Voltage Subtraction

[Sambrose]

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.

 

[Skogsgurra]

The classic way is to use an op-amp with an offset. But if you want to do it very simple, then just buy yourself a DC output wallwart and add a resistor plus a zener. Use a 6.8 V zener for best stability and then use a potentiometer to sdjust voltage to exactly 2.000 V. Putting this voltage in series with your signal will either add or subtract 2.000 V from it. It depends on how you connect it.

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

http://www.gke.org

 

[logbook]

Sambrose,
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.