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 +V
CC
.
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.
+2.7V to +5V
1µF
+
to
10µF
(Optional)
ADS7843
0.1µF
1
2
3
Touch
Screen
4
5
6
7
Auxiliary Inputs
8
+V
CC
X+
Y+
X–
Y–
GND
IN3
IN4
DCLK 16
CS 15
DIN 14
BUSY 13
DOUT 12
PENIRQ 11
+V
CC
10
V
REF
9
0.1µF
100kΩ (optional)
Serial/Conversion Clock
Chip Select
Serial Data In
Converter Status
Serial Data Out
Pen Interrupt
FIGURE 1. Basic Operation of the ADS7843.
–IN
(1)
GND
GND
GND
GND
+REF
(1)
+V
REF
+V
REF
+V
REF
+V
REF
–REF
(1)
GND
GND
GND
GND
A2
0
1
0
1
A1
0
0
1
1
A0
1
1
0
0
X+
+IN
Y+
+IN
IN3
IN4
X SWITCHES
OFF
ON
OFF
OFF
Y SWITCHES
ON
OFF
OFF
OFF
+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
0
1
0
1
A1
0
0
1
1
A0
1
1
0
0
X+
+IN
+IN
+IN
+IN
Y+
IN3
IN4
–IN
(1)
–Y
–X
GND
GND
X SWITCHES
OFF
ON
OFF
OFF
Y SWITCHES
ON
OFF
OFF
OFF
+REF
(1)
+Y
+X
+V
REF
+V
REF
–REF
(1)
–Y
–X
GND
GND
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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