ADS7843
SBAS090B – SEPTEMBER 2000 – REVISED MAY 2002
TOUCH SCREEN CONTROLLER
FEATURES
DESCRIPTION
● 4-WIRE TOUCH SCREEN INTERFACE
The ADS7843 is a 12-bit sampling Analog-to-Digital Converter (ADC) with a synchronous serial interface and low onresistance 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.
● 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
Low power, high speed, and onboard switches make the
ADS7843 ideal for battery-operated systems such as personal 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
US Patent No. 6246394
PENIRQ
+VCC
X+
X–
SAR
DCLK
Y+
Y–
Four
Channel
Multiplexer
CS
Comparator
CDAC
IN3
IN4
Serial
Interface
and
Control
DIN
DOUT
BUSY
VREF
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.
Copyright © 2001, Texas Instruments Incorporated
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.
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ELECTROSTATIC
DISCHARGE SENSITIVITY
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.
This integrated circuit can be damaged by ESD. Texas Instruments 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.
PACKAGE/ORDERING INFORMATION
PRODUCT
MAXIMUM
INTEGRAL
LINEARITY
ERROR (LSB)
ADS7843E
"
PACKAGE-LEAD
PACKAGE
DESIGNATOR(1)
SPECIFIED
TEMPERATURE
RANGE
±2
SSOP-16
DBQ
–40°C to +85°C
"
"
"
"
PACKAGE
MARKING
ORDERING
NUMBER
TRANSPORT
MEDIA, QUANTITY
ADS7843E
ADS7843E
ADS7843E
ADS7843E/2K5
Rails, 100
Tape and Reel, 2500
NOTES: (1) For the most current specifications and package information, refer to our web site at www.ti.com.
PIN CONFIGURATION
PIN DESCRIPTION
Top View
2
SSOP
PIN
NAME
+VCC
X+
Y+
X–
Y–
GND
IN3
IN4
VREF
+VCC
PENIRQ
+VCC
1
16
DCLK
X+
2
15
CS
Y+
3
14
DIN
X–
4
13
BUSY
12
DOUT
1
2
3
4
5
6
7
8
9
10
11
12
DOUT
13
BUSY
14
DIN
15
CS
16
DCLK
ADS7843
Y–
5
GND
6
11
PENIRQ
IN3
7
10
+VCC
IN4
8
9
VREF
DESCRIPTION
Power Supply, 2.7V to 5V.
X+ Position Input. ADC input Channel 1.
Y+ Position Input. ADC input Channel 2.
X– Position Input
Y– Position Input
Ground
Auxiliary Input 1. ADC input Channel 3.
Auxiliary Input 2. ADC input Channel 4.
Voltage Reference Input
Power Supply, 2.7V to 5V.
Pen Interrupt. Open anode output (requires 10kΩ
to 100kΩ pull-up resistor externally).
Serial Data Output. Data is shifted on the falling
edge of DCLK. This output is high impedance
when CS is HIGH.
Busy Output. This output is high impedance when
CS is HIGH.
Serial Data Input. If CS is LOW, data is latched on
rising edge of DCLK.
Chip Select Input. Controls conversion timing and
enables the serial input/output register.
External Clock Input. This clock runs the SAR conversion process and synchronizes serial data I/O.
ADS7843
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SBAS090B
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
PARAMETER
ANALOG INPUT
Full-Scale Input Span
Absolute Input Range
CONDITIONS
MIN
Positive Input – Negative Input
Positive Input
Negative Input
0
–0.2
–0.2
Capacitance
Leakage Current
VREF
+VCC +0.2
+0.2
V
V
V
pF
µA
12
0.1
0.1
30
70
±2
±6
1.0
±4
1.0
12
500
30
100
100
5
6
Ω
Ω
125
VIN = 2.5Vp-p at 50kHz
1.0
CS = GND or +VCC
Bits
Bits
LSB(1)
LSB
LSB
LSB
LSB
µVrms
dB
Clk Cycles
Clk Cycles
kHz
ns
ns
ps
dB
3
+VCC
5
13
2.5
0.001
fSAMPLE = 12.5kHz
CS = +VCC
DIGITAL INPUT/OUTPUT
Logic Family
Logic Levels, Except PENIRQ
VIH
VIL
VOH
VOL
PENIRQ
VOL
Data Format
UNITS
11
SWITCH DRIVERS
On-Resistance
Y+, X+
Y–, X–
REFERENCE INPUT
Range
Resistance
Input Current
MAX
25
0.1
SYSTEM PERFORMANCE
Resolution
No Missing Codes
Integral Linearity Error
Offset Error
Offset Error Match
Gain Error
Gain Error Match
Noise
Power-Supply Rejection
SAMPLING DYNAMICS
Conversion Time
Acquisition Time
Throughput Rate
Multiplexer Settling Time
Aperture Delay
Aperture Jitter
Channel-to-Channel Isolation
TYP
40
3
V
GΩ
µA
µA
µA
CMOS
| IIH | ≤ +5µA
| IIL | ≤ +5µA
IOH = –250µA
IOL = 250µA
+VCC • 0.7
–0.3
+VCC • 0.8
+VCC +0.3
+0.8
TA = 0°C to +85°C, 100kΩ Pull-Up
0.4
V
V
V
0.8
V
3.6
650
3
V
µA
µA
µA
1.8
mW
+85
°C
Straight Binary
POWER-SUPPLY REQUIREMENTS
+VCC
Quiescent Current
Power Dissipation
Specified Performance
2.7
280
220
fSAMPLE = 12.5kHz
Shutdown Mode with
DCLK = DIN = +VCC
+VCC = +2.7V
TEMPERATURE RANGE
Specified Performance
–40
NOTE: (1) LSB means Least Significant Bit. With VREF equal to +2.5V, 1LSB is 610µV.
ADS7843
SBAS090B
3
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TYPICAL CHARACTERISTICS
At TA = +25°C, +VCC = +2.7V, VREF = +2.5V, fSAMPLE = 125kHz, and fCLK = 16 • fSAMPLE = 2MHz, unless otherwise noted.
POWER-DOWN SUPPLY CURRENT
vs TEMPERATURE
400
140
350
120
Supply Current (nA)
Supply Current (µA)
SUPPLY CURRENT vs TEMPERATURE
300
250
200
100
80
60
40
150
20
100
–40
–20
0
20
60
40
80
–40
100
–20
0
20
40
60
80
100
Temperature (°C)
Temperature (°C)
SUPPLY CURRENT vs +VCC
MAXIMUM SAMPLE RATE vs +VCC
320
1M
300
Sample Rate (Hz)
Supply Current (µA)
fSAMPLE = 12.5kHz
280
VREF = +VCC
260
240
220
100k
10k
VREF = +VCC
200
180
1k
2
2.5
3.5
3
4
4.5
5
2
2.5
3
+VCC (V)
0.15
0.6
0.10
0.4
Delta from +25˚C (LSB)
Delta from +25˚C (LSB)
4
4.5
5
CHANGE IN OFFSET vs TEMPERATURE
CHANGE IN GAIN vs TEMPERATURE
0.05
0.00
–0.05
0.2
0.0
–0.2
–0.4
–0.10
–0.6
–0.15
–40
–20
0
20
40
60
80
100
–40
–20
0
20
40
60
80
100
Temperature (°C)
Temperature (°C)
4
3.5
+VCC (V)
ADS7843
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SBAS090B
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 TEMPERATURE
18
12
16
Reference Current (µA)
Reference Current (µA)
REFERENCE CURRENT vs SAMPLE RATE
14
10
8
6
4
14
12
10
8
2
6
0
0
25
50
75
100
–40
125
–20
0
20
40
60
80
Sample Rate (kHz)
Temperature (°C)
SWITCH-ON RESISTANCE vs +VCC
(X+, Y+: +VCC to Pin; X–, Y–: Pin to GND)
SWITCH-ON RESISTANCE vs TEMPERATURE
(X+, Y+: +VCC to Pin; X–, Y–: Pin to GND)
8
100
8
Y–
7
7
Y–
X–
6
X–
RON (Ω)
RON (Ω)
6
5
4
5
Y+
4
X+
Y+
3
3
X+
2
2
1
1
2
2.5
3
3.5
4
4.5
5
–40
0
–20
+VCC (V)
20
40
60
80
100
Temperature (°C)
MAXIMUM SAMPLING RATE vs RIN
2
1.8
INL: R = 2k
INL: R = 500
DNL: R = 2k
DNL: R = 500
1.6
LSB Error
1.4
1.2
1
0.8
0.6
0.4
0.2
0
20
40
60
80
100 120 140
Sampling Rate (kHz)
ADS7843
SBAS090B
160
180
200
5
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THEORY OF OPERATION
ANALOG INPUT
The ADS7843 is a classic Successive Approximation Register (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 fourchannel multiplexer. A unique configuration of low on-resistance 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 possible to negate the switch’s on-resistance error (should this be
a source of error for the particular measurement).
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 relationship between the A2, A1, A0, and SER/DFR control bits and
the configuration of the ADS7843. The control bits are provided 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.
+2.7V to +5V
1µF
+
to
10µF
(Optional)
ADS7843
0.1µF
Touch
Screen
Auxiliary Inputs
Serial/Conversion Clock
1
+VCC
DCLK 16
2
X+
CS 15
3
Y+
DIN 14
4
X–
BUSY 13
Converter Status
5
Y–
DOUT 12
Serial Data Out
6
GND
7
IN3
+VCC 10
8
IN4
VREF
Chip Select
Serial Data In
Pen Interrupt
PENIRQ 11
100kΩ (optional)
9
0.1µF
FIGURE 1. Basic Operation of the ADS7843.
A2
A1
A0
X+
0
1
0
1
0
0
1
1
1
1
0
0
+IN
Y+
IN3
IN4
–IN(1)
X SWITCHES
Y SWITCHES
+REF(1)
–REF(1)
+IN
GND
GND
GND
GND
OFF
ON
OFF
OFF
ON
OFF
OFF
OFF
+VREF
+VREF
+VREF
+VREF
GND
GND
GND
GND
+IN
+IN
NOTE: (1) Internal node, for clarification only—not directly accessible by the user.
TABLE I. Input Configuration, Single-Ended Reference Mode (SER/DFR HIGH).
A2
A1
A0
X+
0
1
0
1
0
0
1
1
1
1
0
0
+IN
Y+
IN3
IN4
–IN(1)
X SWITCHES
Y SWITCHES
+REF(1)
–REF(1)
+IN
–Y
–X
GND
GND
OFF
ON
OFF
OFF
ON
OFF
OFF
OFF
+Y
+X
+VREF
+VREF
–Y
–X
GND
GND
+IN
+IN
NOTE: (1) Internal node, for clarification only—not directly accessible by the user.
TABLE II. Input Configuration, Differential Reference Mode (SER/DFR LOW).
6
ADS7843
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SBAS090B
PENIRQ
+VCC
VREF
A2-A0
(Shown 001B)
SER/DFR
(Shown HIGH)
X+
X–
Y+
+IN
+REF
CONVERTER
Y–
–IN
–REF
IN3
IN4
GND
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 lownoise reference, and a low-noise input signal.
the ADS7843 as shown in Figure 1. This particular application 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).
+VCC
Y+
The voltage into the VREF input is not buffered and directly
drives the Capacitor Digital-to-Analog Converter (CDAC) portion 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
X+
+IN
+REF
Converter
–IN
–REF
Y–
GND
FIGURE 3. Simplified Diagram of Single-Ended Reference
(SER/DFR HIGH, Y Switches Enabled, X+ is
Analog Input).
ADS7843
SBAS090B
VREF
7
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However, since the resistance between Y+ and Y– is fairly low,
the on-resistance of the Y drivers does make a small difference. 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.
switches. Note that there is an important consideration regarding 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 measurements 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).
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 conversion 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
DIGITAL INTERFACE
+VCC
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 processor 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.
Y+
+IN
X+
+REF
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
Converter
–IN
–REF
Y–
GND
FIGURE 4. Simplified Diagram of Differential Reference (SER/
DFR LOW, Y Switches Enabled, X+ is Analog
Input).
CS
tACQ
DCLK
DIN
1
S
8
A2
A1
1
8
1
8
A0 MODE SER/
DFR PD1 PD0
(START)
Idle
Acquire
Conversion
Idle
BUSY
DOUT
11
10
9
8
7
6
5
4
3
(MSB)
X/Y SWITCHES(1)
(SER/DFR HIGH)
OFF
X/Y SWITCHES(1, 2)
(SER/DFR LOW)
OFF
2
1
0
Zero Filled...
(LSB)
ON
OFF
ON
OFF
NOTES: (1) Y Drivers are on when X+ is selected input channel (A2-A0 = 001B), X Drivers are on when Y+ is selected
input channel (A2-A0 = 101B). Y– will turn on when power-down mode is entered and PD1, PD0 = 00B. (2) Drivers will
remain on if power-down mode is 11B (no power-down) until selected input channel, reference mode, or power-down
mode is changed.
FIGURE 5. Conversion Timing, 24 Clocks per Conversion, 8-bit Bus Interface. No DCLK Delay Required with Dedicated
Serial Port.
8
ADS7843
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SBAS090B
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.
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.
Control Byte
BIT
See Figure 5 for the placement and order of the control bits
within the control byte. Tables III and IV give detailed information 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 singleended (HIGH) or differential (LOW). (The differential mode is
also referred to as the ratiometric conversion mode.) In singleended 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.
Bit 7
(MSB)
Bit 6
Bit 5
Bit 4
S
A2
A1
A0
Bit 3
Bit 2
MODE SER/DFR
Bit 1
Bit 0
(LSB)
PD1
PD0
NAME
7
DESCRIPTION
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.
PD1
TABLE III. Order of the Control Bits in the Control Byte.
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.
CS
DCLK
1
DIN
8
1
8
S
1
8
1
S
CONTROL BITS
CONTROL BITS
BUSY
DOUT
11 10 9
8
7
6
5
4
3
2
1
0
11 10 9
FIGURE 6. Conversion Timing, 16 Clocks per Conversion, 8-bit Bus Interface. No DCLK Delay Required with Dedicated
Serial Port.
ADS7843
SBAS090B
9
www.ti.com
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.
FS = Full-Scale Voltage = VREF(1)
1LSB = VREF(1)/4096
1LSB
11...111
Digital Timing
Figure 7 and Table VI provide detailed timing for the digital
interface of the ADS7843.
Output Code
11...110
11...101
00...010
00...001
SYMBOL
DESCRIPTION
MIN
TYP
MAX
UNITS
tACQ
Acquisition Time
1.5
tDS
DIN Valid Prior to DCLK Rising
100
ns
tDH
DIN Hold After DCLK HIGH
10
ns
tDO
DCLK Falling to DOUT Valid
00...000
µs
200
0V
NOTES: (1) Reference voltage at converter: +REF – (–REF). See Figure 2.
(2) Input voltage at converter, after multiplexer: +IN – (–IN). See Figure 2
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
FIGURE 8. Ideal Input Voltages and Output Codes.
8-Bit Conversion
ns
tBDV
CS Falling to BUSY Enabled
200
ns
tBTR
CS Rising to BUSY Disabled
200
ns
FS – 1LSB
Input Voltage(2) (V)
TABLE VI. Timing Specifications (+VCC = +2.7V and Above,
TA = –40°C to +85°C, CLOAD = 50pF).
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.
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 actually 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.
CS
tCSS
tCL
tCH
tBD
tBD
tD0
tCSH
DCLK
tDS
DIN
tDH
PD0
tBDV
tBTR
BUSY
tDV
tTR
DOUT
11
10
FIGURE 7. Detailed Timing Diagram.
10
ADS7843
www.ti.com
SBAS090B
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 conversion rate is decreased by simply slowing the frequency of the
DCLK input, the two modes remain approximately equal. However, 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 reference 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.
Supply Current (µA)
fCLK = 16 • fSAMPLE
100
fCLK = 2MHz
TA = 25°C
+VCC = +2.7V
1
1k
10k
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 frequency (50Hz or 60Hz) can be difficult to remove.
1000
10
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.
100k
1M
fSAMPLE (Hz)
FIGURE 9. Supply Current versus Directly Scaling the Frequency 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 concerning power, cost, size, and weight. In general, most portable
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.
ADS7843
SBAS090B
11
www.ti.com
PACKAGE OPTION ADDENDUM
www.ti.com
10-Dec-2020
PACKAGING INFORMATION
Orderable Device
Status
(1)
Package Type Package Pins Package
Drawing
Qty
Eco Plan
(2)
Lead finish/
Ball material
MSL Peak Temp
Op Temp (°C)
Device Marking
(3)
(4/5)
(6)
ADS7843E
ACTIVE
SSOP
DBQ
16
75
RoHS & Green
NIPDAU
Level-2-260C-1 YEAR
-40 to 85
ADS
7843E
ADS7843E/2K5
ACTIVE
SSOP
DBQ
16
2500
RoHS & Green
NIPDAU
Level-2-260C-1 YEAR
-40 to 85
ADS
7843E
ADS7843E/2K5G4
ACTIVE
SSOP
DBQ
16
2500
RoHS & Green
NIPDAU
Level-2-260C-1 YEAR
-40 to 85
ADS
7843E
ADS7843EG4
ACTIVE
SSOP
DBQ
16
75
RoHS & Green
NIPDAU
Level-2-260C-1 YEAR
-40 to 85
ADS
7843E
(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)
RoHS: TI defines "RoHS" to mean semiconductor products that are compliant with the current EU RoHS requirements for all 10 RoHS substances, including the requirement that RoHS substance
do not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, "RoHS" products are suitable for use in specified lead-free processes. TI may
reference these types of products as "Pb-Free".
RoHS Exempt: TI defines "RoHS Exempt" to mean products that contain lead but are compliant with EU RoHS pursuant to a specific EU RoHS exemption.
Green: TI defines "Green" to mean the content of Chlorine (Cl) and Bromine (Br) based flame retardants meet JS709B low halogen requirements of
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