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Biasing High Pass Sallen-Key Filter

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truffaldino

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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
 

Attachments

  • Sallen_Key_hp_unbiased.jpg
    Sallen_Key_hp_unbiased.jpg
    39.7 KB · Views: 108
  • Sallen_Key_hp_biased.jpg
    Sallen_Key_hp_biased.jpg
    44.4 KB · Views: 132

KlausST said:
Hi,



Show us the curves.

Klaus
Click to expand...

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
 

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:
 

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  • highpass filter.png
    highpass filter.png
    93.4 KB · Views: 83
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 - - -

Audioguru said:
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:
Click to expand...

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

Attachments

  • 4th_order_S_K_low_pass_biased.jpg
    4th_order_S_K_low_pass_biased.jpg
    61.8 KB · Views: 107

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.
 

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