TINA/Spice/OPA365: output clipping

Vadim Bishtein1

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.

over 5 years ago

0 Karan Kotadia over 5 years ago

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 Michael Steffes over 5 years ago in reply to Karan Kotadia

Guru 48235 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 Vadim Bishtein1 over 5 years ago 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 Karan Kotadia over 5 years ago in reply to Vadim Bishtein1

TI__Genius 16725 points

Hi,

I believe Sean Cashin 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 Vadim Bishtein1 over 5 years ago 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.

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TINA/Spice/OPA365: output clipping