In the below schematics a source is coupled to a differential amplifier.
Rb is a bias resistor for bias currents to flow:
Since in instrumentation amplifiers it is a remedy to add resistors for bias currents to find a return path(See the middle in this link.).
So basically I wanted to see the effect of this resistor value on common mode voltage rejection in a simulation. So I simulated the above schematics in LTspice for Rb = 1 Ohm and Rb = 47k resistors. Source is a 50mV DC and the common mode voltage is 50Hz interference. Here are the results for Vo1-Vo2:
For Rb = 1 Ohm:
For Rb = 47k 0hm:
As you see above in Rb = 1 Ohm case the distortion/common mode is rejected much more than in Rb = 47k case.
On the other hand the manufacturers recommend 10k to 100k resistor for Rb. I couldn't get the logic behind if it is causing more distortion why wouldn't we just ground the negative side or use a very low resistance? And what makes 10k very recommended?