Offset Voltage Indicator?

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This is probably not going to generate much interest but here goes...

If you are designing an amplifier that has potential for DC offsets on the output;

And the offset can be trimmed out;

And you would like to keep it to (nearly) zero offset...

...and somehow you are motivated enough to design this circuit but not design away the offset voltage in your amp... ;)

How about a front panel LED that is an offset indicator? That is to say, when your output has a DC offset, there is an LED winking at you. Then a little knob beside the indicator trims the offset away until the LED goes out. I am thinking more along the lines of a headphone amplifier, but the concept works as well for a main system amp, no?

This would probably be a pretty simple cct to design using any run-of-the-mill comparator setup.

Comments?
 
Yes, it is possible.

IMHO it is quite simple: any OPAMP with a healthy gain should do the trick. Example: If +/- 50mV is max. before indication (LED switches on) then the gain should be around 3V/50mV=60. I say 3V for the LED+resistor to switch on, but all of it should be experimented with, so that the indication comes in an acceptable maner. And if you use a bicolored LED then you will know which way to turn your trimmer.

I hope I make sense . . . and no, I havent tried it myself. Just a suggestion.
 
millwood said:
would a DC servo driving an LED work?

Stocker said:
...and somehow you are motivated enough to design this circuit but not design away the offset voltage in your amp... ;)

If you went to the effort to design in the DC servo, the whole thing would be unneccessary, n'est-ce pas? Although I suppose, If you are a gadget freak or tweaking is in your blood (surely no one here ! ) It could be another gee-whiz feature to show off...


sreten said:
Better to use the DC servo to make offset a non-issue.

Yes, if you like a DC servo :D

Boholm: seems like if you are going to do it, do it all the way: go for <10mV if you are making the effort. Heck, go for 0 if you want. but yes, this was the idea. A high-gain opamp would definately be a must. A bicolor LED would be one way, and a great idea for amps with dual supplies, BUT it would always be on. If I did it I would go for a single LED so I could have the satisfaction of seeing it fade to black as I zeroed out the offset. :cool: maybe an integrator...

It would be :cool: :cool: if you put it behind a translucent face plate and you didn't see anything at all besides the adjustment knob when the LED winks out.
 
Re: Re: Offset Voltage Indicator?

Stocker said:

Boholm: seems like if you are going to do it, do it all the way: go for <10mV if you are making the effort. Heck, go for 0 if you want. but yes, this was the idea. A high-gain opamp would definately be a must. A bicolor LED would be one way, and a great idea for amps with dual supplies, BUT it would always be on. If I did it I would go for a single LED so I could have the satisfaction of seeing it fade to black as I zeroed out the offset. :cool: maybe an integrator...

It would be :cool: :cool: if you put it behind a translucent face plate and you didn't see anything at all besides the adjustment knob when the LED winks out.

Well, I just left a little headroom, so that it wouldn't be triggered too often if the amplifier had some fluktuations (spelling? meaning:D ) But yes, you could go so low - I would test it out before final assembly.

Why would it always be on?

And if you go for a single LED then you would only catch the DC going in one direction depending on how you soldered in your LED . . . . oh, maybe you actually mean two LEDs (how to say?) opposite coupled? (sorry my technical english - needs brushing off:blush: )

And the fading would look good, but it shouldn't fade too slow. Otherwise you could turn the trimmer too far and past 0.
 
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What you would want is an integrator that senses the DC, basically a servo but without the servo output used to null the offset (because you want to do that yourself, right). Then take this output and stick it into what is called a window comparator. Basically two opamps in open loop that switch to HI or LO depending whether the offset is below or above a threshold. Combine the two to drive a LED and you have a bi-directional offset indicator.
Look up window comparators in Walt Jung's opamp cookbook, or ask google to look it up on the net.

Jan Didden
 
This is close to a scheme I described a few weeks back here using an LED to indicate offset & bias. For offset, the output would be fed into a full wave 'ideal' rectifier using op amps which then drives an op amp configured as a current source driving the LED whose brightness is approximately proportional to the current passing through it. You can, of course, add integration or a LP filter to the above to reject audio band energy.
 
Re: Re: Re: Offset Voltage Indicator?

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! :eek:

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... :D ...another excellent observation.


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

thoriated [/i] [B] For offset said:
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... :)
 
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Re: Re: Re: Re: Offset Voltage Indicator?

Stocker said:
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
 
Re: Re: Re: Re: Re: Offset Voltage Indicator?

janneman said:

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
 
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jwb said:
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
 
janneman said:
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




:eek: 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...
 
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