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_rf wrote on Dec 6^th, 2013, 12:41am:
A 3dB frequency in the range of tens of MHz? For an ECG amplifier? Even a unity-gain BW in that range seems generous to me....

Which approach did you use for sizing the compensation cap?

- B O E