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OPA2189: Amplifiers forum

kurata-k
Prodigy 10 points
Part Number: OPA2189
Other Parts Discussed in Thread: LM6171, , OPA2192, OPA189, OPA192

Hi.


I have created a composite amplifier using OPA2189 and LM6171, and when I created a PWM modulator using this amplifier, I encountered a problem where the output voltage fluctuates every time the power is turned on and off.

I checked the experimental system shown below and found that the output of the PWM modulator fluctuates by about 12μV, and that changing the OPA2189 to the OPA2192 tends to improve the symptom.

Other things I am aware of are as follows.

Fluctuation does not always occur, but sometimes does not occur.

The magnitude and polarity of the fluctuations are not constant.

The fluctuation is not constant in magnitude and polarity.

All experiments were conducted in a thermostatic chamber set at 20°C, so it is unlikely that the temperature had any effect on the drift.

This is the first time I have used a zero-drift op amp, so I do not fully understand many aspects of it.

  • 0
    TI__Guru 52486 points

    Hi Kurata-san,

    As you have mentioned, the OPAx189 is a zero-drift or chopper amplifier. Zero-drift / chopper amplifiers can be sensitive to high source impedance, or high impedance mismatch at the inverting and non-inverting inputs of the op-amp. Since the OPA189 is a chopper, the input bias current does exhibit periodic switching transients. These input bias current transient become transformed to a voltage offset when the bias currents flow through the high value circuit resistors. Hence, in order to eliminate the noise generated by IB spikes across the input resistors and offset shift caused by large unmatched input impedances in chopper amplifiers, we recommend to attempt to match the equivalent impedances at the inverting and non-inverting input terminals, as well at keeping the equivalent impedances relatively low, if possible, around the ~10kΩ range. In addition to reducing the impedances and feedback components, adding and/or matching the equivalent input capacitance can help improve results.

    Below are a couple of posts related to this topic:

    https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/1190764/effect-of-the-input-impedance-of-the-source-to-the-op-amp/4494822

    https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/1165061/opa2188-noise-reduction-in-howland-current-source/4382915

    On the OPAx189, matching the equivalent resistanceand 4700pF capacitance of the inverting terminal, and placing these components on the non-inverting terminal may help mitigate the issue, see example/figure below)

    As you have mentioned, the OPA192 is a high-precision non-chopper amplifier and the device will not be sensitive to the impedance mismatch.

    Thank you and Kind Regards,

    Luis



  • 0 in reply to Luis Chioye
    Prodigy 10 points

    Hi, Mr.Chioye

    thank you for your reply.

    I tried added a resistance and capacitance at non-inverting input, but it's not effect to the problem.

    after that, I trie some ideas and I found that reducing the resistance at the inverting input terminal to 2.2 kΩ reduced the variation by 2/3.

    However, it not resolve this ploblem.

  • 0 in reply to kurata-k
    TI__Guru 52486 points

    Hi Kurata-san,

    Yes, the OPAx189 tends to be sensitive to mismatched impedances, specially impedances above 10k.  To reduce the error, the total equivalent resistance looking into the inverting terminal will need to be reduced, and in our tests, matching the inverting and non-inverting impedance helps reduce errors.  This would mean reducing the total series resistance on the inverting trace path, and then matching the equivalent impedance on the non-inverting path. 

    If the circuit requirements do not allow reducing the series resistances, as you have discussed, the OPAx192 is a high-precision, non-chopper amplifier that will not be sensitive to the high-impedance at the inputs.

    Thank you and Regards,

    Luis