opamp bandwidth requirement in continous time filter

[carl_chao]

op amp ugb

Hi all,

I'm designing a continous time (active RC) filter. The input signal has a highest speed of 20MHz. What's the minimum opamp bandwidth I should design for this filter?

Thanx in advance!

Added after 23 minutes:

I meant the input has a bandwidth of 20MHz.

 

[rameshprakash]

filter and op-amp bandwidth

To determine the GBW, the loop gain (Aol) has to be maintained @ highest frequency of the input signal (w= 20 MHz).

Aol = GBW/w

For example, your open loop gain requirement = 46 dB (~200)

Then your GBW > Aol* w

GBW > (200)*20e6
GBW ~ 4 GHz

Regards,

Ramesh

 

[FvM]

opamp gbw, filter

This depends on the filter frequency and the intended characteristic. Generally a limited amplifier GBW distorts the filter characteristics. This may be considered to a certain extent in filter dimensioning, but it could be impossible to achieve an exact filter behaviour. E. g. a flat Bessel or Gaussian filter may show peaking and overshoot in time domain. GBW above 100 MHz is generally advisable for a 20 MHZ filter.

 

[carl_chao]

bandwidth requirement to time domain

Thanks FvM and rames,

Yes, I think the UGB of the opamp should be determined by the application (especially the distortion) requirement of the filter. If the UGB is too low, at 20MHz the opamp gain is too low to make the closed loop (thst is, the filter) effective, which will lead to high frequency component peaking. So, the opamp UGB design is a kind of iterative process: start with a common sense (like 5 times the filter bandwidth requirement) value, test the circuit, if pass the spec, decrease the opamp UGB (to save power); if not, increase it. Correct me if I'm wrong here.

rames, I think you mis-understood my question.

 

[rameshprakash]

There are two things you should be concerned abt.

1) The GBW of opamp affects the pole/zero frequencies (arise from the filter).
You could model the opamp as verilogA (include GBW parameter in verilog-A) and simulate the filter characteristics (with verilog-A opamp)
and chose the GBW - which doesn't affect the pole/zero frequencies

2) Distortion requirement

Regards,

Ramesh

 

[jecyhale]

Hi all,

I'm designing a continous time (active RC) filter. The input signal has a highest speed of 20MHz. What's the minimum opamp bandwidth I should design for this filter?

Thanx in advance!

Added after 23 minutes:

I meant the input has a bandwidth of 20MHz.

about 100MHz

 

[rameshprakash]

Hi jecyhale,

Can you give the relationship(equation) how did you arrive @ GBW of opamp for the requirement ?

Or we should just rely on verilog-a modeling of opamp (with arbitary min GBW to start with) and observe the pole frequency changes, and change the GBW (to finally arrive @ sweet spot) for which the pole frequency doesn't var.

Thanks,

Ramesh