I'm working on the above mentioned circuit. I'm a hobbyist with little electronics training so don't laugh if something terribly dumb comes out of my keyboard please.
Anyway, a different question on stackexchange had a comment that opamps don't need a negative rail in order to invert a signal. I'd always thought a negative rail was required to handle a negative signal so this was like a bolt of lightning. instead of messing with voltage inverters etc, this just uses plain old single supply. Circuitlab seems to like it.
Anyway, I'm concerned with pH 4-10 (just enough to sneak in standard pH solutions for calibration). I've add enough gain so that 4-10pH @ 40C almost hits the entire span (which is plenty more resolution that I really need).
What circuitlab can't really help with (or at least I don't know to to do it) is handling noise and instability. How do you think this would hold up in the real world? I figure I'll need an RC filter on the output and I'm just reading up on those. Like the description says, if it takes 2 minutes to go from 0 to 1.8V that's actually Speedy Gonzalas territory in this application.
So, is this simply not going to work the way I think or is there modifications I can do to make it work (better)?
Thanks!
simulate this circuit – Schematic created using CircuitLab
P.S. BBB = BeagleBone Black. 3.3v supply, 1.8v ADC and 1.8v analog supply limited to about 10mV which is why the voltage follower/buffer to power the opamps.
Edit: Spotted a tip about matching input resistances between the two inputs. Reduced R7 and R8 to 1k and 4.3k to get 811 ohms on – and 779 ohms on +. Should I add the extra 30ish ohms to the + input or do you think the 30ish ohms makes no difference?
Best Answer
Well now that I've been made aware of the m vs M error I can see this simply won't be practical. While I can get resistors of appropriate values to make the above work for gain and offset, I probably can't be sure what source impedance I'll get from probe to probe, even from the same manufacturer so proper gain is likely to be big problem. Looks like it's back to two op-amps.
Still, I can use this without gain as a buffer then a second amp for the gain and still not need a negative rail.