simple output filter question

This is exactly why I said virtual ground.

"With the inverting op amp your cap's grounded pin is at virtual

It makes a big difference. Else we are simply driving a capacitor. With a low impedence voltage source. Only the internal resistance of that source is at play in filtering.

Because the capacitor is frequency dependent we obviously can't simply tie it directly to ground instead of virtual ground. If we think of the capacitor as frequency dependent resistance then the gain of the op omp configuration is simply frequency dependent. Because it's a gain parameter it must be in the negative feedback loop to have any effect.

Again, there is a difference between ground and virtual ground and the reason I explicitly mentioned it.

True. Even if they have exactly the same voltage, the virtual ground isn't connected to ground.

--
The movie \'Deliverance\' isn\'t a documentary!

The Lancaster book uses "equal component Sallen-Key", which is a slightly different beast from the standard S-K. It uses (as the name implies) equal Rs and equal Cs to determine frequency tuning, but the damping ("Q") is set by the (non-inverting) gain.

So the gain isn't unity (for most cases), but in exchange for that little detail it is *much* easier to tune. It's also much easier to buy parts, because everything is equal. The standard S-K requires the Cs to be computed according to the damping, with one of the Cs going up and one down as the damping changes. With the equal-component S-K, you only need to adjust the gain of the stage... a standard trimpot can even be used.

The upshot is that (IMHO) the standard S-K is best for a production design where you are going to build a bazillion and can order custom parts. The equal-component S-K is best for one-offs and limited production, since you can tweak the stage gains to get the desired response.

Best regards,

Bob Masta DAQARTA v4.51 Data AcQuisition And Real-Time Analysis

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I agree that you said "virtual ground". However you changed the circuit to show the capacitor connected to "ground" not "virtual ground". That is why I put my comments immediately below your circuit diagram.

I agree that simply driving a capacitor (as your diagram shows) is not the same thing as the original circuit. That is why I replied to your posting.

With an ideal op amp, i.e. one with no output impedance and infinite gain, the capacitor, in the circuit that you drew, would have no effect upon the gain of the circuit. The gain is simply -Rf/Rin.

With a real op amp, i.e. one with a non zero output impedance and probably its own internal pole, the capacitor (in your circuit) would add a second pole and could easily make the circuit unstable.

I agree that there is a big difference between ground and virtual ground. Why did you then draw a circuit in which the capacitor is connected to ground? It does not help the discussion.

Dan

Well, I didn't use the term Virtual ground in the graphic because it was too much trouble to draw all that stuff.

Again, this is why I explicitly said virtual ground right above it. Anyone that is going to response should at least read what is said

">>>> With the inverting op amp your cap's grounded pin is at virtual

to me that makes complete sense even if I put the GND symbol. The reason is that if it were a mistake on my part to use virtual ground then it would be highly unlikely to insert a random word that made the situation make complete sense as opposed to leaving off the term in the graphic.

Anyone reading the post with any care should have no problem understanding what I was talking about. Even if thats not the case it shouldn't matter too much. This is the internet. Also it's not that big a deal. If anyone was confused before then they are not now ;)

Sure it would have helped(since it couldn't have hurt) to add VGND or something but I didn't... it's over and done. I guess when I start getting paid to reply to people I'll be more cautious.

Well, the DC gain is a function of resistor ratios. Cheap delta-sigma ADCs have drifts of a few PPM per degree C, and you'll have to pay dollars per resistor, four resistors typically, to build a state-variable or biquad filter that doesn't wreck that.

John

Hi Phil, I guess I do need the 'bleeding obvious' pointed out again and again.

OK Let's assume some generic DC signal with a bunch of white noise up to 1MHz, after which it drops off quickly. I want to filter this signal and feed it to a DMM or some other voltmeter. I'd like to be able to measure the DC level with high accuracy as some other parameter is changed. I can short the input and measure the DC offset. If I want only single pole roll-off, would you recommend using a passive filter with buffer, an inverting active filter, or some other configuration.

Thanks,

George H.

Ahhh, Thanks John, I guess I've just started using $0.20 0.1% resistors, which seem cheap compared to the rest of the components (I think I'm a man living in the past switches are the most expensive component I use). If I can 'zero' my measurement do I care so much about temperature drift?

I must admit that SV and biquad filters are a bit 'freaky'. Who figured you could put two integrators in a row, close the loop and everything would be OK. But they seem to work just great!

George H.

Hey I wanted to thank you all for the stimulating discussion.

I think the obvious point I was missing. (Besides the active versus passive difference.. Phil A.) Is that the inverting configuration's accuracy depends on a resistor ratio. (As implied by John L.) Yeah, this is probablly so obvious that no one thought to mention it. Duh!

The resistors in an SV or biquad don't contribute zero error (the opamps take care of that for you) but they do donate gain error.

Only one of the integrators is a real integrator; the other has sort-of-local negative feedback. Two true cascaded integrators would indeed be indeterminate and would saturate one way or another.

Here's a little 6-pole S-K filter. It uses chopper opamps so dc offset is just a few microvolts and DC gain is 1.0 +- a PPM or so.

ftp://jjlarkin.lmi.net/Filter1.jpg

16 of them plug into this:

ftp://jjlarkin.lmi.net/Filter2.jpg

John

I worked through the 'math' of the SV filter once upon a time. And it 'behaves' just like a harmonic oscillator. And you are free to choose any resonant frequency and damping. But still when you first look at the circuit there are the two integrators... I just never would have thought to put two of them in a row.

Wow. well I'll have to wait till monday till I can download the pics, but how do you measure a DC gain to a few PPMs? I have a hard time with just 0.1%

thanks, George H.