level shifting and amplifying signal from temp sensor with LM324
I want to measure the output of an LM34 temp sensor and use and ADC with 0->5v. I'm only interested in the output of the LM34 from 0.8 to 2.2 volts ~(100F->220F). So I can use an LM324 as a non-inverting amp with a gain of about 2.2 and I convert 0->2.2v to 0->5v. But I want to add an offset of 0.8v and when I try to add a level-shifter to the non-inverting input I get results that don't make any sense.
Would I be better off using another opamp to do the level-shifting or do I need to go from 5v from R3 instead of gnd? I'm using +15v as the positive rail on the LM324 and gnd as the - rail.
Re: level shifting and amplifying signal from temp sensor with LM324
You actually want a negative output offset. It can be achieved by adding a resistor from inverting OP input to a positive reference voltage, e.g. the 5V ADC reference. Noninverting OP input should be directly connected to sensor output.
Your input and output specs are feasible for a single transistor (if you don't mind the output dropping when your input rises).
Subtract the output voltage from 4.35V...
Or, add a second transistor stage if you need the output to be right-side up.
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The potentiometers are to indicate that a lot of experimentation is needed, to get the desired range of output. Repeated adjustment will probably be required anyway if you decide to go with an op amp.