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OPA2189: Op Amp low noise solution

Daviel Almonte
Expert 3526 points

Part Number: OPA2189
Other Parts Discussed in Thread: OPA2140, OPA2197

Hello

How can I reproduce the specified noise figures for the OPA2189 if  source impedances are not well matched at both inputs? or is there another part where I can have similar or lower noise? 

  • 0
    TI__Guru* 93105 points

    Hi Daviel,

    The OPA2189 uses auto-zeroing techniques to minimize voltage offset and voltage offset drift to the very low numbers seen the datasheet. There is active switching occurring at the OPA2189 frontend, and charge transfer taking place among the input switch elements. The current noise generated by the switching is very low and is taken into account in the noise specifications. However, the noise is derived with the op amp in a lower impedance, well matched circuit.

    If the impedance paths seen by the two inputs contain high impedances and are not matched the charge transfer current noise can degrade the otherwise low noise performance. Therefore, if there is any question about there being high impedances and mismatches in the circuit it is better to use an op amp that doesn't employ auto-zeroing or chopping to obtain the lowest levels of noise performance from the circuit.

    Here's an example of a precision, non-auto-zero JFET input op amp that delivers low-noise performance, the OPA2140:

    www.ti.com/.../opa140.pdf

    It will tolerant a source impedance mismatch and not generate any additional noise itself due to it.

    Another op amp possibility if very low voltage offset and drift are required along with low noise are required is one of the high precision CMOS e-trim op amps, the OPA2197:

    www.ti.com/.../opa197.pdf

    Regards, Thomas

    Precision Amplifiers Applications Engineering