What’s U102 being powered from?
Can you disconnect the divider from U102 to check that it has the correct resistors and that your DAC can drive it?
Also, what are 102A and 102B doing, also be sure that they are tied off and not oscillating.
(Shot in the dark but usual lizard brain review stuff)
You probably did, but I have to ask anyway because I have missed this step. Did you physically measure the resistors in the circuit?
Here is an interesting plot from that datasheet
which is a clue as to why I’d avoid using opamps as comparators.
Usually because they recommend that you don’t provide more than a certain differential voltage lest the part draws excessive current / fries.
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Thanks everyone!
Extra info:
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I measured the resistors. They’re OK.
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U102 is powered from the same 3.3V supply as the DAC. It’s a supply provided by the Teensy board, generated from the 5V USB input. It’s supposed to be good for 250 mA, which seems well within range here.
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The other units in U102 are all in use, all in “inoffensive” opamp roles, i.e. the only unit being used without (obvious) negative feedback is the one that’s giving the problem here.
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The DAC output impedance is 1 ohm. I need to take a closer look at the datasheet to see whether there might be something up with the resistor network there.
Oh bugger. I think you might win the prize there. It just seemed so convenient: quad opamp, one left over, need a comparator, off you go. But the equivalent circuit shown in the datasheet for what happens when you overdrive the differential voltage might explain what I’m seeing:
I was really wondering how the opamp input could be showing an input impedance in the 10s of kOhm. Looks like that might be the answer!
That figure also explains why things start getting weird when the DAC voltage gets up to around 1V. The bias currents are negligible until the input differential voltage is greater than about 0.8V.
Looks suspicious, no worries it happens to all of us 
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If you have the space to patch in resistors in both traces running to the opamp inputs, you could try to insert large value resistors. I’m thinking an extra 1MOhm-10MOhm on each leg would reduce the current through the amplifier inputs by a factor 100 - 1000, and should not affect the normal operating range.
That’s an interesting idea. I just had a look at the layout and I don’t think I could do it very easily though. One of the inputs to that opamp comes from a via under the package, so I’d have to dead-bug the whole thing to get at it. I was planning on a respin of this project at some point though, so it’s not a tragedy. I’ll use a real comparator next time!
Lessons learned:
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Read the datasheets.
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Understand the datasheets.
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Opamp != comparator.
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Check the assumptions your ideal models of components are based on. Real components aren’t like the “perfect” components in the textbooks!
Thanks everyone! That was incredibly useful. I’d buy you all a beer if we were in the same town!
Also:
- you did you math and checked it twice
- you built a circuit without analysis paralysis (and it mostly worked)
- you tested your circuit and Identified the problem
- you summarized you problem well and asked for help
- you had an open mind👍
- etc, etc, etc
Also focus on all the things that you did right!
If it was easy, everyone would do it!
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Chopper-stabilized opamps like this one are great for offset but require particular attention to details and quirks as we’ve seen here. Using a standard opamp as a comparator will rarely result in a problem like this.
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OK, one more thing to add to my “things to learn about” list! I don’t think I’d even internalised the fact that the TLVx333 opamps are chopper-stabilised. (Before about 10 minutes ago, I didn’t even know what that term meant, so that’s sort of an excuse. I can’t remember why I chose that part for this project.)
Ah, good that you found it.
My initial thought when I looked was that I couldn’t tell what your supply range for the opamp was, and that maybe you were leaking through ESD protection diodes into the rail…
I used to think hidden connections to rails on the schematic was a reasonable thing, but especially with many mixed-voltages these days, being explicit about the rails is helpful.
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Chopper amps has incredible performance on some parameters, horrible on others
Since you cannot change the layout, find another pin for pin compatible part, and swap that in
It does not seem that your performance of the load depends using this opamp, an opamp with higher Vos could probably do the job, if nessesary combined with some tricks in SW
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TSV994A perhaps?
LMV654 if you don’t need full CM range to the positive rail
Not TSV994A - it’s not unity gain stable. That is, at least U102D will most likely oscillate.
Actually not LMV654 either: it has an absolute maximum rating for the differential input of +/- 0.3V.
I suspect that EMI protection diodes start to conduct at that voltage. Yes, if you knew the details around those diodes, you could perhaps add protection resistors in front, but it’s already decided that this layout prevents that.
I’d be more interested in examining e.g. TLV4379. Unity gain, rail-to-rail (25mV typical), 0.8mV offset (typical) - with a caveat that the offset increases 10-fold when the input common mode increases above Vcc-1V.
The datasheet even lists a “typical application” wherein it’s used as a comparator.
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Maybe I should have pointed out I was just suggesting parts, but up to the OP to examine the details himself
The real point is that the layout was fixed. Well, then change the part instead 
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CMOS opamp usually don’t have this kind of input differential voltage protection diodes, which usually occurs at the bipolar input opamps. These diodes are here due to the input chopper for this particular opamp I suppose.