[sdiy] Single supply op-amp filter biasing question

[Tom Wiltshire]

Hi All,

I've got another, related, question with single supply filter biasing.

If I were biasing a Sallen-Key filter, how would I achieve it?

For the Highpass filter, it's very easy, since you can replace the ground connection of the resistor from the +ve input with your bias level and bingo! you're done. Like this:

http://electricdruid.net/wp-content/uploads/2017/05/SallenKeyHPFilter.png

For a *lowpass* sallen-key filter though, it doesn't seem so easy. The resistor to ground is now a capacitor, and putting a bias level on the bottom of that is pointless. Thus by elimination, I'd say we've got to feed our bias level into the input along with the signal. If the signal were audio, I'd bias the input with resistors to 9V and 0V, and then AC-couple the input to the bias point with a capacitor. But since the signal that I'm trying to retrieve with this filter is a DC level, using a capacitor (AKA highpass filter) to connect the input doesn't seem feasible.
So could I do something like this, and how would I calculate the value of X?

http://electricdruid.net/wp-content/uploads/2017/05/SallenKeyFilter.png

Thanks,
Tom




On 27 Apr 2017, at 19:37, Tom Wiltshire <tom at electricdruid.net> wrote:

> Hi All,
> 
> I have the following situation:
> 
> A PIC produces a PWM output representing an LFO waveform. This is then sent to an op-amp MFB filter for smoothing. The circuit looks like this:
> 
> 	http://www.electricdruid.net/images/SingleSupplyLFO.jpg
> 
> If I'm running the PIC on 0-5V, and running the op-amp on 0-9V, the op-amp can't handle the raw PWM output, since it clips below about 1.6V (for the TL07x - other typical op-amps are not terribly dissimiliar). So the PWM signal needs biasing to move it to the correct range.
> 
> I found the following useful document discussing biasing, both AC and DC, for the various op-amp configurations:
> 
> 	https://ocw.mit.edu/courses/media-arts-and-sciences/mas-836-sensor-technologies-for-interactive-environments-spring-2011/readings/MITMAS_836S11_read02_bias.pdf
> 
> I've used this guide with success on other circuits. However, in this case I can't make theory and practice match up. For a start, the MFB filter isn't a typical DC-coupled inverting op-amp application, although that's the closest. The filter is inverting and has no overall gain, so gain = -1.
> 
> How would I calculate the required bias to get the LFO output signal (0-5V) in the centre of the op-amps range (2-7V)?
> 
> I know the answer to this question from practical experiments, but I'd like to know how I'm supposed to derive it from the theory.
> 
> Thanks,
> Tom
> 
> 
> 
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