LSB
Gain Error ±4LSB
Gain Error Match 0.1 1.0 LSB
Noise 30 µVrms
Power-Supply Rejection 70 dB
SAMPLING DYNAMICS
Conversion Time 12 Clk Cycles
Acquisition Time 3 Clk Cycles
Throughput Rate 125 kHz
Multiplexer Settling Time 500 ns
Aperture Delay 30 ns
Aperture Jitter 100 ps
Channel-to-Channel Isolation VIN = 2.5Vp-p at 50kHz 100 dB
SWITCH DRIVERS
On-Resistance
Y+, X+ 5Ω
Y–, X–6Ω
REFERENCE INPUT
Range 1.0 +VCC V
Resistance CS = GND or +VCC 5GΩ
Input Current 13 40 µA
fSAMPLE = 12.5kHz 2.5 µA
CS = +VCC 0.001 3 µA
DIGITAL INPUT/OUTPUT
Logic Family CMOS
Logic Levels, Except PENIRQ
VIH | IIH | ≤ +5µA+V
CC • 0.7 +VCC +0.3
VIL | IIL | ≤ +5µA–0.3 +0.8 V
VOH IOH = –250µA+V
CC • 0.8 V
VOL IOL = 250µA 0.4 V
PENIRQ
VOL TA = 0°C to +85°C, 100kΩ Pull-Up 0.8 V
Data Format Straight Binary
POWER-SUPPLY REQUIREMENTS
+VCC Specified Performance 2.7 3.6 V
Quiescent Current 280 650 µA
fSAMPLE = 12.5kHz 220 µA
Shutdown Mode with 3 µA
DCLK = DIN = +VCC
Power Dissipation +VCC = +2.7V 1.8 mW
TEMPERATURE RANGE
Specified Performance –40 +85 °C
ELECTRICAL CHARACTERISTICS
At TA = –40°C to +85°C, +VCC = +2.7V, VREF = +2.5V, fSAMPLE = 125kHz, fCLK = 16 • fSAMPLE = 2MHz, 12-bit mode, and digital inputs = GND or +VCC, unless otherwise
noted.
ADS7843E
NOTE: (1) LSB means Least Significant Bit. With VREF equal to +2.5V, 1LSB is 610µV.
ADS7843
4SBAS090B
www.ti.com
TYPICAL CHARACTERISTICS
At TA = +25°C, +VCC = +2.7V, VREF = +2.5V, fSAMPLE = 125kHz, and fCLK = 16 • fSAMPLE = 2MHz, unless otherwise noted.
SUPPLY CURRENT vs +V
CC
3.5252.5 4
+V
CC
(V)
Supply Current (µA)
320
300
280
260
240
220
200
180 4.53
f
SAMPLE
= 12.5kHz
V
REF
= +V
CC
MAXIMUM SAMPLE RATE vs +V
CC
3.5252.5 4
+V
CC
(V)
Sample Rate (Hz)
1M
100k
10k
1k 4.53
V
REF
= +V
CC
SUPPLY CURRENT vs TEMPERATURE
20–40 100–20 0 40
Temperature (°C)
Supply Current (µA)
400
350
300
250
200
150
100 60 80
CHANGE IN GAIN vs TEMPERATURE
20–40 100–20 0 40
Temperature (°C)
Delta from +25˚C (LSB)
0.15
0.10
0.05
0.00
–0.05
–0.10
–0.15 60 80
CHANGE IN OFFSET vs TEMPERATURE
20–40 100–20 0 40
Temperature (°C)
Delta from +25˚C (LSB)
0.6
0.4
0.2
0.0
–0.2
–0.4
–0.6 60 80
POWER-DOWN SUPPLY CURRENT
vs TEMPERATURE
20–40 100–20 0 40
Temperature (°C)
Supply Current (nA)
140
120
100
80
60
40
20 60 80
ADS7843 5
SBAS090B www.ti.com
TYPICAL CHARACTERISTICS (Cont.)
At TA = +25°C, +VCC = +2.7V, VREF = +2.5V, fSAMPLE = 125kHz, and fCLK = 16 • fSAMPLE = 2MHz, unless otherwise noted.
REFERENCE CURRENT vs SAMPLE RATE
750 12525 50 100
Sample Rate (kHz)
Reference Current (µA)
14
12
10
8
6
4
2
0
REFERENCE CURRENT vs TEMPERATURE
20–40 100–20 0 40
Temperature (°C)
Reference Current (µA)
18
16
14
12
10
8
660 80
SWITCH-ON RESISTANCE vs +V
CC
(X+, Y+: +V
CC
to Pin; X–, Y–: Pin to GND)
3.5252.5
X+
Y+
Y–
X–
4
+VCC (V)
R
ON
(Ω)
1
8
7
6
5
4
3
2
4.53
SWITCH-ON RESISTANCE vs TEMPERATURE
(X+, Y+: +V
CC
to Pin; X–, Y–: Pin to GND)
20–40 100–20
X+
Y+
Y–
X–
40
Temperature (°C)
R
ON
(Ω)
1
8
7
6
5
4
3
2
60 800
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
LSB Error
20 40 60 80 100 120 140 160 180 200
Sampling Rate (kHz)
MAXIMUM SAMPLING RATE vs R
IN
INL: R = 2k
INL: R = 500
DNL: R = 2k
DNL: R = 500
ADS7843
6SBAS090B
www.ti.com
+V
CC
X+
Y+
X–
Y–
GND
IN3
IN4
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
DCLK
CS
DIN
BUSY
DOUT
PENIRQ
+V
CC
V
REF
Serial/Conversion Clock
Chip Select
Serial Data In
Converter Status
Serial Data Out
+
1µF
to
10µF
(Optional)
+2.7V to +5V
ADS7843
Auxiliary Inputs
Touch
Screen
0.1µF
Pen Interrupt
100kΩ (optional)
0.1µF
THEORY OF OPERATION
The ADS7843 is a classic Successive Approximation Regis-
ter (SAR) ADC. The architecture is based on capacitive
redistribution which inherently includes a sample-and-hold
function. The converter is fabricated on a 0.6µs CMOS
process.
The basic operation of the ADS7843 is shown in Figure 1.
The device requires an external reference and an external
clock. It operates from a single supply of 2.7V to 5.25V. The
external reference can be any voltage between 1V and +VCC.
The value of the reference voltage directly sets the input
range of the converter. The average reference input current
depends on the conversion rate of the ADS7843.
The analog input to the converter is provided via a four-
channel multiplexer. A unique configuration of low on-resis-
tance switches allows an unselected ADC input channel to
provide power and an accompanying pin to provide ground for
an external device. By maintaining a differential input to the
converter and a differential reference architecture, it is pos-
sible to negate the switch’s on-resistance error (should this be
a source of error for the particular measurement).
ANALOG INPUT
See Figure 2 for a block diagram of the input multiplexer on the
ADS7843, the differential input of the ADC, and the converter’s
differential reference. Table I and Table II show the relation-
ship between the A2, A1, A0, and SER/
DFR
control bits and
the configuration of the ADS7843. The control bits are pro-
vided serially via the DIN pin—see the Digital Interface section
of this data sheet for more details.
When the converter enters the hold mode, the voltage
difference between the +IN and –IN inputs (see Figure 2) is
captured on the internal capacitor array. The input current on
the analog inputs depends on the conversion rate of the
device. During the sample period, the source must charge
the internal sampling capacitor (typically 25pF). After the
capacitor has been fully charged, there is no further input
current. The rate of charge transfer from the analog source
to the converter is a function of conversion rate.
A2 A1 A0 X+ Y+ IN3 IN4 –IN(1) X SWITCHES Y SWITCHES +REF(1) –REF(1)
0 0 1 +IN GND OFF ON +VREF GND
1 0 1 +IN GND ON OFF +VREF GND
0 1 0 +IN GND OFF OFF +VREF GND
1 1 0 +IN GND OFF OFF +VREF GND
TABLE I. Input Configuration, Single-Ended Reference Mode (SER/
DFR
HIGH).
NOTE: (1) Internal node, for clarification only—not directly accessible by the user.
A2 A1 A0 X+ Y+ IN3 IN4 –IN(1) X SWITCHES Y SWITCHES +REF(1) –REF(1)
001+IN –Y OFF ON +Y –Y
101 +IN –XON OFF+X –X
0 1 0 +IN GND OFF OFF +VREF GND
1 1 0 +IN GND OFF OFF +VREF GND
NOTE: (1) Internal node, for clarification only—not directly accessible by the user.
TABLE II. Input Configuration, Differential Reference Mode (SER/
DFR
LOW).
FIGURE 1. Basic Operation of the ADS7843.
ADS7843 7
SBAS090B www.ti.com
FIGURE 2. Simplified Diagram of Analog Input.
REFERENCE INPUT
The voltage difference between +REF and –REF (shown in
Figure 2) sets the analog input range. The ADS7843 will
operate with a reference in the range of 1V to +VCC. There are
several critical items concerning the reference input and its
wide voltage range. As the reference voltage is reduced, the
analog voltage weight of each digital output code is also
reduced. This is often referred to as the LSB (least significant
bit) size and is equal to the reference voltage divided by 4096.
Any offset or gain error inherent in the ADC will appear to
increase, in terms of LSB size, as the reference voltage is
reduced. For example, if the offset of a given converter is
2LSBs with a 2.5V reference, it will typically be 5LSBs with a
1V reference. In each case, the actual offset of the device is
the same, 1.22mV. With a lower reference voltage, more care
must be taken to provide a clean layout including adequate
bypassing, a clean (low noise, low ripple) power supply, a low-
noise reference, and a low-noise input signal.
The voltage into the VREF input is not buffered and directly
drives the Capacitor Digital-to-Analog Converter (CDAC) por-
tion of the ADS7843. Typically, the input current is 13µA with
VREF = 2.7V and fSAMPLE = 125kHz. This value will vary by a
few microamps depending on the result of the conversion. The
reference current diminishes directly with both conversion rate
and reference voltage. As the current from the reference is
drawn on each bit decision, clocking the converter more
quickly during a given conversion period will not reduce overall
current drain from the reference.
There is also a critical item regarding the reference when
making measurements where the switch drivers are on. For
this discussion, it’s useful to consider the basic operation of
the ADS7843 as shown in Figure 1. This particular applica-
tion shows the device being used to digitize a resistive
touch screen. A measurement of the current Y position of
the pointing device is made by connecting the X+ input to
the ADC, turning on the Y+ and Y– drivers, and digitizing the
voltage on X+ (shown in Figure 3). For this measurement,
the resistance in the X+ lead does not affect the conversion
(it does affect the settling time, but the resistance is usually
small enough that this is not a concern).
FIGURE 3. Simplified Diagram of Single-Ended Reference
(SER/
DFR
HIGH, Y Switches Enabled, X+ is
Analog Input).
Converter
+IN +REF
Y+
+V
CC
V
REF
X+
Y–
GND
–REF
–IN
CONVERTER
–REF
+REF
+IN
–IN
IN3
IN4
GND
A2-A0
(Shown 001B)SER/DFR
(Shown HIGH)
X+
X–
+VCC
PENIRQ
Y+
Y–
VREF
ADS7843
8SBAS090B
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FIGURE 5. Conversion Timing, 24 Clocks per Conversion, 8-bit Bus Interface. No DCLK Delay Required with Dedicated
Serial Port.
However, since the resistance between Y+ and Y– is fairly low,
the on-resistance of the Y drivers does make a small differ-
ence. Under the situation outlined so far, it would not be
possible to achieve a 0V input or a full-scale input regardless
of where the pointing device is on the touch screen because
some voltage is lost across the internal switches. In addition,
the internal switch resistance is unlikely to track the resistance
of the touch screen, providing an additional source of error.
This situation can be remedied as shown in Figure 4. By setting
the SER/
DFR
bit LOW, the +REF and –REF inputs are
connected directly to Y+ and Y–. This makes the A/D conver-
sion ratiometric. The result of the conversion is always a
percentage of the external resistance, regardless of how it
changes in relation to the on-resistance of the internal
Converter
+IN +REF
Y+
+VCC
X+
Y–
GND
–REF
–IN
FIGURE 4. Simplified Diagram of Differential Reference (SER/
DFR
LOW, Y Switches Enabled, X+ is Analog
Input).
switches. Note that there is an important consideration regard-
ing power dissipation when using the ratiometric mode of
operation, see the Power Dissipation section for more details.
As a final note about the differential reference mode, it must be
used with +VCC as the source of the +REF voltage and cannot
be used with VREF. It is possible to use a high precision
reference on VREF and single-ended reference mode for mea-
surements which do not need to be ratiometric. Or, in some
cases, it could be possible to power the converter directly from
a precision reference. Most references can provide enough
power for the ADS7843, but they might not be able to supply
enough current for the external load (such as a resistive touch
screen).
DIGITAL INTERFACE
Figure 5 shows the typical operation of the ADS7843’s digital
interface. This diagram assumes that the source of the digital
signals is a microcontroller or digital signal processor with a
basic serial interface. Each communication between the pro-
cessor and the converter consists of eight clock cycles. One
complete conversion can be accomplished with three serial
communications, for a total of 24 clock cycles on the DCLK
input.
The first eight clock cycles are used to provide the control byte
via the DIN pin. When the converter has enough information
about the following conversion to set the input multiplexer,
switches, and reference inputs appropriately, the converter
enters the acquisition (sample) mode and, if needed, the
internal switches are turned on. After three more clock cycles,
the control byte is complete and the converter enters the
conversion mode. At this point, the input sample-and-hold goes
into the hold mode and the internal switches may turn off. The
t
ACQ
AcquireIdle Conversion Idle
1DCLK
CS
81
11
DOUT
BUSY
X/Y SWITCHES
(1)
(SER/DFR HIGH)
X/Y SWITCHES
(1, 2)
(SER/DFR LOW)
(MSB)
(START)
(LSB)
A2S
ON
ON
OFF OFF
OFF OFF
DIN A1 A0
MODE SER/
DFR
PD1 PD0
1098765 4 3210 Zero Filled...
81 8
NOTES: (1) Y Drivers are on when X+ is selected input channel (A2-A0 = 001
B
), X Drivers are on when Y+ is selected
input channel (A2-A0 = 101
B
). Y– will turn on when power-down mode is entered and PD1, PD0 = 00
B
. (2) Drivers will
remain on if power-down mode is 11
B
(no power-down) until selected input channel, reference mode, or power-down
mode is changed.
ADS7843 9
SBAS090B www.ti.com
1
DCLK
CS
81
11
DOUT
BUSY
S
DIN
CONTROL BITS
S
CONTROL BITS
1098765 43210 11 10 9
81 18
next 12th clock cycles accomplish the actual A/D conversion.
If the conversion is ratiometric (SER/
DFR
LOW), the internal
switches are on during the conversion. A 13th clock cycle is
needed for the last bit of the conversion result. Three more
clock cycles are needed to complete the last byte (DOUT will
be LOW). These will be ignored by the converter.
Control Byte
See Figure 5 for the placement and order of the control bits
within the control byte. Tables III and IV give detailed informa-
tion about these bits. The first bit, the ‘S’ bit, must always be
HIGH and indicates the start of the control byte. The ADS7843
will ignore inputs on the DIN pin until the start bit is detected.
The next three bits (A2-A0) select the active input channel or
channels of the input multiplexer (see Tables I and II and
Figure 2). The MODE bit determines the number of bits for
each conversion, either 12 bits (LOW) or 8 bits (HIGH).
The SER/
DFR
bit controls the reference mode: either single-
ended (HIGH) or differential (LOW). (The differential mode is
also referred to as the ratiometric conversion mode.) In single-
ended mode, the converter’s reference voltage is always the
difference between the VREF and GND pins. In differential
mode, the reference voltage is the difference between the
currently enabled switches. See Tables I and II and Figures 2
through 4 for more information. The last two bits (PD1-PD0)
select the power-down mode as shown in Table V. If both
inputs are HIGH, the device is always powered up. If both
inputs are LOW, the device enters a power-down mode
between conversions. When a new conversion is initiated, the
device will resume normal operation instantly—no delay is
needed to allow the device to power up and the very first
conversion will be valid. There are two power-down modes:
one where
PENIRQ
is disabled and one where it is enabled.
PD1 PD0 PENIRQ DESCRIPTION
0 0 Enabled Power-down between conversions. When each
conversion is finished, the converter enters a low
power mode. At the start of the next conversion,
the device instantly powers up to full power.
There is no need for additional delays to assure
full operation and the very first conversion is
valid. The Y– switch is on while in power-down.
0 1 Disabled Same as mode 00, except PENIRQ is disabled.
The Y– switch is off while in power-down mode.
1 0 Disabled Reserved for future use.
1 1 Disabled No power-down between conversions, device is
always powered.
TABLE V. Power-Down Selection.
FIGURE 6. Conversion Timing, 16 Clocks per Conversion, 8-bit Bus Interface. No DCLK Delay Required with Dedicated
Serial Port.
BIT NAME DESCRIPTION
7 S Start Bit. Control byte starts with first HIGH bit on
DIN. A new control byte can start every 16th clock
cycle in 12-bit conversion mode or every 12th clock
cycle in 8-bit conversion mode.
6-4 A2-A0 Channel Select Bits. Along with the SER/DFR bit,
these bits control the setting of the multiplexer input,
switches, and reference inputs, see Tables I and II.
3 MODE 12-Bit/8-Bit Conversion Select Bit. This bit controls
the number of bits for the following conversion: 12
bits (LOW) or 8 bits (HIGH).
2 SER/DFR Single-Ended/Differential Reference Select Bit. Along
with bits A2-A0, this bit controls the setting of the
multiplexer input, switches, and reference inputs, see
Tables I and II.
1-0 PD1-PD0 Power-Down Mode Select Bits. See Table V for
details.
TABLE IV. Descriptions of the Control Bits within the Control
Byte.
Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
(MSB) (LSB)
S A2 A1 A0 MODE
SER/DFR
PD1 PD0
TABLE III. Order of the Control Bits in the Control Byte.
16-Clocks per Conversion
The control bits for conversion n + 1 can be overlapped with
conversion ‘n’ to allow for a conversion every 16 clock cycles,
as shown in Figure 6. This figure also shows possible serial
communication occurring with other serial peripherals between
each byte transfer between the processor and the converter.
ADS7843
10 SBAS090B
www.ti.com
FIGURE 8. Ideal Input Voltages and Output Codes.
Output Code
0V
FS = Full-Scale Voltage = VREF(1)
1LSB = VREF(1)/4096
FS – 1LSB
11...111
11...110
11...101
00...010
00...001
00...000
1LSB
NOTES: (1) Reference voltage at converter: +REF – (–REF). See Figure 2.
(2) Input voltage at converter, after multiplexer: +IN – (–IN). See Figure 2
Input Voltage(2) (V)
This is possible provided that each conversion completes
within 1.6ms of starting. Otherwise, the signal that has been
captured on the input sample-and-hold may droop enough to
affect the conversion result. Note that the ADS7843 is fully
powered while other serial communications are taking place
during a conversion.
Digital Timing
Figure 7 and Table VI provide detailed timing for the digital
interface of the ADS7843.
SYMBOL DESCRIPTION MIN TYP MAX UNITS
tACQ Acquisition Time 1.5 µs
tDS DIN Valid Prior to DCLK Rising 100 ns
tDH DIN Hold After DCLK HIGH 10 ns
tDO DCLK Falling to DOUT Valid 200 ns
tDV CS Falling to DOUT Enabled 200 ns
tTR CS Rising to DOUT Disabled 200 ns
tCSS CS Falling to First DCLK Rising 100 ns
tCSH CS Rising to DCLK Ignored 0 ns
tCH DCLK HIGH 200 ns
tCL DCLK LOW 200 ns
tBD DCLK Falling to BUSY Rising 200 ns
tBDV CS Falling to BUSY Enabled 200 ns
tBTR CS Rising to BUSY Disabled 200 ns
TABLE VI. Timing Specifications (+VCC = +2.7V and Above,
TA = –40°C to +85°C, CLOAD = 50pF).
FIGURE 7. Detailed Timing Diagram.
PD0
t
BDV
t
DH
t
CH
t
CL
t
DS
t
CSS
t
DV
t
BD
t
BD
t
TR
t
BTR
t
D0
t
CSH
DCLK
CS
11
DOUT
BUSY
DIN
10
Data Format
The ADS7843 output data is in Straight Binary format, as
shown in Figure 8. This figure shows the ideal output code for
the given input voltage and does not include the effects of
offset, gain, or noise.
8-Bit Conversion
The ADS7843 provides an 8-bit conversion mode that can be
used when faster throughput is needed and the digital result
is not as critical. By switching to the 8-bit mode, a conversion
is complete four clock cycles earlier. This could be used in
conjunction with serial interfaces that provide 12-bit transfers
or two conversions could be accomplished with three 8-bit
transfers. Not only does this shorten each conversion by four
bits (25% faster throughput), but each conversion can actu-
ally occur at a faster clock rate. This is because the internal
settling time of the ADS7843 is not as critical—settling to
better than 8 bits is all that is needed. The clock rate can be
as much as 50% faster. The faster clock rate and fewer clock
cycles combine to provide a 2x increase in conversion rate.
ADS7843 11
SBAS090B www.ti.com
POWER DISSIPATION
There are two major power modes for the ADS7843: full power
(PD1-PD0 = 11B) and auto power-down (PD1-PD0 = 00B).
When operating at full speed and 16 clocks per conversion ( see
Figure 6), the ADS7843 spends most of its time acquiring or
converting. There is little time for auto power-down, assuming
that this mode is active. Therefore, the difference between full
power mode and auto power-down is negligible. If the conver-
sion rate is decreased by simply slowing the frequency of the
DCLK input, the two modes remain approximately equal. How-
ever, if the DCLK frequency is kept at the maximum rate during
a conversion but conversions are simply done less often, the
difference between the two modes is dramatic.
Figure 9 shows the difference between reducing the DCLK
frequency (“scaling” DCLK to match the conversion rate) or
maintaining DCLK at the highest frequency and reducing the
number of conversions per second. In the later case, the
converter spends an increasing percentage of its time in
power-down mode (assuming the auto power-down mode is
active).
Another important consideration for power dissipation is the
reference mode of the converter. In the single-ended refer-
ence mode, the converter’s internal switches are on only
when the analog input voltage is being acquired (see Figure
5). Thus, the external device, such as a resistive touch
screen, is only powered during the acquisition period. In the
differential reference mode, the external device must be
powered throughout the acquisition and conversion periods
(see Figure 5). If the conversion rate is high, this could
substantially increase power dissipation.
devices have fairly “clean” power and grounds because most
of the internal components are very low power. This situation
would mean less bypassing for the converter’s power and
less concern regarding grounding. Still, each situation is
unique and the following suggestions should be reviewed
carefully.
For optimum performance, care should be taken with the
physical layout of the ADS7843 circuitry. The basic SAR
architecture is sensitive to glitches or sudden changes on the
power supply, reference, ground connections, and digital
inputs that occur just prior to latching the output of the analog
comparator. Thus, during any single conversion for an ‘n-bit’
SAR converter, there are n ‘windows’ in which large external
transient voltages can easily affect the conversion result.
Such glitches might originate from switching power supplies,
nearby digital logic, and high-power devices. The degree of
error in the digital output depends on the reference voltage,
layout, and the exact timing of the external event. The error
can change if the external event changes in time with respect
to the DCLK input.
With this in mind, power to the ADS7843 should be clean and
well bypassed. A 0.1µF ceramic bypass capacitor should be
placed as close to the device as possible. A 1µF to 10µF
capacitor may also be needed if the impedance of the
connection between +VCC and the power supply is high.
The reference should be similarly bypassed with a 0.1µF
capacitor. If the reference voltage originates from an op amp,
make sure that it can drive the bypass capacitor without
oscillation. The ADS7843 draws very little current from the
reference on average, but it does place larger demands on
the reference circuitry over short periods of time (on each
rising edge of DCLK during a conversion).
The ADS7843 architecture offers no inherent rejection of
noise or voltage variation in regards to the reference input.
This is of particular concern when the reference input is tied
to the power supply. Any noise and ripple from the supply will
appear directly in the digital results. While high frequency
noise can be filtered out, voltage variation due to line fre-
quency (50Hz or 60Hz) can be difficult to remove.
The GND pin should be connected to a clean ground point.
In many cases, this will be the “analog” ground. Avoid
connections which are too near the grounding point of a
microcontroller or digital signal processor. If needed, run a
ground trace directly from the converter to the power-supply
entry or battery connection point. The ideal layout will include
an analog ground plane dedicated to the converter and
associated analog circuitry.
In the specific case of use with a resistive touch screen, care
should be taken with the connection between the converter
and the touch screen. Since resistive touch screens have
fairly low resistance, the interconnection should be as short
and robust as possible. Longer connections will be a source
of error, much like the on-resistance of the internal switches.
Likewise, loose connections can be a source of error when
the contact resistance changes with flexing or vibrations.
FIGURE 9. Supply Current versus Directly Scaling the Fre-
quency of DCLK with Sample Rate or Keeping
DCLK at the Maximum Possible Frequency.
LAYOUT
The following layout suggestions should provide the most
optimum performance from the ADS7843. However, many
portable applications have conflicting requirements concern-
ing power, cost, size, and weight. In general, most portable
10k 100k1k 1M
f
SAMPLE
(Hz)
Supply Current (µA)
100
10
1
1000
f
CLK
= 2MHz
f
CLK
= 16 • f
SAMPLE
T
A
= 25°C
+V
CC
= +2.7V
PACKAGE OPTION ADDENDUM
www.ti.com 16-Aug-2012
Addendum-Page 1
PACKAGING INFORMATION
Orderable Device Status (1) Package Type Package
Drawing Pins Package Qty Eco Plan (2) Lead/
Ball Finish MSL Peak Temp (3) Samples
(Requires Login)
ADS7843E ACTIVE SSOP DBQ 16 75 Green (RoHS
& no Sb/Br) CU NIPDAU Level-2-260C-1 YEAR
ADS7843E/2K5 ACTIVE SSOP DBQ 16 2500 Green (RoHS
& no Sb/Br) CU NIPDAU Level-2-260C-1 YEAR
ADS7843E/2K5G4 ACTIVE SSOP DBQ 16 2500 Green (RoHS
& no Sb/Br) CU NIPDAU Level-2-260C-1 YEAR
ADS7843EG4 ACTIVE SSOP DBQ 16 75 Green (RoHS
& no Sb/Br) CU NIPDAU Level-2-260C-1 YEAR
(1) The marketing status values are defined as follows:
ACTIVE: Product device recommended for new designs.
LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.
NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design.
PREVIEW: Device has been announced but is not in production. Samples may or may not be available.
OBSOLETE: TI has discontinued the production of the device.
(2) Eco Plan - The planned eco-friendly classification: Pb-Free (RoHS), Pb-Free (RoHS Exempt), or Green (RoHS & no Sb/Br) - please check http://www.ti.com/productcontent for the latest availability
information and additional product content details.
TBD: The Pb-Free/Green conversion plan has not been defined.
Pb-Free (RoHS): TI's terms "Lead-Free" or "Pb-Free" mean semiconductor products that are compatible with the current RoHS requirements for all 6 substances, including the requirement that
lead not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, TI Pb-Free products are suitable for use in specified lead-free processes.
Pb-Free (RoHS Exempt): This component has a RoHS exemption for either 1) lead-based flip-chip solder bumps used between the die and package, or 2) lead-based die adhesive used between
the die and leadframe. The component is otherwise considered Pb-Free (RoHS compatible) as defined above.
Green (RoHS & no Sb/Br): TI defines "Green" to mean Pb-Free (RoHS compatible), and free of Bromine (Br) and Antimony (Sb) based flame retardants (Br or Sb do not exceed 0.1% by weight
in homogeneous material)
(3) MSL, Peak Temp. -- The Moisture Sensitivity Level rating according to the JEDEC industry standard classifications, and peak solder temperature.
Important Information and Disclaimer:The information provided on this page represents TI's knowledge and belief as of the date that it is provided. TI bases its knowledge and belief on information
provided by third parties, and makes no representation or warranty as to the accuracy of such information. Efforts are underway to better integrate information from third parties. TI has taken and
continues to take reasonable steps to provide representative and accurate information but may not have conducted destructive testing or chemical analysis on incoming materials and chemicals.
TI and TI suppliers consider certain information to be proprietary, and thus CAS numbers and other limited information may not be available for release.
In no event shall TI's liability arising out of such information exceed the total purchase price of the TI part(s) at issue in this document sold by TI to Customer on an annual basis.
OTHER QUALIFIED VERSIONS OF ADS7843 :
TAPE AND REEL INFORMATION
*All dimensions are nominal
Device Package
Type Package
Drawing Pins SPQ Reel
Diameter
(mm)
Reel
Width
W1 (mm)
A0
(mm) B0
(mm) K0
(mm) P1
(mm) W
(mm) Pin1
Quadrant
ADS7843E/2K5 SSOP DBQ 16 2500 330.0 12.4 6.4 5.2 2.1 8.0 12.0 Q1
PACKAGE MATERIALS INFORMATION
www.ti.com 16-Aug-2012
Pack Materials-Page 1
*All dimensions are nominal
Device Package Type Package Drawing Pins SPQ Length (mm) Width (mm) Height (mm)
ADS7843E/2K5 SSOP DBQ 16 2500 367.0 367.0 35.0
PACKAGE MATERIALS INFORMATION
www.ti.com 16-Aug-2012
Pack Materials-Page 2
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