OPA189: input current noise density spectrum of OPAx189

Hi Gerhard,

The OPA189 is a zero-drift or chopper amplifier.  Yes, the current noise density remains flat at low frequencies.

Zero-drift amplifiers such as the OPAx189 use switching on the inputs to correct for the input offset and drift of the op-amp. Charge injection from the integrated switches on the inputs can introduce extremely short current transients in the input bias current of the amplifier. These input bias currents cause no issues when using relatively low source impedances at the inverting and non-inverting op-amp inputs.

However, chopper amplifiers can be sensitive to high source impedance, or high impedance mismatch at the inverting and non-inverting terminals of the op-amp. We recommend keeping the equivalent input impedances at the inverting and non-inverting inputs of the op-amp relatively low, if possible, around the ~10kΩ range.  Another recommendation, in order to mitigate any noise generated by IB spikes across high value input resistors, and unmatched input impedances in the chopper amplifier, attempt to match the equivalent impedances at the inverting and non-inverting input terminals.

Below are a couple of posts that you may find helpful related to this topic:

e2e.ti.com/.../4494822

e2e.ti.com/.../4382915

Please let me know if you have questions,

Kind Regards,

Luis

Thanks, that helps.

With best greetings

Gerhard