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Design >> Analog Design >> Precision Analog Transimpedance Gain Control
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Message started by dannybeckett on Apr 15^th, 2013, 6:20am

Title: Precision Analog Transimpedance Gain Control
Post by dannybeckett on Apr 15^th, 2013, 6:20am

Hi guys,

First time posting here so, hello y'all =] Basically 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

Title: Re: Precision Analog Transimpedance Gain Control
Post by raja.cedt on Apr 22^nd, 2013, 12:35pm

hello,
yes you are correct, some how you need to change the FB res, but a transistor with continuous may not be a good idea due to vds sensitivity and non-linear control. Your 2nd option digital control resister was very good from practical point. Otherwise use logarithmic clipping which compress gain as function of log(current). You can refer the following pap.

130dB-DR Transimpedance Amplifier with Monotonic Logarithmic
Compression and a High-Current Monitor

Thanks,
Raj.