Biasing High Pass Sallen-Key Filter

[truffaldino]

Hello,

I simulated biased and unbiased Sallen-Key high-pass filters and got different responce curves.
I will be grateful if someone could point out on my error with biasing.

Please, find attached unbiased (double source V-=-V+) and biased (single source V-=GND) schematics.

Thanks in advance
Truffaldino

 

[KlausST]

Hi,

and got different responce curves.

Show us the curves.

Klaus

 

[truffaldino]

Hi,



Show us the curves.

Klaus

Sorry Guys,

I have re-simulated the filter.

The curves are identical. It seems that last time I did sometsing wrong with simulator.

It was a false alert.

Sorry again

Truffaldino

 

[Audioguru]

The results with the single supply voltage depend on the output impedance of the power supply. The circuit you show feeds half of any hum and noise on the positive supply to the reference input of the opamp.
Do it liked this:

 

[truffaldino]

Thanks Audioguru,

Although responce curve of circuit you proposing is somehow different at very low frequencies (-110db vs -140db at 10Hz which does not make real difference), attenuation of noice is much more important!

Truffaldino

- - - Updated - - -

The results with the single supply voltage depend on the output impedance of the power supply. The circuit you show feeds half of any hum and noise on the positive supply to the reference input of the opamp.
Do it liked this:

And for low-pass filter: is the following biasing correct?

 

[Audioguru]

My mistake, only one voltage divider is needed to make the half-supply bias voltage. The filter capacitor for it must have a high enough value for it to be almost a short circuit at your lowest frequency.
Your biasing for the lowpass filter is correct.