Datasheet

R
SOURCE
C
BYPASS
+
-
Signal Source
AVDD
GND
ADS8634/8
AINx
AINGND
GND
R
1
ADS8634
ADS8638
SBAS541A –MAY 2011– REVISED AUGUST 2011 www.ti.com
DRIVING ANALOG SIGNAL INPUT WITHOUT AN OPERATIONAL AMPLIFIER
There are some low input signal bandwidth applications, such as general-purpose programmable logic controllers
(PLCs) I/O, where it is not required to operate an ADC at high sampling rates and it is desirable to avoid using a
dedicated driving op amp from a cost perspective. In this case, the ADC input recognizes the impedance of the
signal source (such as signal conditioning circuit, PGA, or sensor). This section elaborates on the effects of
source impedance on sampling frequency.
Equation 5 can be rewritten as Equation 10:
Sampling Time = Filter Time Constant × Settling Resolution × ln(2) (10)
As shown in Figure 94, it is recommended to use a bypass capacitor across the positive and negative ADC input
terminals.
Figure 94. Driving Without an Operational Amplifier
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