Qustion: IIP3 of active complex filter

Qustion: IIP3 of active complex filter

Dear all,

I am designing a complex complex filter for wireless communication these days, which use OPAMP-RC topology;
Let use a 1st order OPAMP-RC filter to simplify the problem, the transfer function is (Rf||C)/R1, actually there is a series switch with R1, let's call it SW1;
As for the linearity of filter, I found that there is tens of dB difference for the IIP3 of filter between the case with SW1 and the case without SW1! No matter how I increase the size of SW1, there is no improvement for IP3, once I remove SW1, the IIP3 is quite good.

How to explain this? is it reasonable?

Pls don't delete my post, because the analog guys and RF guys are not in the same sub-forum. And hence I post in both sub-forums.

BR
Buckaroo

IIP3 is a measure of non-linearity. Perhaps your SW1 is very non-linear.
For high frq. shielding is essential with characterisitic impedance.

Hi SunnySkyguy,

I am very sorry that it is due to the common mode voltage setting...
The common mode voltage is generated by I*R, for some reason, I change the bias current, but I forget to change the R...

Thanks！