Measuring Phase Margin in TINA

Emkay Kim

Prodigy 270 points

Other Parts Discussed in Thread: LMH6644, TINA-TI

Hi,

I’ve been trying to measure PM, but Aol never cross the 0dB point.

Please help me out to measure phase margin of LMH6644.

the oscillation wave figure and tina-ti schem file are attached.

the oscillation in real board looks worse but when i change the Co from 1u to bigger(or remove), the oscillation peak level decreased and is gone

LMH6644 Phase Margin Measurement.pptx LMH6644 PM Measurement.TSC

Thanks.

over 9 years ago

0 Samir Cherian over 9 years ago

TI__Genius 12631 points

Hello Emkay,
I think you mean to connect R3 between Vcc and the non-inverting terminal, to set the common-mode bias at the inputs of the amplifier. Once you do that and run an ac simulation you can increase the End Frequency of the sweep to 100MHz and also make sure to select the "Amplitude & Phase" option on the ac simulation window.
-Samir

0 Hooman Hashemi over 9 years ago in reply to Samir Cherian

TI__Mastermind 22825 points

Hi Samir and Emkay,

I have used TINA-TI to measure the LMH6644 phase margin with a heavy cap load. For this to work, I had to estimate the device output impedance (R + L) that would cause excess phase shift with a heavy cap load. I did not spend too much time on this but a better estimate can be derived by looking at the output impedance vs. frequency (Figure 31 datasheet):