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Old 22nd April 2004, 03:29 PM   #31
moamps is offline moamps  Croatia
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Quote:
Originally posted by peranders
I'm sorry, but this won't work (I might be wrong).
Hi,
I have this on breadboard. Works reasonably good.
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1 LP-filter, preferable up to 3th or even 4th order, cut frequency 0.01-1 Hz. Those filters most also have low offset voltages. This is dependent how low DC voltage you want to detect, how advanced the filter should be.
Too complex.
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Fullwave rectifier or two halfway rectifiers, one for positive and one for negative signals
Too many additional IC's.
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3 Comparator with HYSTERESES!
...will destroy windows margine.
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Filtering out a couple of millivolts from several volts or tenths of volts is a tricky business. Don't forget that DC is low frequency AC!
What about simple (one IC) DC servos in amps? Doesn't look so tricky.
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Old 22nd April 2004, 03:30 PM   #32
moamps is offline moamps  Croatia
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Quote:
Originally posted by peranders
Don't forget that DC is low frequency AC!
Oximoron?
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Old 22nd April 2004, 03:45 PM   #33
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Quote:
Originally posted by janneman
Per-anders,

You may remember that this comparator stuff is preceded by an integrator like the servo loop normally used to null the offset. Please look up the previous posts.

Jan Didden
OK, but even so the integrator must be extremely slow and/or you need extra filters.
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Old 22nd April 2004, 03:48 PM   #34
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Quote:
Originally posted by peranders

OK, but even so the integrator must be extremely slow and/or you need extra filters.

Not different from 'normal' servo loops. The fact that they are used in many 10.000's of amps is some indication that most probably it works. I concede that often the implementation isn't very good, but we have beaten that particular horse to death in this thread I would think.

Jan Didden
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Old 22nd April 2004, 03:50 PM   #35
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OK, I'll back off a little but if you take signal from the DC servo intergrator you will not get an exact measure of the offset. The servo output is only a signal inside a feedback loop and tell you not much really.
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Old 22nd April 2004, 04:18 PM   #36
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Quote:
Originally posted by peranders
OK, I'll back off a little but if you take signal from the DC servo intergrator you will not get an exact measure of the offset. The servo output is only a signal inside a feedback loop and tell you not much really.

You obviously have not taken my advice to read the former posts. The servo loop is not closed, because he wants to regulate the offset by hand. I said "an integrator LIKE the servo loop NORMALLY used to....". There is no closed servo loop and the integrator output is the offset voltage, at least above the low pass freq. And indeed that should be low enough so that there is no lf audio to slowly vary the apparent offset.

Can I go now?

Jan Didden
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Old 22nd April 2004, 04:20 PM   #37
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If one is not sure the amp being nulled may not dc offset slightly in the presence of some musical or test signals, best to dc null with no input signal applied or with a mute function enabled at the same time.
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Old 22nd April 2004, 04:21 PM   #38
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Quote:
Originally posted by janneman
Can I go now?
yes,
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Old 22nd April 2004, 04:53 PM   #39
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Default Here's some initial doodlings of my linear idea

This is meant to be implemented with a quad op amp. The first section is a differential integrator which will probably change - I drew it this way because I wanted to be able to select this circuit as a bias monitor as well as a offset monitor circuit. The second and third op amp sections and associated components comprise a high speed 'ideal' full wave rectifier. The final op amp stage acts as a bipolar current source to linearly control the LEDs intensity. 'Off' would correspond to 0 volts offset, in this case. I threw in D5 and R16 to limit sensitivity more than several tenths of to a couple volts away from the null range. All component values hypothetical
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Old 22nd April 2004, 05:23 PM   #40
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Originally posted by peranders

yes,

LOL



guys you are making my day(s) by actually putting some effort into this whim of mine.

Thanks everybody!
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