PGA281
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SBOS664A –MARCH 2013–REVISED JUNE 2013
Input Bias Current Return Path
The input impedance of the PGA281 is extremely high; greater than 1 GΩ. However, make sure to provide a
path for the input bias current of both inputs. This input bias current is typically 300 pA. A high input impedance
means that this input bias current changes very little with varying input voltage.
Input circuitry must provide a path for this input bias current for proper operation. Figure 47 shows various
provisions for an input bias current path. Without a bias current path, the inputs float to a potential that exceeds
the common-mode range of the PGA281, and the input amplifiers saturate. If the differential source resistance is
low, connect the bias current return path to one input (as shown in the thermocouple example in Figure 47). With
higher source impedance, using two equal resistors provides a balanced input with the possible advantage of
lower input offset voltage as a result of bias current and better high-frequency common-mode rejection.
Microphone,
hydrophone,
and so on.
PGA281
47 kꢀ
47 kꢀ
Thermocouple
PGA281
10 kꢀ
PGA281
Center-tap provides
bias current return.
Figure 47. Providing an Input Common-Mode Current Path
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