Voltage Subtraction
Voltage Subtraction
(OP)
What is the best way to offset an analogue voltage. I have a voltage (from a measurement) that I want to "zero" so that 2 volts is read as 0V and anything above this is given as Vout=Vin-2V. Below 2V does not matter (prefer it all to be zero but if it just goes negative that is fine.





RE: Voltage Subtraction
A battery can work for short time duty, but it does not keep voltage constant for ever.
If the resulting signal is heavily loaded, you may need to use the op-amp approach. The reason is that the internal resistance of the potentiometer will cause an additional voltage drop when loaded.
National Semiconductor has a very comprehensive collection of op amp circuits in their application note AN-31.
http://www.national.com/an/AN/AN-31.pdf
Gunnar Englund
www.gke.org
RE: Voltage Subtraction
you really haven’t given us enough information to help you. You have a DC offset of 2V on your signal. Fine. But how big is the signal? If it is a 2V offset on a 200V signal the approach is necessarily different to that if you have a 1mV signal with a 2V offset. This has to do with dynamic range, signal range, over-voltage, and so forth.
Is this 2V fixed for all time, temperature dependant, variable from unit to unit, drifting slowly?
A modern approach is to use a low-speed inexpensive serial DAC to do such offset work under processor control. If it is stable then a pot from a reference voltage may be adequate.
I bet you have a tiny signal and a huge offset and you have not considered the drifting aspect.