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Stable bode plot and still opamp oscillating!! (Read 4632 times)
AAOAA
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Stable bode plot and still opamp oscillating!!
Oct 31st, 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]
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Transient.jpg
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AAOAA
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Re: Stable bode plot and still opamp oscillating!!
Reply #1 - Oct 31st, 2013, 3:24pm
 
Bode plot
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Bode_Plot.jpg
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AAOAA
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Re: Stable bode plot and still opamp oscillating!!
Reply #2 - Oct 31st, 2013, 3:25pm
 
The circuit
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The_circuit.jpg
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wave
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Re: Stable bode plot and still opamp oscillating!!
Reply #3 - Oct 31st, 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.


Undecided
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aaron_do
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Re: Stable bode plot and still opamp oscillating!!
Reply #4 - Oct 31st, 2013, 5:43pm
 
Hi,


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


regards,
Aaron
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ywguo
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Re: Stable bode plot and still opamp oscillating!!
Reply #5 - Oct 31st, 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
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AAOAA
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Re: Stable bode plot and still opamp oscillating!!
Reply #6 - Nov 1st, 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)?
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tm123
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Re: Stable bode plot and still opamp oscillating!!
Reply #7 - Nov 1st, 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.
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