How to perform xf analysis to calculate opamp's CMRR?

[teraze]

hello,
when I perform xf analysis on a fully differential opamp circuit to
calculate its CMRR.
I let the output to be: db20(1/DATA("/Vic/PLUS" "xf-xf"))
I use the calculator to write this equation,am i right(I began to
learn how to use spectre recently)??
but when i run it,i am getting the following errors:


*Error* Evaluating expression (dB20((1 / DATA("/Vicm/PLUS"
"xf-xf")))).
*Error* ("quotient" 2 t nil ("*Error* quotient: can't handle (1 /
nil)"))

 

Teraze,

I have another question for you. How are you measuring CMRR?
In general, CMRR is the ratio of the common-mode loop gain to the
differential mode loop gain. In this case the numerator is 1, why?
Are you using a balun to generate the input for the op-amp?
d - differential input signal
c - common-mode input signal
p - connect to non-inverting input
n - connect to inverting input
Then you calculate the CMRR as the ratio of gain from common-mode
input to differential output to differentianl mode input to
differential
mode output. BTW, you will need a balun on the output.

In general, using the environment avoids the syntax issues you are
experiencing since you just point and click.

Best
Regards,

Brand
Sheldon


teraze wrote:

hello,
when I perform xf analysis on a fully differential opamp circuit to
calculate its CMRR.
I let the output to be: db20(1/DATA("/Vic/PLUS" "xf-xf"))
I use the calculator to write this equation,am i right(I began to
learn how to use spectre recently)??
but when i run it,i am getting the following errors:


*Error* Evaluating expression (dB20((1 / DATA("/Vicm/PLUS"
"xf-xf")))).
*Error* ("quotient" 2 t nil ("*Error* quotient: can't handle (1 /
nil)"))