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OPA350: Using the OPA350 for amplifying small signals

Nicholas Razo
Prodigy 20 points
Part Number: OPA350

Hello all,

I am trying to use the OPA350PA and take advantage of its low input bias current to amplify a small signal of around a nA of current. I have it set up in a simple negative feedback loop with a resistance value of 100 mega ohms. The output current from this configuration shows a linear relationship with the input current. The input signal is around 0.2V and 2nA and the output current is around 0.12mA. I have not been able to find a reliable way to test if the voltage out of the OPA350 varies with the input current (the input voltage varies with the input current). I need to feed the output through a buffer and into an analog to digital converter to take measurements so I need the output voltage to vary with the input current. Does this op amps voltage vary with small signal changes on the order of 1nA of current? Is there anyway I can measure the output voltage without changing the characteristics of the circuit? 

  • 0
    TI__Guru 62700 points

    Nicholas,

    It is always the best to show a schematic instead of describing the circuit - a picture is worth thousand words.  I believe you are using a trans-impedance configuration but in your writeup you keep mixing input current and input voltage - the input must be current while the resulting output must be voltage.  Assuming that you use 5V single supply, this would allow the linear output voltage range from 50mV to 4.95V (max 50mV from either rail).  Therefore, a linear input current range of the configuration would have to be from 0.5nA (vout=50mV) to 49.5nA (Vout=4.95) - see below.

    If you need to measure a positive and negative input current, you must reference the input and output voltage to mid-supply (see Vref below) or to use a dual supply - for example +/-2.5V (see at the bottom).  With RF of 100M, this would allow you to measure the input current in the linear range from -24.5nA to +24.5nA - see below.

    Nicholas OPA350 TIA.TSC