PENIRQ
+V
CC
V
REF
A2-A0
(Shown 001
B
)
SER/DFR
(Shown HIGH)
X+
X–
Y+
Y–
+IN
–IN
+REF
CONVERTER
–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 +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 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 V
REF
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
V
REF
= 2.7V and f
SAMPLE
= 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).
+V
CC
V
REF
Y+
X+
+IN
–IN
+REF
Converter
–REF
Y–
GND
FIGURE 3. Simplified Diagram of Single-Ended Reference
(SER/DFR HIGH, Y Switches Enabled, X+ is
Analog Input).
ADS7843
SBAS090B
7
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