Help Needed !! Instrumentation Amplifier - Phase Margin

Hi,

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.

I did pole-zero analysis:

Poles:

1. -3.22+02 Hz
2. -5.417+07 hz
3. -2.513+07 Hz
4. -9.684e+07 Hz
5. -3.19e+08 Hz
6. -5.27+e08 Hz

Zeros

1. -2.7941e+06 Hz
2. -4.922e+07 Hz
3. -9.679 e+07 Hz
4. -3.14e+08 Hz
5. 6.94232e+07 Hz
6. 2.43 e+09 Hz
7. 9.01661 e+09 Hz

Can you pls tell me how to improve the Phase margin?

shaikss said:

I observed phase margin in negative value and 3dB cut off frequency at ten's of MHz.

Can you pls tell me how to improve the Phase margin?

Click to expand...

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.

LvW said:

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.

Click to expand...

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

- - - Updated - - -

LvW said:

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.

Click to expand...

Yaa, I have determined the margin correctly. The system has a feedback.

shaikss said:

Yaa, I have determined the margin correctly. The system has a feedback.

Click to expand...

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?

The feedback configuration shows a single ended feedback network. What's the ground level related to OP supply voltages?

shaikss said:

Yaa, I have determined the margin correctly. The system has a feedback.

Click to expand...

Do you need further help?
If yes - you should answer questions as contained in my last post.