Hi guys,

I need to control the gain of a transimpedance amplifier in a very clean and precise manor, for a capacitance meter I am designing. I'm going to be sensing currents down in the nA range, with an excitation signal of up to about 100KHz. The reason for such small currents is because the excitation amplitude cannot exceed ~10mV. This would be fine, but for my application the current flowing is about 20nA at low frequencies and up to about 50uA or greater at higher frequencies. This means I need to be able to control the gain of the transimpedance amplifier to select different current ranges. Does anyone have an expertise in this area of electronics design? I am currently considering using JFETs as resistors, reed relays to psychically swap out fixed precision resistors and possibly the use of a digitally programmable potentiometer. Sensitivity, capacitance and frequency response need to be considered!

Cheers for any help,

Dan

 

If you want fixed step gain values, an op amp with switched precision resistors would work. You can use relays or analog switches to switch in the desired resistor.

If a digital potentiometer has sufficient accuracy for your requirements then you could also use one of those with an op amp.

 

Cheers, I've found some low thermal offset reed relays which high end multimeters use to switch between small voltage & current ranges. I think these will suffice in switching the gain of the transimpedance amplifier