Tricky op-amp based VR problem

matejS · 19th July 2007, 10:11 PM

Hi, all!

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

I have op-amp based voltage regulator:

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

aymanme · 20th July 2007, 12:11 PM

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.

SY · 20th July 2007, 12:15 PM

Separate opamps for each of the outputs (as opposed to duals or quads)?

matejS · 20th July 2007, 12:21 PM

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

matejS · 20th July 2007, 12:22 PM

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

SY · 20th July 2007, 12:45 PM

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.

jan.didden · 20th July 2007, 02:01 PM

Hmmm. That battery for -5V is grounded at it's pos terminal, right?

Jan Didden

matejS · 20th July 2007, 02:06 PM

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

SY · 20th July 2007, 02:27 PM

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.

PigletsDad · 20th July 2007, 02:48 PM

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

19th July 2007, 10:11 PM	#1
matejS diyAudio Member Join Date: May 2007	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: 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

20th July 2007, 12:15 PM	#3
SY diyAudio Moderator Join Date: Oct 2002 Location: Chicagoland Blog Entries: 1	Separate opamps for each of the outputs (as opposed to duals or quads)? __________________ If there's a sucker born every minute, where do the rest of them come from?

20th July 2007, 12:45 PM	#6
SY diyAudio Moderator Join Date: Oct 2002 Location: Chicagoland Blog Entries: 1	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. __________________ If there's a sucker born every minute, where do the rest of them come from?

20th July 2007, 02:01 PM	#7
jan.didden diyAudio Member Join Date: May 2002 Location: Great City of Turnhout, Belgium Blog Entries: 6	Hmmm. That battery for -5V is grounded at it's pos terminal, right? Jan Didden __________________ /Yes! Its out: Linear Audio Vol 5! I'm not an "accademic", just a plodder who loves a challenge - Ian Hegglun

20th July 2007, 02:27 PM	#9
SY diyAudio Moderator Join Date: Oct 2002 Location: Chicagoland Blog Entries: 1	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. __________________ If there's a sucker born every minute, where do the rest of them come from?


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20th July 2007, 12:11 PM	#2
aymanme diyAudio Member Join Date: Aug 2002 Location: Austin, TX	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.

20th July 2007, 02:48 PM	#10
PigletsDad diyAudio Member Join Date: Jul 2007 Location: South Worcestershire	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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