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level shifting and amplifying the signal from temp sensor with LM324

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Build-A-Burger

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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.
non-inverting-opamp-with-offset.jpg
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

There's no offset in your circuit. It's simply a non inverting amp with a voltage divider on the input.
 

Re: level shifting and amplifying signal from temp sensor with LM324

Hi,

The key word is "differential amplifier circuit".
Search the forum. I've (and others) posted circuits and calculation examples.

Or try a search with "signal conditioning"

Klaus

Btw:
But I want to add an offset of 0.8v
You don't want to "add", but "subtract" 0.8V (input referenced).
 
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.
 
Re: level shifting and amplifying signal from temp sensor with LM324

output of the LM34 from 0.8 to 2.2 volts
...
convert 0->2.2v to 0->5v.

Your input and output specs are feasible for a single transistor (if you don't mind the output dropping when your input rises).

NPN 3 pot adj signal 800-2200mV  gain 2x output 400-4300 mV.png

Subtract the output voltage from 4.35V...
Or, add a second transistor stage if you need the output to be right-side up.

- - - Updated - - -

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.
 
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