HI
Thanks for answering, you might be right, I will think about it more. I do know there is no DC feedback, but it seems is necessary for a charge pump of PLL, not sure about it yet. Can you comment more on the DC feedback and why it should not working for any pratical circuits?
Thanks a lot
LeeX wrote on Apr 28th, 2009, 11:02am:I think I may have found how you get the 4 mV "offset".
First of all, I do not think this configuration can work for any practical design, as no DC feedback is present.
However, for the ideal op-amp, I assume it works properly and by "offset" you refer to the difference between zero cross points for the input and output.
The transfer function between input and output is
Vout/Vin=1+1/(jw/(1/RC))
where w is the angular frequency, in your case it is 2G*2*Pi, R=10K and C=200fF, according to your circuit.
Thus
Vout/Vin=1-j0.03978
So Vout is actually shifted in phase and amplified in magnitute, comparing to Vin,
the phase shift is
-0.03978
so if Vin = 0.1*Sin(wt)
Vout is approximately
Vout = 0.1*Sin(wt-0.03978)
let t=0,
Vin = 0
Vout = -0.00397≈4 mV
Actually, by definition the word offset should refer to the voltage difference between the op-amp positive and the negative input when the output is zero (common mode).
If my guess is right, you are measuring the voltage difference between the output node and input node, non of them are the opamp input.