switched capacitor filter

[krivan]

capacitor filter

Hi,

I do chopper-stabilization in my circuit and I want to low-pass filter the output signal.
I want to set the corner frequency of the filter to 100 Hz. In a normal first-order LPF it would mean 320 MOhm && 5 pF. I implement the resistor as a simulated [switched-cap] resistor using two non-overlapping switching signals [t_period=1.6ms].

The filter is set up like you see in the first picture. So I assumed the sw_cap_resistor to behave like the "normal" one but it obviously does not happen, the whole block filters practically nothing... :-/

Could somebody please explain me how to improve this to get the required filter out of it and why the bode plot is like a slope-down? It is like the freq. response of an integrator, however it does not meant to behave like that.

Thx,
Krivan

 

[LvW]

chopper switched capacitor filter

...........................
In a normal first-order LPF it would mean 320 MOhm && 5 pF. I implement the resistor as a simulated [switched-cap] resistor using two non-overlapping switching signals [t_period=1.6ms].
The filter is set up like you see in the first picture. So I assumed the sw_cap_resistor to behave like the "normal" one but it obviously does not happen, the whole block filters practically nothing... :-/
Krivan

Here are two comments which may be helpful for you:
1.) Your hope that the "sw_cap resistor" behaves like a "normal" one is unrealistic.
You cannot replace each resistor in a passive circuit by a S/C unit, because of the following restrictions (which result from the S/C theory):
a) Each S/C circuit based on the EULER approximation must be placed between two nodes which are either signal input, opamp virtual ground (inv. input) or real ground. In your circuit. one node of each s/C unit is floating (capacitor).
Therefore, in most cases an active filter is transferred to a s/C filter.
b) But you can use the bilinear s/C circuit approximation, which has some other drawbacks (parasitics).

2.) I don´t know which program you have used for the BODE diagram. But realize that each S/C circuit is time variant and nonlinear. Therefore you cannot run the normal ac analysis of a simulation package. Either you need a specific feature of the program or you must convert the whole circuit into a linear time-continuous system in advance.

Good luck.
Regards LvW