Design of switch cap filter?

[hebu]

This is my first time to design sc filter, any mistake is easy to make?
and anything I should pay attenation?

 

[willyboy19]

Be more specific if you want specific help or advise.

 

[hebu]

OK, I would like to design a low pass filter with 1MHz sampling clock,
and second order butterworth I will try first. the 3dB bandwidth of this filter
is at 15KHz.

1)My question is if I use two first order cascaded rather than one biquad,
is this OK?
2)If I analyze the sc filter as same as active RC filter rather than in discrete
time domain? is this OK?
3)And do I need to do the internal compensation in OPamp, or just use
the cap of filter to do the compensation?
4)Which kind of CMFB, do you suggest?

Thanks,

 

[ablue]

For 1, I think we should also consider the power dissipation or other spec. I prefer biquad since you implemented filter in sc filter.
For 2, I think discrete time domain will give you similar result with continous time domain, and the accurate response maybe need to be verified in transient .
For 3,4 . I think it depends on ur opamp structure. If the capacitor is large enough to compensate ur op, selfcompensation is a good choice. and since u implement the opamp in sc filter, cmfb can also be the sc structure. That's my opnion

Ablue

 

[hebu]

Thanks,

By the way, how I check the THD with cadence because the THD will count
the harmonics caused by swithcing ckt so the total THD is bad.
Any more correct method to measure this?

Added after 1 minutes:

Thanks,

By the way, how I check the THD with cadence because the THD will count
the harmonics caused by swithcing ckt so the total THD is bad.
Any more correct method to measure this?

 

[ablue]

So u should chose switch size so that the distortion caused by switching ckt is lower than distortion required. The most sensitive switch is at sampling node and other switches should be placed at signal independent nodes.

 

[hebu]

How about the selection of OPamp, the gain and the ft?
Uasually, the current of OPamp is around? 0.35um process, 1MHz sampling
clock.

Thanks,

 

[jiangwp]

The characteristic parameters of op amp is relative to the loading and feedback factors of integrator.
In the meanwhile, the settling accuracy and noise requriement are the factor to determine the op amp.

I think the design of op is the core design for SCF.

 

[hebu]

Thanks for reply,
How about the Slew Rate of Opamp?

1) What kind of simulation I have to do to make sure the SR is adequate to
the SCF?

2) I am confused will the slewing condition happen? if we precede a anti-aliasing
filter. does the aa filter ease the Slewing?

Thanks,

 

[swicap]

how can i know that the settling accuracy is ok.
judge from transient analysis?
is there any general method?