A customer using this INA118 amplifier in an EEG pre-amplifier application. In the section in the datasheet about bias current return path, it looks like a 47kOhm resistor is specified for the case of higher differential source impedance. He is wondering what is the reason for a 47K resistor here, does the bias return resistor need to be in a certain resistance range for proper operation?
Naser,
There is a range of resistor values that may be used to provide the necessary input bias current return path in the applications with high differential source impedance such as EEG; on the low end, the resistors may not be too low in order to prevent overloading of the signal coming from the EEG electrodes. On the high end, one may have to limit the size of the resistors in order to minimize the resistor thermal noise (btw, 47k resistor generates 27nVrms/rt-Hz or 164nVpp/rt-Hz noise) as well as to minimize the input voltage offset error generated by the input bias offset current (up to +/-10nA - see below) and the resistor mismatch.
