need help to understand the offset / bias current / input impedance of op amp

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glias

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Hello all,
I would want to understand why my input current of the minus input of my op amp vary with the resistor MCT sensor... ? For me input current of the op amp is composed of an offset current and a bias current : I-=Ibias-Ioffset/2, and this current should be a constant .. .?
How can I deduce the ac value of this current and thus the voltage error of my minus input ?

Is there any relation between the input current and input impedance of the op amp (Rdiff=5k ; Rcm=30M) to have a total input offset voltage ? or is this parameter is taken in account in the Vos voltage of the op amp ?

regards.


here is the schematic and plots (the blue one is the minus input current of the op amp)

 

This opamp seems to have a MOS input stage.
If that's the case, the current at the input is simply the current used to charge/discharge the input capacitance of the opamp. The amplitude depends on the input capacitance, your input frequency and the source impedance as seen from the inputs. It seems conformant with specs, which state <0.1uA, which really isnt that much.

You should do some calculations to see if this 0.1uA is really the major source of any "errors" you are seeing.
 

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