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THS4631: THS4631-based integrator output operation's large delay

Yatao Ling1
Expert 1840 points
Part Number: THS4631

Dear TI expert,

I design a PCB in which a THS4631-based integrator is used. The circuit is as Fig1. The power supply for 4631 is +16V and -12V, feedback capacitor C2 is 20pF, R1 is 100 ohms, which leads to high output voltage slopes.

Fig1

I step VG1 from -1V to +1. The waveform is as Fig2, where blue waveform is VG1, red one voltage at inverting node and green one is the output voltage Vout. Note that at time point 1, the VG1 steps to 1V, however, after about 60ns, the inverting and output node begins to act at time 2.

Fig2

I then step VG1 from -1V to +1.5V, and the waveform is as Fig3. Note that the delay is reduced to about 40ns but is still there.

Fig3

I then try to change R1 in Fig1 to 499ohms, and VG1’s step to +5V. The experiment shows the delay is very small and negligible. The waveform is not given here.

I do the test in TINA and finds no such interval between time 1 and 2 as observed in Fig2,3. In the simulation, - and output node voltage act very quickly.

In theory, when VG1 steps to 1V or 1.5V(large signal), given the inverting node is 0V, THS4631 should immediately rise at a slope of several hundred V/us.

I wonder what may lead to the delay. 

If you need further detail about my PCB design of this part of integrator or other information, just inform me.

      

 

Best Regards

Yatao

 

  • 0 in reply to kai klaas69
    Expert 1840 points
    Hi Kai,
    The 2 threads are different. The link you give here is mainly about the input's difference voltage.
    This one is about the op amp delay, which is not normal. Because at the moment of the step, the output should slew quickly. However, in my experiment, the output has tens of ns of delay.Please help me!

    Regards
  • 0 in reply to Yatao Ling1
    Guru 140200 points

    Yes, but if you start a new thread without giving a link to the original one, nobody knows what we have discussed so far. Can you imagine how much time and efforts we have spend up to now to answer your questions?

    Kai

  • 0 in reply to kai klaas69
    Expert 1840 points

    Hi Kai,
    I've marked "solved" of last thread. Sorry to forget that.
    This thread is independent and I wonder what causes the phenomenon. You can also observe that when + node voltage steps +1V, then the difference voltage is +1V. The output should slew quickly, however , it hardly rises until after 60ns.
    Please help me!
    Regars

  • 0 in reply to Yatao Ling1
    Guru 140200 points

    Hi Yatao,

    if you look very carefully at your scope plots, you will notice that there's no such delay. The signal at the -input of THS4631 immediately rises:

    The reason why you don't see it, is because the output slew rate is still low at the begin of input pulse. It increases, the more the output comes out of saturation.

    You must not drive the output into the rails, because this results in saturation and considerably slows down the THS4631!

    Kai

  • 0 in reply to kai klaas69
    Expert 1840 points

    Hi Kai,
    Thanks for you patience, and I think I get it.
    Then I want to confirm 2 points from you.
    1. When 1V is changed to 1.5V, the delay is reduced.When 1V changes to 5V with R1 from 100 ohms to 499 ohms, the delay is hardly seen. Based on the phenomenan, can I draw the conclusion that the larger the input step, the faster the op amp leaves the saturation zone?
    2. How can I ensure THS4631 not work in saturation(say, op amp stabilizes at +15V, -10V, noting the supply voltage is +16,-12V)? The circuit should be modified or some other ways may help?

    Regards
    Yatao