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OPA2189: Impedance matching precision amplifier OPA2189

Part Number: OPA2189
Other Parts Discussed in Thread: OPA2192

hello,

I looked at figure 43b of the OPA2189 Datasheet and was wonder if the input impedance of the plus terminal should be matched to the input of the minus terminal.

I need to invert 1 PPM 5 volt signals and was wondering if the inputs should be matched to terminate biases or that it's better to have the plus terminal grounded.

Awaiting responses

Tom

  • Hi Tom,

    The least amount of noise will be had by minimizing the number of resistances added to the circuit. For example, if you use the OPA2192 Figure 43B inverting amplifier, adding a resistor equal to the parallel resistance of R1 and R2 to the non-inverting input would only result in higher noise, than if you don't include it. The inclusion of that resistor was sometimes done to reduce the voltage offset introduced by the input current flowing through the resistors in the inverting input circuit.

    Additionally, regarding voltage offset, the input current into either input can be plus, or minus, and can be different from one amplifier to the next. You could make the output-referred voltage offset better, or worse, depending on the polarity of the input currents.

    Sometimes the simplest approach is best!

    Regards, Thomas
    Precision Amplifiers Applications Engineering