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Stability OPA1632 with closed loop gain <1

Marcin Kuczma
Prodigy 60 points
Other Parts Discussed in Thread: OPA1632, OPA627

Dear Sir/Madam

I am designig an audio single ended/differential to differential amplifier with second order 100 kHz low pass filter. I decided to use opa1632. I need to use gain less than 1 (0,6 or 0,3; I do not know yet). I can not see open loop gain and phase characteristics in datasheet, so i need to ask about stability this amplifier with closed loop gain as low as 0,3V/V.

Thanks for your help.

  • 0
    TI__Genius 12631 points

    Hello Marcin,

     The simulation shows a bit of peaking, however we can tame that by adding feedback capacitance. Please see attached TINA simulation.

    -Samir

    TestCkt.TSC

  • 0 in reply to Samir Cherian
    Prodigy 60 points 
    Thank you for your quick response. I have one more question. Why you recomend low pass filter for ADC like figure 14 in datasheet of opa1632 instead of schematic from "filter pro"?
  • 0 in reply to Marcin Kuczma
    TI__Genius 12631 points
    The filters in filter pro are MFB (multiple feedback) filters. Those are 2nd order filters in a single amplifier stage.

    The filter shown here is a first-order filter with in a multiple feedback loop configuration. This can be confusing compared to what I mentioned previously about the MFB filter so I will try to clarify the difference.

    The main difference is that the circuit shown in the datasheet utilizes a multiple feedback loop in order to be able to drive the large 2.7nF (C3) output cap without losing phase margin. The way to analyze the circuit in the datasheet is as follows:

    1. At low frequencies, the signal gain is through R1 and R3 (I will only analyze 1/2 the circuit). At the low frequencies C1 (and C2) will be "open".

    2. As the frequency increases the impedance of C1 will reduce and this will become the dominant signal path and R3 will act as an "open" circuit. In this condition the output pin of the amplifier will be isolated from the 2.7nF load cap. through the 40ohm (R5) resistor. If that resistor was not there, then at high frequencies, close to the amplifiers loop-gain crossover point, the amplifier output would directly be driving the large output cap. This is not safe and can result in instability.

    One way for you to check this out is to reduce the value of that 40-ohm resistor to say 4 ohms and run an ac frequency response simulation in TINA. You will notice that the frequency response will show a lot of peaking - thus indicating marginal stability.

    Hope this makes sense. If this is still confusing see section 7.3.6 of the OPA627 datasheet where they show a similar circuit for a single-ended opamp.

    -Samir