The photos were very helpful. It looks like I was wrong about the 24-pin SOIC (part #1821-1690) being the DAC. The DAC is the AD7846.
The AD7846 is a single output DAC, but we know we need a bunch more outputs than that.
To get multiple outputs, I believe the 1821-1690 is an analog MUX with 12 or maybe 16 outputs. On each output of the MUX you can see a 100k resistor going to a holding capacitor. The should sequence through all the MUX outputs while setting the DAC to the value it wants for each output. It's a multi-channel sample & hold.
You're welcome! And thanks for your quickly response!
So yes you're right again! But i've double checked the DAC Output, and as you said as it's shared, if it was broken then all channels are suposed to get wrong values, isn't it?
Most, if not all, of the outputs go to the dual LT1112 opamps. Looking the symmetry of the connections and components around the opamps, it appears there are 3 distinct groupings of opamp pairs. Since it's a dual opamp, each pair of packages would be involved with all four channels for offset or trigger level (or possibly something else). I have outlined them in the attached photo below. All this is my first guess, subject to revision as we poke around.
I would examine the output of each opamp group (2 packages = 4 outputs) to see what function they perform and whether you can find one output in the group that's not consistent with the others.
You mention you already found one that's not consistent. Can you be more specific? What are the voltage levels on chips/pins? Also, before measuring stuff, if there's a way to do it, you should set all the calibration constants to their default value.
Group A.Left controls CH1 and CH2 offsets, and Group A.Right controls CH3 and CH4 offsets. I think all LT's are in a good working condition, all inputs have same values except for CH1, so giving a wrong output value.
The inputs 2 and 3 of "groupA.Left" are receiving 0.966, while all other inputs from CH2, 3 and 4 are on 1.045 aprox. So CH1 trace is not on 0v offset level as result.
I don't either think the Mux is the problem.. So I don't know at this point how to continue with that! That's weird.
There are 3 LT1112 that don't seem to have symmetric circuitry around them. They may be used for one-off functions such as the calibrator output, external trigger level, etc.
Probably, I will check them too, but I'm sure they're not affecting to offset problem.
Perhaps I'm not understanding the problem completely, but are you saying the trigger level is offset too much, or the trace is offset too much? Are you able to control them?
Trigger levels are correct I think, at least with all the tests I did. The problem is the trace as you said! But in any case I'm able to move it, there's no control problem, and if I put the trace on 0v offset point then I get the correct values! But obviously offset is -250mV.
I also reinstalled the firmware twice, because maybe was a software failure, but still the same! The only thing I can do maybe is to swap the board with the other working one.. to ensure that the problem is located on that board.
Thank you so much Mark!
Best,
Oriol.