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THS4561: Peculiar output waveform

Chet Bassani
Prodigy 10 points
Part Number: THS4561

201016 Preamp 6363 Amp - PCB.zip

  • 0
    TI__Mastermind 23196 points

    Hi Chet,

    What is the "peculiarity" specifically? Can you show me the input signal that induced this output, and show me what the LTC6363 output looks like with the same input?

    1. With a coil attached, the FDA is in a 93.1k/200= 465V/V gain. This will gain the input offset voltage up to a very high value. Do you need the gain to be this high? What is the input signal amplitude range which you are trying to measure? 

    2.  What is the signal frequency? Maybe we can try to AC couple the amplifier circuit so that the DC gain is not so very high, but the signal gain is still high.

    Are you using the amplifier to multiply voltage or is this an integrator? If the former, if you remove the 33pF feedback capacitors, the bandwidth of this circuit will increase and the gain will be more stable. Would that solve the problem?

    Best regards,

    Sean

  • 0
    Guru 48395 points

    Well Chet, 

    Lot going on there, One thing you might try is putting a cap across the THS4561 inputs, 

    That 33pF feedback takes the noise gain to 1 for the FDA and the THS4561 is actually decompensated a bit. Try 50pF right across the the two input pins as close close as possible to the pins. 

    It would be nice to have a full schematic in simulation - if you zip a .asc file, i can work with that. 

  • 0 in reply to Michael Steffes
    Prodigy 10 points

    Hi Michael,

    I have 47 pF caps and I will attempt to install those directly across the input pins.  I will let you know what happens.

    Thanks,

    Chet

  • +1 in reply to Sean Cashin
    Prodigy 10 points
    Hi Sean,

    The peculiarity is shown in the yellow circles of the waveform screenshot.  You can see how the output signal offsets and then the signal rides on top of this  DC offset until the offset disappears.  The LTC6363 output looks exactly like that shown but does NOT exhibit the DC offset.  Its output is one continually smooth shape without the DC offsets (shown in yellow circles).

    1) Yes, we need that high gain and the system requires DC coupling as we are measuring step responses of a magnetic field induced in metal targets.  The input signal amplitude is in the order of 10s to 100s of uVolts.

    2) Input is step responses so DC coupling is a necessity.  Maybe a very small AC coupling capacitor may work ( pFs).

    I can definitely remove the 33 pF feedback capacitors and try.

    Chet
  • 0 in reply to Chet Bassani
    Guru 48395 points

    yea I was looking at the LTC6363, that was one of those real odd ones with a 2nd pole zero pair in the Aol curve, this gives them that 500MHz GBP, but 35MHz unity gain bandwidth - point is they try to stabilize it internally for low gain with that trick - The THS4561 is a little touchy to feedback and load caps, but that cap across the inputs is a noise gain shaping trick that might help. 

  • 0 in reply to Michael Steffes
    Prodigy 10 points

    Removing the 33 pF feedback capacitors has solved the issue.  After having done this on 6 different preamp circuits the DC offset peculiarity has been eliminated.  The feedback cap removal has also eliminated any initial latching to rail upon power up.