Electronics > Projects, Designs, and Technical Stuff
Building an Electronic Load inspired by Louis Scully
MasterT:
--- Quote from: kalj on August 27, 2020, 08:38:43 pm ---I did manage to find a similar low TCR resistor with 100mOhm, which was quite cheap too: https://www.mouser.com/ProductDetail/605-RMCJ3U000R1FS
I'll play around with gate resistors and the feedback circuit once I have something built up.f
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I get myself from here, https://www.arrow.com/en/products/msr5-0r1f1/riedon
Build recently load using dual mcp4802, jumping two outputs with 1:256 divider got 16-bits resolution
pqass:
--- Quote from: kalj on August 27, 2020, 10:15:07 pm ---
Okay, that actually made a bit of sense. So just to confirm that I understood properly; the precision, stability, and the exact value of the negative rail is not crucial, just that it is well below any value we want to pass through, i.e. -2mV in your case.
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Yes
--- Quote ---On the other hand, it is crucial that we can produce accurate and stable values at the input to the op amp, including all the way down to say -10mV.
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Yes.
ie. Don't rely on the rails since if they fluctuate (due to dynamic power demands) so does anything that uses them as a reference.
--- Quote ---With an accurate DAC, and a proper calibration procedure, the exact value of the negative offset should also not be of importance, only that it is rock stable and reproducible, since the values put out through the DAC are going to be adjusted based on calibration anyways.
Correct?
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Yes.
It starts with the +Vref connected to the DAC (Vref in) whose output flows to down-stream op amps. So the accumulated errors go: +Vref temp co+noise, the DAC output adds further errors, and down-stream op amps have offset errors. These totalled should be constant with temperature and aging and should be very small compared to a DAC step.
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Regarding applying an offset to a non-inverting buffer, I cooked up this:
Does that not make sense? It seems simpler than the inverting one.
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Well, I'd describe that as a differential op amp configuration. Maybe it's me but I'm just not seeing how it fits into the larger circuit. I mean, if the sawtooth wave is your control voltage, then it looks to be about 4V in range (guessing) which will need its own +Vref in addition to the +1V feeding into it to be subtracted. But we only want to lower the control voltage by ~2mV so instead of a +1V it needs to be at ~+2mV. So you'll at least need a resistor divider (2000:1 off the main +Vref) and a buffer to achieve that.
I agree my inverting stage is a bit odd but I did get it to work (well, modeled on falstad.com and attached below); was able to dial-in any desired negative output via op-amp feedback resistor/pot). Which is what I thought you wanted; a non-software-calibrated op amp offset compensation.
However, while looking at the datasheet for your chosen 16-bit DAC (https://datasheets.maximintegrated.com/en/ds/MAX5215-MAX5217.pdf page 24) I stumbled over a method for the DAC to emit bipolar output. Since the DAC won't go below ground by itself, a non-inverting gain=2 stage with offset set to +Vref makes the output swing between +Vref and -Vref.
Now, instead of a gain of 2, what if it's set to just above 1? That is, R1=47K and R2=1K where gain=Vin*(1+R2/R1)=Vin*(1+1k/47k)=1.02 Given a +Vref of 1V, now the bipolar output will go from +1V to -20mV! That's enough negative output to compensate for DAC errors, and down-stream op amp offset errors.
However, this method now relies on a software-based compensation but simplifies the overall circuit; no pots, accomodates component tolerances, requires only one quality +Vref, negative rail can still be -1V and not a mirror of the positive rail. A DAC constant is saved in EEPROM where, if any lower, will drive the final CC op amp stage output negative (if a 12bit DAC then 20mV/(1/2^12) = ~82). To keep more usable DAC steps, you could raise R1 to 100K but probably no higher.
See attached falstad.com screenshot and model file (should you want to play around).
With regards to a current monitoring op amp(s), I chose a non-inverting summing amp configuration since using two inverting amps will require a negative rail to be at least as negative (+extra) as the what it's measuring. Also, the software will need to save in EEPROM a constant to subtract the monitoring op amp offset at the 0A.
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