sorry for my english , i will add some clarification , the full scale of voltage is 20mV and it correspand at 20mOhm , the resolution of the instrument is 10µOhm so the lowest voltage to be amplified is 10µV , the gain of amplification have to be 350 , the BW is about 20Hz or bellow , i tested a non inverting amplifier with DAC to compensate Offset like HP34420a but that work correctly the gain have a exponontial form , also a standard instrumentation Amplfier don't give correctly at low voltage input ,