Recently, i'm working on a design which needs to design an INA front end. The most used circuit like this (please omit the component values):
You see, the two 10M resistors which provide the path for biasing current are at the most front of the circuit. But occasionally, i saw designs like this:
The 'biasing resistors' come to the back, just before the INA. Which one is right, or which one is better?
In ADI's application note: Designing Amplifier Circuits: How to Avoid Common Problems, they give a similar circuit:
And they suggest:
In these circuits, there will be a small offset-voltage error due to mismatches between the resistors and/or the input bias currents. To minimize such errors, a third resistor, about 1/10 th their value (but still large compared to the differential source resistance), can be connected between the two in-amp inputs (thus bridging both resistors).
They suggest to parallel a smaller resistor to bridge the unbalanced input. But why not reduce the value of the 'biasing' resistors directly. Are there any differences?