What is the differences between Gm-C and SC filter?

[byoungson]

switched

What is the differences between Gm-C and SC filter?

I'm designing the Gm-C filter. But I don't know why I use this type of filter.

Could you explain about the differences?

 

[LvW]

gm-c filter concept

1.) in gm-C filters a current source (OTA with its transfer gm) plays the role of an ohmic resistor (with the aim to create an RC time constant)
2.) in SC filters a periodically switched capacitor (loaded/deloaded via a fet switch) simulates the resistor functin.

 

[byoungson]

gm-c filter

Thanks. LvW

But I've already known about your answer.
The things that I wanna know are weak points about SC or Gm-C filter.

Why do people use SC filter, not Gm-C filter?
or opposite situation.

Could U reply? I really wanna know.

 

[LvW]

It is not easy and not possible to give a final answer in short.
It really depends on your needs and on specific application conditions.
Both filters are suitable for integration purposes. Both offer the possibility for external tuning, but tuning the SC filter via the clock frequency is more linear and simpler as several time constants can be tuned at the same time.
But, of course, the disadvantage of the SC concept is the switched nature of the system with Nyquist limits and the necessity of pre- and after filtering.

Perhaps somebody else has some additional remarks.
Regards

 

[eld03]

lets be clear:
1. Gm/C filter has limited dynamic range.
2. Sability analysis in case of higher order gm/c filter would require analytical skills. Cad tools are of not use as breaking loop is guess work.
23. Gm/C filter is easy to design and control and used when your c is big in value so get R big by adjusting gm. low on silicon area.
4. Gm/C filter would be have higher noise ( double ended)

refer to gassiplex ASIC on web .