I am working on the design of Instrumentation amplifier for ECG system. Initially, I designed a Difference differential amplifier. When I checked the Gain and Phase margin response, the system is highly unstable.
I observed phase margin in negative value and 3dB cut off frequency at ten's of MHz.
I tried to add a resistor in series with cap but there is no change in Phase margin.
No - at first we should clarify if you have determined the margin correctly. I doubt.
At first, problems with stability margins arise for systems with feedback only - and to determine the margin you have to investigate the LOOP GAIN.
Did you? Where is your feedback loop?
Secondly, all the poles you have shown have a NEGATIVE real part. This indicates stability of your system.
No - at first we should clarify if you have determined the margin correctly. I doubt.
At first, problems with stability margins arise for systems with feedback only - and to determine the margin you have to investigate the LOOP GAIN.
Did you? Where is your feedback loop?
Secondly, all the poles you have shown have a NEGATIVE real part. This indicates stability of your system.
I don't know why the attachments are missing in my earlier post....
untitled.jpg is the Difference differential amplifier and 12.jpg is the feeback circuit
No - at first we should clarify if you have determined the margin correctly. I doubt.
At first, problems with stability margins arise for systems with feedback only - and to determine the margin you have to investigate the LOOP GAIN.
Did you? Where is your feedback loop?
Secondly, all the poles you have shown have a NEGATIVE real part. This indicates stability of your system.
OK - so you have opened the loop and terminated correctly?
Where is the loop ? Can you show the point of loop opening in your diagram?
Another question: The given poles and zeros are for closed or open loop?
EDIT: As I cannot read the axis of your magnitude/phase diagram: What is the phase shift for very low frequencies?