Q
What are the disadvantages of Indirect Current Feedback?
A
There is a limit to the differential voltage at the inputs, which is typicallynot the case in traditional in-amps. This sometimes limits circuits which are
in a gain of 1 configuration. For AD8237, this limit is approximately ±(Vs –
1.2V), for example, a ±2.5 V supply would allow for ±(5V – 1.2V) = ±3.8V.
There is some dependence of the CMRR on the differential input voltage. This is
typically shown in either a CMRR vs. Vin graph, or a gain-error vs. Vcm graph
(AD8237 figure 34).
The noise and supply current of an ICF in-amp may change with the input signal
because of the biasing techniques used to increase the differential input
range. This does not happen with AD8237 at low frequencies because the ALS
blocks and their cross-coupled connection prevent the input gm amplifiers from
seeing a steady-state differential input.