Comparator oscillation in meter circuit - pollutes audio

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ilya
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Comparator oscillation in meter circuit - pollutes audio

Post by ilya » Fri Sep 01, 2017 3:43 am

I'm using a rather standard metering circuit with full wave op-amp rectifier that feeds a string of LM339 comparators. Comparators outputs are connected to the string of LEDs that are fed from the constant current source. All well and good. I had this circuit working on the dual rails supply with no problems at all.
However, in this device I have a single supply, so I'm using an op-amp rail-splitter and I'm deriving virtual ground reference from there. Now, here's what happens. When the LED just starts to turn on (that is, input V at the comparator = Vref), I see a high order harmonics apearing on the FFT analyzer. Once the LED is fully on, these harmonics disappear.
I hunted this problem down to LM339 oscillation between on and off and this somehow couples into audio path. The only solution that I found is to put a cap in the NFB loop of each comparator. Here're the quesions:
1. Why does this happen? Is it because of split rails? Didn't have any issues with regular dual supplies.
2. Is there any more elegant solution than slapping 16 additional caps on the LM339 (I've got 4 chips on the board).

Thanks for any comments!

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mediatechnology
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Re: Comparator oscillation in meter circuit - pollutes audio

Post by mediatechnology » Fri Sep 01, 2017 4:59 am

Can you post a schematic?
If it didn't oscillate before and it does with a rail splitter I'm suspicious of the rail splitter.
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Re: Comparator oscillation in meter circuit - pollutes audio

Post by JR. » Fri Sep 01, 2017 8:39 am

Look for ground corruption which will look like positive feedback.

May be as simple as decoupling cap on v/2 buffer output.

JR
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Re: Comparator oscillation in meter circuit - pollutes audio

Post by ilya » Fri Sep 01, 2017 3:32 pm

I'm attaching the schematic below.

The circuit has negative power supply, so MTR_GND is 0V, _B- is -18V, VIRT_GND is something like -9V

The grounding scheme might be the culprit as I'm using a V/2 as a decoupling point and now I don't think it was such a good idea. But if you see anything that may cause this behaviour, please speak up.
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meter_drv.PDF
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JR.
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Re: Comparator oscillation in meter circuit - pollutes audio

Post by JR. » Fri Sep 01, 2017 3:41 pm

decouple the op amps and comparators to "real" ground. Virt ground, only wishes it was ground.

JR
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Re: Comparator oscillation in meter circuit - pollutes audio

Post by ilya » Fri Sep 01, 2017 3:51 pm

Well, this will actually reduce parts count (2 caps per chip to 1 cap per chip). Will try this when I get back home from tour. Anything else to look at?

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mediatechnology
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Re: Comparator oscillation in meter circuit - pollutes audio

Post by mediatechnology » Sat Sep 02, 2017 5:52 am

What JR said. Decouple to "real" ground.

Also remove those two 22uF at the output of the TL07X rail splitter.
It's not going to like driving 44uF.
Add a 47R in series with the rail splitter output and then decouple the rail splitter line with a single C to "real" ground.
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Re: Comparator oscillation in meter circuit - pollutes audio

Post by ilya » Sat Sep 02, 2017 1:12 pm

mediatechnology wrote:
Sat Sep 02, 2017 5:52 am
What JR said. Decouple to "real" ground.

Also remove those two 22uF at the output of the TL07X rail splitter.
It's not going to like driving 44uF.
Add a 47R in series with the rail splitter output and then decouple the rail splitter line with a single C to "real" ground.
Something like this?
splitter.jpg
Should I lower the value of Cm101 to 0.1uF?

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Re: Comparator oscillation in meter circuit - pollutes audio

Post by mediatechnology » Sat Sep 02, 2017 1:59 pm

I would lower it to 100 nF.
Also bypass the voltage divider node at the non-inverting input.
Be aware of the potential of TL07X latch-up as a follower.
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Re: Comparator oscillation in meter circuit - pollutes audio

Post by ilya » Sat Sep 02, 2017 2:10 pm

mediatechnology wrote:
Sat Sep 02, 2017 1:59 pm
I would lower it to 100 nF.
Also bypass the voltage divider node at the non-inverting input.
Be aware of the potential of TL07X latch-up as a follower.
Will do. What would be a better ic for this job? NE553x?

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