Offset Voltage Indicator?

[jan.didden]

Fred:

Give a man a fish and he has a great meal.
Learn a man how to fish and he eats for the rest of his life.

Jan Didden

[jwb]

You gotta wonder, if you are going through the trouble of all those amps, comparators, etc etc etc why not just build the stupid servo? Worried about stability issues?

[Stocker]

Originally posted by boholm
Well, I just left a little headroom, so that it wouldn't be triggered too often if the amplifier had some fluctuations

Excellent point.

Why would it always be on?

It wouldn't! I was thinking about another project I have been contemplating for a while!

if you go for a single LED then <snip>

Your idea of a bicolor LED seems like exactly what would be called for. That way you don't have to fool with an integrator at all.

And the fading would look good, but it shouldn't fade too slow. Otherwise you could turn the trimmer too far and past 0.

Well, you can always adjust it back... ...another excellent observation.


Janneman, I think Boholm nailed it with his one bi-color LED.

Originally posted by thoriated For offset,<snip> the LED whose brightness is approximately proportional to the current passing through it.

If you set the thing up right a plain comparator would (might? )be able to vary the voltage so the brightness variance would be controlled by it.

Originally posted by jwb
You gotta wonder, if you are going through the trouble of all those amps, comparators, etc etc etc why not just build the stupid servo? Worried about stability issues?

Depending on the amp, perhaps. Then again some people seem to despise servos, and others just like to tweak. Nobody ever said this was the easiest, simplest, most cost effective, most automatic or most elegant solution available. Depending on how unstable our theoretical amp's offsets are, this could end up being a pain in the booty to keep adjusting...

It's just an idea...

[jan.didden]

Originally posted by Stocker
Janneman, I think Boholm nailed it with his one bi-color LED.



If you set the thing up right a plain comparator would (might? )be able to vary the voltage so the brightness variance would be controlled by it.

[/B]

A comparator by definition is an on/off switch, controlled by a reference. For instance, you may decide you can tolerate 10mV offset, you would have a comparator (really an opamp without feedback) with one input at 10mV and the other input at the offset. Whenever the offset gets above the reference, the output of the comparator switches. Depending on which inputs (- or +) have the ref or offset, the switching goes pos or neg. So if you take two comparators with opposite polarity connections, and wire-or the outputs (combining them with two diodes) you have a window comparator of which the combined output switches ON if the offset is outside the window (+ or - 10mV in this example).

The example in the quoted part above is no comparator at all but a linear amp whose output follows the offset to drive a variable current through the LED. That's not what he wanted.

In any case, you need an integrator to extract the offset from the amp output, which has a huge AC component completely swamping the relatively tiny offset. So, you need 3 opamps (yes I know you can do it with 2, but let's leave it at this for now).

Jan Didden

[Steven]

Originally posted by janneman

In any case, you need an integrator to extract the offset from the amp output, which has a huge AC component completely swamping the relatively tiny offset. So, you need 3 opamps (yes I know you can do it with 2, but let's leave it at this for now).

Make it a damped integrator. A normal integrator has an (almost) infinite gain for DC. Remember in this case, i.e. not being a servo, there is no feedback, so the LED wil go on after a while, even with a fraction of a mV offset.

Steven

[jwb]

Does it even need to be an integrator? I'll show my ignorance by suggesting you could just low pass the amp's output with a f(-3dB) ~ 1Hz and pass that to your DC-detecting window comparator

[jan.didden]

Indeed Steven, good point.

Jan Didden

[jan.didden]

Originally posted by jwb
Does it even need to be an integrator? I'll show my ignorance by suggesting you could just low pass the amp's output with a f(-3dB) ~ 1Hz and pass that to your DC-detecting window comparator

No false modesty please!
Hmm yes, why not. The nice point of the integrator is its low output impedance to drive the comparators, but a low-pass filter for very low frequencies would need either a large cap, which may be relatively large and expensive, or a large R, which may influence the comparators. But other than that, yes sure.

Jan Didden

[Stocker]

Originally posted by janneman
A comparator by definition is an on/off switch, <snip>

The example in the quoted part above is no comparator at all but a linear amp whose output follows the offset to drive a variable current through the LED. That's not what he wanted.

In any case, you need an integrator to extract the offset from the amp output, which has a huge AC component completely swamping the relatively tiny offset. So, you need 3 opamps (yes I know you can do it with 2, but let's leave it at this for now).

Jan Didden

that stuff on my face would be egg.

Thank you and you are of course right about the comparator/ linear amp business. I am being too sloppy with my terminology in this thread!

also, when you use a quad op-amp like a TL074 or LM324, there are 4 in one package...

[hjelm]

Wouldn't be a nice retro touch if you had a low pass filter and an analog voltmeter, + and - indication, instead of a LED? That way you have a relative feel for the actual offset and can trim it down.