The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> Analog Design >> Stable bode plot and still opamp oscillating!!
https://designers-guide.org/forum/YaBB.pl?num=1383258257

Message started by AAOAA on Oct 31^st, 2013, 3:24pm

Title: Stable bode plot and still opamp oscillating!!
Post by AAOAA on Oct 31^st, 2013, 3:24pm

Hello,

Could someone please help me with this. The bode plot seems to be OK @100kHz. But I cant figure out why the signal oscillates in the transient analysis?
[img][/img]

Title: Re: Stable bode plot and still opamp oscillating!!
Post by AAOAA on Oct 31^st, 2013, 3:24pm

Bode plot

Title: Re: Stable bode plot and still opamp oscillating!!
Post by AAOAA on Oct 31^st, 2013, 3:25pm

The circuit

Title: Re: Stable bode plot and still opamp oscillating!!
Post by wave on Oct 31^st, 2013, 5:36pm

Hello - What is the goal of this circuit?

Your schematic is confusing if not completely wrong.
The left side of the Wheatstone bridge seems connected to both VDD/VSS supplies of your amp. The R6 gain resistor seems like it should go to VSS too.

It is not clear what loop you are simulating for the Bode plot.

However, the gain starts off -12dB, so it seems like a wrong test.
The phase does not quite move 180deg, but if you are in the wrong loop, it is meaningless.

:-/

Title: Re: Stable bode plot and still opamp oscillating!!
Post by aaron_do on Oct 31^st, 2013, 5:43pm

Hi,

your power supply is hooked up wrongly, and your bode plot is showing no gain.

regards,
Aaron

Title: Re: Stable bode plot and still opamp oscillating!!
Post by ywguo on Oct 31^st, 2013, 6:44pm

Hi AAOAA,

The gain in your bode plot is less than 0 dB, which means no gain. So the schematic must be wrong.

And you need to check the phase margin, not the gain and phase at 100 kHz.

Best Regards,
Yawei

Title: Re: Stable bode plot and still opamp oscillating!!
Post by AAOAA on Nov 1^st, 2013, 7:01am

Thanks for your replays. But is the Bode plot useless at this point (since the phase margin doesnt indicate about oscillation @100KHz)?

Title: Re: Stable bode plot and still opamp oscillating!!
Post by tm123 on Nov 1^st, 2013, 7:16am

The Bode plot is meaningless because the circuit is not connected correctly. It seems like the node where the OP AMP VDC sources connect together should go to ground not the feedback node. Also it is not clear how you generate this plot and where you break the feedback, but a circuit can not be unstable if the open loop gain is <0dB.