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1.2.1 Signal Conditioning
The factory recommends the analog input to any SAR-type converter be buffered and low-pass filtered. The
input buffer on the ADS78x1EVM uses the
THS4031 (as shown in Figure 1-1 ) configured as an inverting
gain of one. However, the amplifier is not stable at a gain of one, and is thus configured for inverting gain of
one. The THS4031 was selected for its low noise, high slew rate, and fast settling time. The low-pass filter
resistor and capacitor values were selected such that the ADS78x1EVM meets the 1-MHz AC performance
specifications listed in the data sheet. The series resistor works in conjunction with the capacitor to filter the input
signal, but also isolates the amplifier from the capacitive load. The capacitor to ground at the input of the ADC
works in conjunction with the series resistor to filter the input signal, and functions as a charge reservoir. This
external filter capacitor works with the amplifier to charge the internal sampling capacitor during sampling mode.
Resistors R1 and R12 were selected to reduce offset.
The EVM has a provision to offset the input voltage by adjusting R25, a 10-kΩ potentiometer.
130 pF
C0G
+2.5 V
THS4031
1000 pF
C0G
(+)IN
(−)IN
C35
C16
R13
U2
C33
R25
C7
C31
C37
C15
R10
R6
R1
R12
604
W
−V
CC
0.1
m
F
1
m
F
604 W
21
W
21
W
12
W
1
m
F
0.1 m F
+V
CC
1 m F
V
IN
10 k
W
Figure 1-1. ADS7881 Input Buffer Circuit
www.ti.com Introduction
SLAU150A – DECEMBER 2004 – REVISED OCTOBER 2021
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