LMP2021: Prediction of Ib temperature drift

Hello Hugh,

A low offset voltage and offset drift, or a low IB, does not necessarily means a low IB drift - this is especially true in the case of zero-drift amplifiers like LMP2021 and OPA387.  In order to provide you with a best option, we would need to see your circuit schematic including the resistor values used to gain up the signal.  Also, what is the required temperature range of your application and power supply voltage used?  

The design can be flexible to meet the requirement.  As the input sensor needs to feed into a high impedance, I had planned to connect it to the non-inverting input and have a 1MOhm resistor from the non-inverting input to ground to provide a path for bias current (the sensor impedance is higher than 1MOhm and varies with the signal but I can allow for the variation).  The gain would then be as in a standard non-inverting amplifier.  I can choose the other resistors around the amplifier to optimise noise and drift.  As only low frequency operation is required, the 1MOhm resistor on the input could have a capacitor of up to 1uF across it.

If an inverting configuration would give better results, I can do that with the input being fed through a 1MOhm resistor to the inverting input.  This would then require a feedback resistor of around 150MOhm.  Any resistor connected between the non-inverting input and ground can be chosen to minimise noise and, again, capacitors that limit the bandwidth can be added to the circuit. I have assumed this configuration would give worse results as the bias current on the inverting input would be feeding into much more than 1MOhm.

I can be flexible with power supply voltage from 2.5V (or +/- 1.25V) to +/- 15V.

The design is for a laboratory based instrument and the temperature will be room temperature and fairly stable, say between 15 and 25 degrees at any one time as a worst case.  As I mentioned before, day to day variations can be calibrated out by the operator before using the instrument.

My problem is that it is difficult to design for the high impedances in the circuit as the data sheets give very little information about low frequency current noise and bias current temperature drift.

Thanks.  There are some useful insights there. 

I am still not getting something about the bias currents.  My understanding is that in these types of amplifiers the bias currents to the inverting and non-inverting inputs have opposite signs.  This can be seen in the data sheets for the two devices we have been looking at where the offset current is quoted as being twice the bias current.  Therefore the trick of using similar resistors to the two inputs to cancel bias currents doesn't work.  Are you saying then that, rather than cancelling the effect of the bias currents, the similar resistors actually reduce both bias currents and/or make them more stable.

In short - yes. The input bias currents in chopper amplifiers do have opposite magnitude at any given instant of time BUT they keep switching back and forth at every clock auto-correction cycle  Thus, the error comes NOT from a DC components of the said currents but only from the mismatch between the positive and negative magnitude of those currents.  For this reason, matching the input impedances cancels error created by the IB's mismatch just as it does in the bipolar input op amps with no IB cancellation.

One could also get around extrapolation of Fig 41 in LMP2021 by matching the input impedances as shown below.