Blown opamps in chaos circuit

Maybe the reset button kills the amp. It might discharge one of the integrating caps back into the front end.

You could also get some (appropriately random) voltage multiplication effects from playing with SW1.

Why do you have switches S1 and SW1?

S is a switch

U is an IC (not IC3, not IC3POW)

Q is a transistor (not T, not TR) (T is transformer)

L is an inductor (not IND, not CHO)

R is a resistor (not RV, not POT, not RN)

D is a diode (not Z or anything) (CR if you're old enough)

If I was named IC3POW, I might blow up too.

rating

ects

No, it arrives on my desk for testing already blown. I can't make a working circuit fry the opamp. (I tried all sorts of switch flipping, power on /off cycles.. no problem.)

Well S1 resets the 'thing' You can then watch how the signal changes with time from a given starting point. SW1 just changes the time scale for the circuit. It's easier to see on the 'scope with a fast time scale, but slower has less noise and it's easier to find the period doubling points.

One thing that does stink about digital 'scopes is the X-Y mode.

Grin... It was easier just to dump it out of eagle than to try and redraw it.

George H.

.highlandtechnology.com  jlarkin at highlandtechnology dot com

That looks like Eagle libraries, which are quite inconsistent on nomenclature, except for using "IC" instead of "U".

re

be

ould

st

e quoted text -

Yup Eagle, Different people add different parts to the libs and end up calling things whatever they want. I doesn't bother me... but heck I use i for sqrt(-1).

"Damn it Tim, I'm a physicist not an engineer!" (Channeling Deforest Kelly :^)

George H.

It's claimed to be able to withstand +/-30V differential input voltage (1K internal series resistors), but nothing else leaps out.

(I assume you've got a fixture or something to guarantee the +/-15 isn't mixed up.. there are six different ways to connect 3 power supply wires, and not all of them are benign- you're also close to the

If nothing else comes up, maybe consider back-to-back diodes (eg. one BAV99 each) from the inverting inputs to ground.

Seems unlikely.

Looks as if closing reset would try to dump all of the charge built up in integration cap C6 into the inverting input of IC2A in an instant. It would also short IC2A's output to ground. I can imagine the IC being rather unhappy with either or both of these things.

(The switch notation seems to imply that S1 is normally closed, and opens on a push... this seems backwards but even if correct the problem would remain... when the contacts close, the op amp is likely being asked to do what it doesn't wanna do).

P.S. Also suggest you check where the op-amps came from and make sure that they were not returns or otherwise in question.

Grin... I guess I'm the fixture. (potted into my chair perhaps.) I'm the first person to supply power to the boards and I always give them a visual inspection first... power correct, and check that any tant's are in the right way.

?? Do you mean back-to-back zeners? Or does back-to-back mean in parallel? I could do that on the integrators.

Everything seems unlikely. I thought maybe we got a bad lot of IC's and tested a bunch from the tube with the same date code... they were all fine. We use this opamp in ~10 different instruments and I've never had a problem.

Thanks for the input,

George H.

grating

Hmm, Thanks I hadn't thought about the other end of the cap. (the one tied to inverting input.) So hitting the reset causes the inverting input to have a spike (pulse) that's mostly about 1 volt but sometimes up to 2 volts. (This is with the circuit configured for the biggest voltage output... again in the 1-2 volt range) Shorting the opamp output to ground is not that much of an issue.

Sorry about the NC/NO switch screw-up. The switch has to be open for the circuit to run.

George H.

Yes, definitely! I bought a bunch of AD275 opamps from a VERY large but non-franchised distributor quite a few years ago. 50% of them blew up at power on! I went to AD, and they of course wouldn't support parts from that source, but they did confirm that it would be absolutely impossible for parts with different date codes to be on the piece of tape. So, that clearly proved the parts had at least been off the original tape. So, possibly ESD by re-reeling the parts, but my guess is they were recycled off boards. This was right around the time that AD changed their markings from pin 1 dimples to the chamfered edge on SOIC parts. Their triangle logo could easily be mistaken for a pin 1 mark, and would get you to load the parts backwards. So, my guess is these went on boards backwards, the ones with the lowest voltage parasitic diode on a board would blow, and the whole production run of something went out as scrap, and I ended up with them.

(Sorry for the long war story, but that's why I'll NEVER, EVER buy from a grey-market source again!)

Jon

Have you noted which of the op-amps have blown? Are the blown characteristics all the same?

I see the datasheet says the output can be shorted indefinitely so S1 should be fine when closed.

It does say the "inputs can withstand ±30V differential inputs without damage" and also "The protection diodes will, of course, conduct current when the inputs are over-driven". Later I see the inputs are "protected with 1k? series input resistors and diode clamps"

I'm therefore surprised that there is indeed an issue as the device should be fairly rugged.

Grin.. I checked a bunch, they were all fine. Most likely came from newark. I got some bad 79L12 volt reg from Jameco... a light gray? I don't go there any more.

George H.

Youi're leaving the caps open, allowing them to charge to what ever.

It's been a while since I've looked at the specs on that op but something tells me you could be over doing it on the input, depending on what the other input is doing...

Personally, I would put high value bleeder R's crossing the switch, at least this way the caps are more than likely to be close to the existing value when switched in.

You should be able to select proper values that won't conflict with the current values selected. 1 MEG. 5 MEgs etc..

Jamie

or

So I?m

ut

cted

Yeah! I've been swapping 'em in and out on my bench (no wrist strap most of the time.) and hooking up the power backwards every once in a while. And never any problems.

So all of the time it's IC2 (U2) the Y opamp that has been fried, but once the Z opamp was also fried. The Y opamp does have the biggest feed back R.

I'm done testing the latest batch, there'll be more coming in a few weeks or so, waiting on a part. I'm guessing it won't happen again... Some one time ESD event that took out a handful? (doesn't explain why it's IC2?)

George H.

It is generally good practice to use a resistor to connect an op-amp input to ground. The value should be similar to the impedance of the other input circuit, so for IC1A through IC3A about 5k would be good. This is mostly to balance the offset voltage due to input current, but it may also avoid having high current spikes through the input protection diodes to the supply rails. Another source could be the direct connection from the AD633s to IC2B and IC3B. Also, as others have pointed out, the reset switch may cause a

current surge from the capacitor discharge. A 10-100 ohm resistor might be wise.

I use the OPA2277UA (SOIC8) in my Ortmaster product and I have not had any failures.

The spec sheet indicates that the inputs have internal 1k series resistors for the diode clamps, and also bias cancellation circuitry which makes the bias cancellation resistor unnecessary. But I still think it's a good idea.

Paul

he

So the opamps arrive blown. (or blow on the first power up... but never on any power up after that once they have been replaced.)

Not much room for more parts, and adding those R's would 'screw-up' the math a bit.

I've done several switched cap. circuits, I've never had a problem.

Say, how big a voltage can a 1uF film cap get with one side open? (would it be different for a ceramic cap?)

George H.

IC2 is the op-amp that has its output connected to the reset switch to GND. It may be able to withstand a short to GND, but it seems like poor practice to short an output for reset.

Also, a high precision low power Op-amp is likely optimized for other parameters and not so much for ruggedness.

Paul

I think that speaks volumes.

I was about to suggest monitoring your power supply rails around switch on, to see if there are any over-voltage transients. But I think you've hit the nail on the head.

D.

ce

Yeah I know... I had a different opamp in there to start with and then was confronted with this big DC offset. (note the trimmer pots on the multipliers*) The circuit has a gain of ~200 or so, and a millivolt of DC offset was a problem. This opamp has more noise than Phil H would like, (8nV) certainly part of that is the 1 k ohm R's in the input protection circuit.

George H.

• They were added after the first spin, I never get my pcb's right on the first try. (I haven't used a trimmer pot in more than ten years.)