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LM6172 slewrate problem????????

Ronald Dekker
Prodigy 90 points
Other Parts Discussed in Thread: LM6172, THS3122, THS3062, THS3061, THS3091, THS3092, THS3096, THS3095

Hello,

 

I have designed a circuit based on the LM6172. Now I am running in some problems.

Abnormal distortion of the signal above 10MHz sine wave input signal and Limited slew rate.(was not the limit of my test equipment)

I have changed the feedback circuit 1Kohm//1.8pF or just 1Kohm or 560Ohm and 0 Ohm and 1kOhm load or no load.

There were some changes but the problem was still there. The input signal was good.

PCB layout: 8mm of 10mil trace on output.

Can you help me and tell me what is going on.

This is my test circuit.

The LM6172 is powered by +15V and -15V

These are the measurement results. Measured with a FetProbe on the output of the LM6172 .

10MHz sinewave signal: Output seems ok:

17MHz sinewave signal: Output seems ????

20MHz sinewave signal: Output seems ????

  • 0
    TI__Mastermind 22825 points

    Hello Ronald,

    Sorry about the delay in getting back to you.

    I have also observed what you have shown in your scope photos of the LM6172 when running the device at high swing / slew rates.

    May I suggest that you try the following to see if it resolves the issue:

    Add back to back Schottky diodes, two diodes total, across the device inputs as shown below, to limit the differential input voltage (which becomes larger at higher frequencies and / or larger swings) to +/-0.3V. There is already silicon diodes (0.7V) internally and these may not be low enough in voltage to prevent this behavior. Schottky diodes may (although I have not tried it myself yet).

    Do you have access to Schottky diodes to try the experiment above?

    Regards,

    Hooman

  • 0 in reply to Hooman Hashemi
    Prodigy 90 points

    Hello Hooman,

     

    Thanks for your response.

    So this is a know issue with the LM6172. If I take a look at the datasheet I cannot find anything about this. Also the spice model does not reveal this problem.

    I will try the sckottky diodes. I am curious if this will work but the confidence in the LM6172 drops. Do you know a suitable replacement?

    Best regard,

    Ronald Dekker 

  • 0 in reply to Ronald Dekker
    TI__Mastermind 22825 points
    Hi Ronald,
    I think THS3062 (current feedback amplifier, 300MHz unity gain bandwidth, 7,000 V/us, +/-15V capable, dual), or THS3122 are good possible candidates to try in place of the LM6172 (although I'm only going by the datasheet specs and I've not tried to bench these under similar operating conditions).

    Regards,
    Hooman
  • 0 in reply to Hooman Hashemi
    TI__Mastermind 22825 points

    Hi Ronald,
    I see the following note within the THS3062 (and a similar one for the THS3122 as well) which probably excludes it as well for your application (high slew rate and repetitive waveform):

    "The THS3061 and THS3062 are not recommended
    for applications with repetitive signals (sine, square, 5 357 Ω 383 Ω
    sawtooth, or other types) that exceed 900 V/μs. Using 10 200 Ω 200 Ω
    this device in these types of applications results in an
    excessive current draw from the power supply and Care must be taken to not set these values too low.
    possible device damage. For applications with a high The amplifier's output must drive the feedback
    slew rate and repetitive signals, the THS3091 and resistance (and gain resistance), and this may place
    THS3095 (singles) or the THS3092 and THS3096 a burden on the amplifier. The end result is that
    (duals) are recommended instead."

    I will look for other options except the ones noted above to be suitable (THS3092, and THS3096), and if I locate any, I'll post it here.

    Regards,
    Hooman

  • 0 in reply to Hooman Hashemi
    Prodigy 90 points

    Hello Hooman,

    I have tried the Schottky diodes but it did not solve my issues.

    I will order the THS3092 and see if that will work.

    Best Regards,

    Ronald