Filtering frequencies from 0Hz to 20Hz

[adrian1232]

Hi again,

I designed the high pass filter with a cutoff of 20Hz and used another op-amp for the variable gain, as suggested.
Is the design good for attenuating frequencies below 20Hz? Are the values selected correct or should I select smaller value capacitors?
Also, when I checked the frequency responce at the output, I got 30Hz at -3dB. Why is this? shouldn't it be 20Hz?

 

[adrian1232]

Sorry forgot to upload the schematic:

 

[FvM]

No good dimensioning for an active filter, why didn't you use one of the filter design tools suggested in this thread.

R1=12k, R2=24k, C1=C2=470n gives a 20Hz butterworth high-pass. See below the comparison of the butterworth filter magnitude characteristic (blue) with your filter (yellow). Besides different cut-off frequency, your filter has lower Q (0.5) than the butterworth filter (0.7)

 

[adrian1232]

It worked much better with those values. Thanks.

 

[Audioguru]

The Sallen-Key filter must be fed from a low impedance like the output of an opamp. Therefore your amplifying opamp should be at its input, not its output.
Is the 2nd-order slope steep enough? The output still has plenty of signal level below 20Hz.

 

[FvM]

Therefore your amplifying opamp should be at its input, not its output.

A buffer may be necessary if the source impedance is high (adrian1232 never told about the signal source), but you don't want additional gain in front of the HP because it brings up a risk of overload by low frequency signals and DC offset.

 

[zorro]

In the schematic, the lower terminal of R2 should be connected to 1.65Vgnd (like the non-inverting input of U1A) instead of ground.
Z

 

[adrian1232]

@Audioguru, The input is sourced from an instrumentational amplifier which has low output impedance.
@zorro, yes the actual circuit has R2 connected to 1.65V. The one attached is the first version on the circuit. Thanks.