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OPA820: pulse fall-time is getteing reduced using 8-to-1 inverting summing amplifier with OPA820

Md Faisal Rahman
Prodigy 170 points
Part Number: OPA820

Tool/software:

Hello

I have built an 8-to-1 inverting summing amplifier using OPA820 where, R_f =  R = 1K  (unity gain). However, I can see a reduction in pulse fall-time which I don't understand. Does anyone who the reason? Is it common with summing amplifier with large number of inputs? One input signal is an analog pulse (anode pulse) and the other seven inputs are at 0 V ( baseline).

Thanks

Faisal

  • 0
    TI__Genius 10560 points

    Hi Faisal,

    One idea that comes to mind is because the device is bandwidth limited. One thing to consider with the additional inputs is that the Rg resistors at the inverting node are technically in parallel if sourcing from low impedance sources. The more inputs, the more Rg resistors in parallel and the effective Rg drops. This causes the amplifier's noise gain to increase, and this increase causes an overall drop in bandwidth. Does the signal get better if you remove some of the Rg resistors thus deducing the noise gain?

    Best Regards,

    Ignacio

  • 0 in reply to Ignacio Vazquez Lam1
    Prodigy 170 points

    Hi Ignacio

    The signal gets betters for lower number of inputs. However,  shouldn't the fall-time increase with noise gain getting increased with increasing number of inputs? I don't understand why the fall-time is getting smaller with increasing noise gain.

    Thank you

    Faisal

  • 0 in reply to Md Faisal Rahman
    TI__Genius 10560 points

    Hi Faisal,

    That is correct, as the noise gain increases your bandwidth will decrease and result in an increase in rise/fall time. It is interesting you are seeing the opposite. Could you share the output signal you are capturing? Is the rise time observable? If so, can you share any changes occurring with that. 

    Best Regards,

    Ignacio

  • 0 in reply to Ignacio Vazquez Lam1
    Prodigy 170 points

    Hi Ignacio

    The rise-time decreases too.

    Thanks

    Faisal

  • 0 in reply to Md Faisal Rahman
    TI__Genius 10560 points

    Hi Faisal,

    Could you share the signals you are seeing where the rise/fall time differ and the circuit configuration used to get those signals?

    Best Regards,

    Ignacio 

  • 0 in reply to Ignacio Vazquez Lam1
    Prodigy 170 points

    Hi Ignacio

    I was thinking if the stray capacitance on a PCB at the input of an inverting summing amplifier can create a high-pass filter effect As the inputs are increased, stray capacitance and inverting input resistance also get added in parallel and thereby, increases the high-pass effect.

    Best

    Faisal

  • 0 in reply to Md Faisal Rahman
    TI__Genius 10560 points

    Hi Faisal,

    That is likely the best direction that can be taken as to why this is happening. There could be some form of parasitic capacitance causing the device to extend its bandwidth which is where you see the better rise time performance. The only concern is that I do think the overall bandwidth is dominated by the noise gain of the circuit and with higher resistors in parallel, the noise gain should increase and reduce the bandwidth. Could you share the results you are see? Are they noticeably different?

    Best Regards,

    Ignacio