Requesting some input on this topic which has already been discussed (partially for my requirement) here:
Negative to Positive Voltage Converter
I also have a positive-only display and I need to measure up to -32v DC. I need to convert the minus voltage to the positive equivalent in a simple manner and I am currently thinking of doing this in TWO op-amp stages as illustrated.
Questions:
1. Is there a more elegant & simple way to do this? Using only one op amp perhaps? The limit on +/- supply is a limiting factor hence my 2-stage design.
2. Does anyone see a problem doing it this way? This is a basic block diagram only.
Thanks for your input.
Steve
Best Answer
If your input ranges from say 0 volts down to -32 volts then you can use a simple inverting op-amp configuration: -
Consider the op-amp chosen will work well with -in and +in close to 0 volts. This is not unusual with rail-to-rail op-amps. Next, as a simple mind-experiment imagine that R1 is 32 x R2. So if Vin is -32 volts then Vout has to be 1 volt to drive -in to 0 volts. I use the word "drive" because this is what happens in an op-amp with negative feedback - the output drives the -in input to match the voltage at +in.
No negative voltages are required to be produced by this op-amp configuration. So, if Vcc is (say) 20 volts, you could make R1 twice as big as R2 and this would convert -32 volts at Vin to +16 volts at Vout. -16 volts at Vin would produce +8 volts at Vout.
There are a few op-amps that work very close to 0 volts but, as always, to widen your scope of choice, it might be better to make a small negative rail (-3 volts is all that is needed) to power the op-amp's negative supply pin: -