Op amp level shift

operational-amplifier

I have an input sensor with a signal level from 2.3 to 3.5 volts that I want to sample with an ADC. I want to be able to shift this range for the ADC from 0 to 3.5 volts to take full advantage of the ADC dynamic range. I used a summing op amp with an LM741 but it seems to be stuck at 2.3 VDC output all the time.

So the sensor voltage range is Rg = 3.5-2.3 = 1.2

From this Gain = 3.5/1.2 = 2.7

Best Answer

Here is a basic circuit that will come close to your requirements.

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I showed using a nice opamp from Linear Technology that can work at 5V and still allow the signal to go down near to GND. To get this to work with older, not so nice opamps like a 741, you would need to use supplies of +/-5V or +/-6V.

This shows the response of the above circuit. The output range does not go quite to the limits that you specified for your A/D converter input. It is a good idea to design with some margin to deal with variation in components and the input signal range.

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Note that the above circuit inverts the range of the sensor. The inverted opamp configuration was convenient in this case but the circuit could just as well have been designed in a non-inverting manner. On the other hand it takes a trivia amount of MCU software to offset the A/D readings to remove the inversion if that proves necessary.