PENIRQ
+VCC
VREF
A2-A0
SER/DFR
(Shown 001B)
(Shown HIGH)
X+
X–
+REF
Y+
Y–
+IN
–IN
CONVERTER
–REF
IN3
IN4
GND
FIGURE 2. Simplified Diagram of Analog Input.
REFERENCE INPUT
the ADS7843 as shown in Figure 1. This particular appli-
cation 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 A/D converter, turning on the Y+ and Y– drivers, and
digitizing the voltage on X+ (see Figure 3 for a block
diagram). 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).
The voltage difference between +REF and –REF (see 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 A/D converter 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 2
LSBs with a 2.5V reference, it will typically be 5 LSBs 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.
+VCC
VREF
Y+
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 conver-
sion. 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 con-
verter more quickly during a given conversion period will not
reduce overall current drain from the reference.
+REF
Converter
–REF
+IN
–IN
X+
Y–
GND
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
FIGURE 3. Simplified Diagram of Single-Ended Reference
(SER/DFR HIGH, Y Switches Enabled, X+ is
Analog Input).
®
7
ADS7843
BB [ BURR-BROWN CORPORATION ]
