Signal Conditioning
2-2
2.1
Signal Conditioning
The factory recommends the analog input to any SAR-type converter be
buffered and low-pass filtered. It is important to note that the input buffer circuit
of the ADS8381EVM, shown in Figure 2−1, uses the THS4031 in an inverting
gain-of-one configuration. The amplifier is not stable in a conventional
gain-of-one configuration. The THS4031 was selected for its low noise, high
slew rate, and fast settling time. The low-pass filter resistor and capacitor
values are selected such that ADS8381EVM meets the 100-kHz AC
performance specifications listed in the data sheet. The series resistor works
with the capacitor to filter the input signal, but also isolates the amplifier from
the 6800-pF capacitive load. The capacitor to ground at the input of the A/D
works with the series resistor to filter the input signal, and acts like a charge
reservoir. This external filter capacitor works with the amplifier to charge the
internal sampling capacitor during sampling mode.
The EVM has a provision to offset the input voltage by adjusting R23, a 10-k
Ω
potentiometer.
Figure 2−1. Input Buffer Circuit
1 k
Ω
1 k
Ω
−V
CC
V
I
0.1
µ
F
1
µ
F
10
Ω
(+) IN
(−) IN
6800 pF
1
µ
F
0.1
µ
F
0.22
µ
F
500
Ω
4.096 V
10 k
Ω
+V
CC
THS4031
−
+