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TINA/Spice/OPA365: output clipping

Vadim Bishtein1
Prodigy 30 points
Part Number: OPA365
Other Parts Discussed in Thread: TINA-TI,

Tool/software: TINA-TI or Spice Models

We're observing clipping on the output voltage when simulating the OPA365. There is no visible clipping of the output current.

The clipping behavior shows dependence on common-mode voltage and the output current. Seems that OPA365 is unable to source >=27mA at VOUT<0.6V. Is this expected ? Attaching portion of the simulation schematic and waveforms.

  • 0
    TI__Genius 16725 points

    Hi,

    The amplifier has a GBW product of 50Mhz. You are trying to have a gain of 20000 with a frequency of 100KHz = 2GHz GBW. This will make the amplifier unstable. You either need to split up the gain in multiple stages or try a faster part.

    Thanks!

    -Karan

  • 0 in reply to Karan Kotadia
    Guru 48165 points

    This is not likely a slew rate issue, the feedback R is a load to the output inverting  mode.

    1. Way too low of Rf values, need to scale that up 

    2. High gain does not lead towards instability for VFA, but more phase margin actually

    But really, those R's are just too low for a low power op amp. And yes, the gain is crazy high in one stage, 

  • 0 in reply to Michael Steffes
    Prodigy 30 points

    Thank you for the reply. Please note that the input signal is a sinusoidal current (swinging 0mA to 20mA) that gets injected into the summing junction of OPA365 which performs current-to-voltage conversion. 

    We have to use the Rfb = 20ohm because the output would always clip near 0.5V when we used 25ohm. 

    Also we have to use 1V on LV_BIAS because the input signal source (current mode DAC, AD9742) has compliance range of <1.2V (we chose 1V for margin).

  • 0 in reply to Vadim Bishtein1
    TI__Genius 16725 points

    Hi,

    I believe  is in email contact with you regarding this circuit. I will close this thread, if you have any questions you can email him directly.

    Thanks!

    -Karan

  • 0 in reply to Karan Kotadia
    Prodigy 30 points

    Hi Karan, yest I received email communication from TI about this topic.

    To share findings with the wider forum: 

    • The clipping observed in simulation appears to match OPA365 datasheet Figure-7 that shows VOUT-vs-IOUT curves.
    • The output current capability of OPA365 scales with output voltage, and in my case OPA365 was being asked to sink >30mA while outputting 0.5V+VSS. According to Figure-7 , at Vout=0.5V (wrt to VSS) the output current capability would be around 25mA. This explains the clipping behavior.