Troubleshooting 'singing' opamp!

I note that they consider 60pF to be a large capacitive load...

Is there another op-amp you can use?

Boss == SWMBO ?:-)

Send me schematic (in private if you wish) and I'll take a look. ...Jim Thompson

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| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Aha. This is a known problem. Don't use the quad opamp as the four stage filter, especially if it is the shaky R-R one. Those things are extremely sensitive to capacitive loads.

Sings in the audio band or somewhere in ~MHz area?

Don't know what kind of performance are you looking for (do you really need MCP6024 for speech? May be LM324 would do), however you can probably try OPA4350 as the direct replacement.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

Hi All, Since I can't do the ascii art thing, I will have to see about posting to my website...

Ok, easier said than done...

While I work on that, the singing is in the audio band. The output load is a series .47uF cap. My present investigations are two caps from their respective IN+ pins to ground. They ended up about 1" and

1.5" from the chip...

Thanks for the help!

Charlie

Ok, now have a hidden downloads page on my website with a couple of PDFs.

Thanks for any insight you can give! The filter section is in the middle of the bottom of the board, to the right of the SOIC14 opamp.

Charlie

Icky! Sallen-key :-( ...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |

                           LOSE the WUSS
                          BRING BACK BUSH

The first thing I'd do is buzz out the board - i.e., check for shorts and opens; does the PCB layout actually match the schematic?

Yes, you can do this safely if your "buzzer" (or ohmmeter) limits the voltage to less than about .5V (so the diodes don't conduct), and the current to microamps (so it doesn't blow anything up).

Do you have a 'scope?

Good Luck! Rich

Yeah, but it worked!

Actually, seem to have fixed it. Moved one of those caps (the farthest one) and tacked it right on to the pin. It is a temporary fix, but these first 10 boards are just for final software and marketing demos. Will have to fix this on the production versions!

Thanks all for your help!

Charlie

That part looks fine. I don't think trace length is a problem. Try a different opamp, something with a lower gain-bandwidth product (*not* an LM324, which can't amplify a 60 Hz sine wave worth spit.)

Microchip is a digital house!

What's the "PWM" thing like? That could have interesting interactions with things.

Are the opamp supply rails clean?

Incidentally, as you twist the pot, the effective value of R3 varies by 5K, if that matters.

John

(*not*

The hell you say.

Jim

Your schematic doesn't indicate any power supply bypass or decoupling -- did you forget the most important caps of all? _Always_ bypass your power supplies -- digital circuits will act weird without them, and analog circuits will oscillate. You should have 10 - 100nF caps as close to the op-amp power supply pins as you can get them, and if the power to the amp isn't clean you should seriously consider decoupling the +V line with a 100-or-so-ohm resistor (after verifying voltage drop, and the knee of the RC network you're making).

The trace lengths that you mention elsewhere on C5 and C10 shouldn't make that big of a difference.

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Any chance the capcitance under the chip from the 'pours' is messing it up? I used to put holes in the ground planes under DIP packages to reduce capacitance. I imagine it would be considerably worse under SOIC.

G=B2

1) Teach it Aida. 2) Cut some traces in one board to isolate the various stages. Haywire (tack) input and out terminators to each stage; at minimum one meg each place (leaded parts become rather useful here). Test each stage separately. Start at output stage and connect previous stage and re-test; stop sequential re-connections like this near middle. Then start at input, connecting second stage and re-test; then connect next stage / retest etc and stop in middle. At this point, if your luck is same as mine, all will be perfect UNTIL you re-connect that magic middle I/O.... 3) Teach it the Ring..

Hi Tim, Yes, there is a bypass of 1.0uF just above the chip. I think it is on the schematic by the A section of the quad opamp. Do you think 1.0uF is too much? I also have ground pour on the bottom and VCC pour on the top...

Charlie

just an update on this MPP (Major Posterior Pain...)

I found that part of my problem was that, for some unknowable reason, when these were being prototyped, the clock was being set incorrectly. the PLL was being turned off, giving me a clock divided by 4. this put the PWM frequency right into my passband, and may have been a part of the problem. While trying to troubleshoot this, I started getting debugging and programming problems i.e. cannot validate the chip. I am now just about ready to drop the PIC and start over with a PSOC...

Charlie