by Tom Maier » Fri Jun 20, 2014 2:48 pm
My cad software uses V- as the designation for the negative supply when I use a generic opamp. That's not uncommon. It could also be labeled Vss, Vee or on parts designed for unipolar supply, "GND". It's the negative rail and for a sungle supply, the ground is the negative rail.
One "gotcha" that hits some people is that they might select an opamp with rail-to-rail output, but the input is not rail-to-rail. Then if they use it in a single supply application like the one I show above, they do not get linearity of the signal because when the sensor signal is near ground the opamp output will have a positive offset, since the ground is the nefative rail and the opamp can't "see" the input signal at that voltage.
A lot of people like to use the old LM324, but if they do then they have trouble at the positive rail because that old chip can handle inputs down to ground, but can't drive to the positive rail. In that case they have to use a V+ that is at least 1.4 volts higher than the Vdd of the ADC. And then it's good to use a clipper to avoid overdriving the ADC.
There is a new version of the LM324 that has rail-to-rail inputs and outputs, but I forgot the part number.
And some people say "hey, you don't need that 4.7 K resistor at the ADC input"", but I like to have something there to protect against accidental misprogramming or run-away code. It that ADC pin flips to an output, then you would have the opamp and ADC pin trying to drive into eachother and possible blow a pin or cause cyclical resets. I don't like having circuits that could self destruct or latch up due to an unlikely accident. Bad design philosophy. That resistor could be lower, like 220 Ohms or something if you need the increased bandwidth, but I made it 4.7 K so you use the same resistor as the input protection resistor and don't have to chase more parts.