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Old 19th July 2007, 10:11 PM   #1
matejS is offline matejS  Slovenia
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Default Tricky op-amp based VR problem

Hi, all!

I have a problem that I am unable to solve...

I have op-amp based voltage regulator:

Click the image to open in full size.

Actually, I have three of them.
One for analog +5V, other for analog -5V and third for the digital +5V.
My power source are 3 SLA (12V) batteries. Each one for each VR.

If only analog +5V and -5V are connected everyhing is OK, but as soon as I connect digital and join DGND with AGND, analog +5V drops by half of the volt!

Is this a VR problem? Or gounding problem?

Any help is very appriciated...

Thanks,
Matej
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Old 20th July 2007, 12:11 PM   #2
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So are you saying you have 3x that circuit shown and they are only tied together at a single ground point? There is no other circuitry past the output?

I just took a cursory look, but it seems it should work. If you have something at the outputs, I would be suspicious about that part.
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Old 20th July 2007, 12:15 PM   #3
SY is offline SY  United States
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Separate opamps for each of the outputs (as opposed to duals or quads)?
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Old 20th July 2007, 12:21 PM   #4
matejS is offline matejS  Slovenia
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Quote:
Originally posted by aymanme
So are you saying you have 3x that circuit shown and they are only tied together at a single ground point? There is no other circuitry past the output?
Well, the negative regulator is a little but different (diodes inverted, etc.). All the the same ground. Analog +5V and -5V are grounded already at the SLA battery. Digital +5V on the PCB (or at the battery - it does not change a thing).

Output circuit is AD1865N DAC for analog, and CS8414 receiver for digital.

Quote:
I just took a cursory look, but it seems it should work. If you have something at the outputs, I would be suspicious about that part.
True, but we are talking about regulator. It should handle the load, right?

Matej
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Old 20th July 2007, 12:22 PM   #5
matejS is offline matejS  Slovenia
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Quote:
Originally posted by SY
Separate opamps for each of the outputs (as opposed to duals or quads)?
I am using separate LF351N for each VR. Actually, all VRs are seperated circuit (they do not share anything) - expect the GND.

Matej
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Old 20th July 2007, 12:45 PM   #6
SY is offline SY  United States
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Then you have a sneak path somewhere in your load circuit, probably a diode which turns on when the commons are connected. Verify this by disconnecting the loads one at a time at the "hot" end.
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Old 20th July 2007, 02:01 PM   #7
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Hmmm. That battery for -5V is grounded at it's pos terminal, right?

Jan Didden
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Old 20th July 2007, 02:06 PM   #8
matejS is offline matejS  Slovenia
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Quote:
Originally posted by janneman
Hmmm. That battery for -5V is grounded at it's pos terminal, right?

Jan Didden
Right. And I guess this is wrong?

Matej
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Old 20th July 2007, 02:27 PM   #9
SY is offline SY  United States
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That's what you want.

Do you have a scope? Sometimes when these puppies oscillate, a simple voltmeter reading shows a voltage change. It would be interesting to see what happens on the rails when the commons are joined.
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Old 20th July 2007, 02:48 PM   #10
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What capacitors are you using as the output bypasses?

If they have really low ESR, you will make the regulator oscillate.

For this type of op-amp based circuit, an ordinary electrolytic is ideal; film caps or exotic audiophile devices can give trouble
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