HI there I need some help ASAP, need to create a circuit using a LM34 sensor, but I need to get a V=100 mV/°C output, an the LM34 works with °F. so far I have no Idea how to do it, so any help is well recived.
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3Why not use an LM35 ? Have you at least looked at what the formula for F to C is ? Can you..model that..with...an opamp ? – efox29 Aug 05 '15 at 02:36
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1This is a simple zero and span problem. – Matt Young Aug 05 '15 at 03:56
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Circuit needs positive gain. Gain is less than 1. Offset is in the negative direction. Optimal topology should be self-evident. – Spehro Pefhany Aug 05 '15 at 16:11
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Assuming that you really wanted to ask how to build an op amp circuit for linear offset and gain correction, otherwise you'd just buy the LM35 (which is the °C output version of the LM34):
Since Fahrenheit and Centigrade are linear temperature scales, this is essentially the same problem as correcting linear offset and gain errors in an analog measurement system. You need to add a constant offset and multiply by a constant voltage gain. For the offset correction, consider that freezing point is +32°F = 0°C, and for the gain consider a difference of 1°C =1.8°F.
One common example circuit for LM34/LM35 is that the analog signal is connected to the analog input of an A/D converter -- if this is what you're doing, you can actually perform the °F to °C conversion in your software. It's the same principle: subtract an offset value and multiply by a gain value. As long as the dynamic range of the signal is enough to handle the minimum and maximum values that you need to measure, it doesn't really matter whether you do the conversion before or after the A/D measurement.
Horowitz and Hill's book The Art of Electronics is widely cited as a good textbook for learning op-amp circuits. From your user profile and the one question you've asked, it's not clear whether you have used op amps before, but they are more complicated to use successfully than the LM34/LM35.
Op amp circuits are a lot more sensitive to proper construction techniques -- they do not work very well when built on solderless breadboard, they almost always require soldering, and for best performance they benefit from a custom printed circuit board layout.
Op amps also require higher total power supply voltage than the output signal -- if you want to drive an output range between 0V and 4.096V, then the op amp will need a negative voltage supply such as -5V to be able to reliably drive 0V output. Op-amps that are advertised as having rail-to-rail outputs can drive their output to very nearly the limits of the supply voltages, but for best performance there still needs to be some "headroom" between the supply voltages and the output signal dynamic range.
Another major issue with op amps is that they are sometimes subject to undesired oscillation, resulting from effects like parasitic capacitance (unintended, naturally occurring capacitive coupling between circuit meshes). This can interfere with the intended operation, whether you are looking for oscillation or not. So in addition to the usual DVM / soldering iron / needle-nose pliers, basic diagnostic lab equipment for op-amp circuits should include a good oscilloscope.
Just to give you a flavor for what the solution could look like, here's a circuit that solves a similar problem: https://electronics.stackexchange.com/a/30744/35022 Note that the resistor values will need to be re-calculated for your application, because you need a different gain. (Note: in this context, "Level shift" is an equivalent term for adding or subtracting an offset voltage.)
