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LM6171: Double Integrator

IVAN SKHEM SAWKMIE
Prodigy 20 points
Part Number: LM6171
Other Parts Discussed in Thread: IVC102

Hi,

I have use the LM6171B for integrating a sinusoidal signal such that I can vary the amplitude and frequency of the input signal. The circuit which I have attached here is from TINA and for example, I have use an input signal with 1Vpp and f=5KHz. Therefore the output of the 1st integrator should be a cosine with an amplitude ~6.24Vpp and the output of the second integrator should be a sine with a phase difference of 180 degree w.r.t the input signal having an amplitude of ~3.9Vpp. However, since the gain setting is very large (R2 and R4), I'm not getting the results. Can you please suggest how to solve this problem?

LM6171_two Integrators.TSC

  • 0
    TI__Genius 14166 points

    Hi Ivan,

    Your rail out problem is unrelated to DC gain caused by R2 and R4. It is Vos and Ib integrating until the capacitors are charged to the rails.

    This could be counteracted in theory by adding a -5.9mV, 5.1k ohm source to your non inverting inputs.

    However, in practice, using bipolar amplifiers like this one for integrator applications is more difficult than just using a lower Ib CMOS amplifier. Additionally, if your source has any DC offset at all, it will cause a DC capacitor current even if Vos and Ib are perfectly compensated. I would recommend an amplifier with a switched reset amplifier like the IVC102 for use as an integrator.

    Best regards,

    Sean

  • 0 in reply to Sean Cashin
    Prodigy 20 points
    Dear Sean,
    Thank you for your reply.
    I have connect a 5.1k to the non-inverting input to compensate Vos and Ib, but it seems that this will not work.
    The output offset voltage of the 1st integrator is less than 1volt in this case. However, the 2nd integrator will see this output offset voltage from the 1st integrator and as a result, the capacitor are charged to the rails at the output of the 2nd integrator. Please correct me if I'm wrong.

    Apart from IVC102, is there any other op amp with pdip package that can perform the integration since with SOIC package, I will have more problems.

    Warm Regards,
    Ivan.
  • 0 in reply to IVAN SKHEM SAWKMIE
    TI__Genius 14166 points

    Hi Ivan,

    Sorry for the late reply. Can you just drop your 1M feedback resistors to 100k? That looks like enough to bleed off the DC error but high enough to not impact 5kHz.

    Best regards,

    Sean

  • 0 in reply to Sean Cashin
    TI__Genius 14166 points

    50k will be even better for decreasing the output DC offset voltage, but it will start to clip off the gain from the integration constant.1263.LM6171_two Integrators.TSC