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OPA625 input offset

Other Parts Discussed in Thread: OPA625


I'm trying to use OPA625 in a current meter design. At first stage, the input signal is passed through a voltage follower to lower the output impedance. Here is a circuit:

According to the datasheet, the input offset of OPA625 is about 15±100 uV. With gain = 1 it translates to a similar output offset. However, this is a simulation result:

The output voltage doesn't seem to drop below 9 mV. OPA625 PSpice model is used (downloaded from WEBENCH® Design Center). I'm now quite sure why is there such a big offset in this circuit, are there any other factors contributing to it?

Thank you,


  • Hello Vlad,

    The additional offset you are seeing is likely caused by the input bias current of the OPA625. The input bias current of the OPA625 has a typical specification of 2uA. Placing R1 (100ohms) in series with the input will create an additional 200uV (2uA*100ohms) offset.

    The reason the output is not going below ~9mV is because of the output voltage swing limitation. The output swing of the OPA625 has a typical specification of 20mV. Therefore the output cannot swing below 20mV.

    For more information on input bias current and input/output limiations of amplifiers, check out our online comprehensive curriculum, TI Precision Labs.

    -Tim Claycomb