Active Filter butterworth 8 order

[LvW]

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But it is look that the SK very sensitive to components tolerance value.

Yes, that is a known disadvantage of the S*K topology. As an advantage, it is less sensitive to the finite and frequency-dependent opamp gain (in contrast to the MFG structure).

 

[TXRX]

Hi,

I order the ADA4896 to replace the ADA4891, The ADA4896 has a noise of 1nV/sqrt(Hz), The ADA4891 has a noise of 12nV/sqrt(Hz).

If put the ADA4896 instead of ADA4891 how much the output voltage noise will be reduce ?

 

[FvM]

ADA4896 is a bipolar OP. You need to reduce the RC impedance by a factor of 10 or 20 to get full advantage of it. With present dimensioning, the increased OP noise current will almost dwart the ADA4896 noise voltage benefit.

You should preferably perform a SPICE noise analysis with the ADA4896 model to evaluate the role of resistance level for total circuit noise.

 

[TXRX]

Hi,

I measure the Noise from the OPA with Scope and BNC Coax, when i disconnect the Coax for the Scope connector the level of the Noise is 1mVptp.

I already choose the BW Limit of the scope to 25MHz.

How i can measure a noise which lower than 1mVptp ?

 

[FvM]

You'll need some kind of amplifier with limited bandwidth. Some digital oscilloscopes (e.g. LeCroy) have additional filter options that can be used as programmable low pass.

 

[TXRX]

Hi FvM,

Do you have recommendation of such amplifier ?

 

[TXRX]

Hi,

I replace the OPA to a low noise: ADA4896-2ARMZ and receive a better reduction of output noise.