Datasheet

®
7
ADS7845
FIGURE 2. Simplified Diagram of ADS7845 Input and Panel Drivers.
REFERENCE INPUT
The voltage difference between +REF and –REF (see Figure
2) sets the analog input range. The ADS7845 will operate
with a reference in the range of 1V to +V
CC
. 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 Least Significant Bit (LSB)
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.
The voltage into the V
REF
input is not buffered and directly
drives the capacitor digital-to-analog converter (CDAC) por-
tion of the ADS7845. Typically, the input current is 13µA
with V
REF
= 2.5V and f
SAMPLE
= 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.
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 ADS7845 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 with the WIPER input to the
A/D converter, turning on the UL and UR drivers to V
CC
,
grounding LL and LR, and digitizing the voltage on the
WIPER (see Figure 3 for a block diagram).
FIGURE 3. Simplified Diagram of Single-Ended Reference
(SER/DFR HIGH).
A/D CONVERTER
–REF
+REF Drivers
+IN
–IN
LL
LR
GND
SER/DFR
(Shown LOW)
UR
UL
+V
CC
PENIRQ V
REF
WIPER
AUXIN
A/D Converter
+V
CC
GND
V
REF HI
WIPER
UR
LR
UL
LL
V
REF LO
GND
GND
GND+V
CC
IN
LOW