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Need formula which increase the ADC value?

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Surender Reddy

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Hi,
I am using RTD sensor which gives less variation when I apply more input. I am getting very less range of digital data.
RTD o/p is connected to ADC pins of PIC18f452 uc.
Please suggest a formula to increase the value from less changes of RTD output.
 

once you have a digital value from the ADC you can multiply it but that will not increase the accuracy
if you want a greater range of digital data while retaining the 10bit ADC accuracy you need an amplifier between the sensor and ADC input. An alternative may be to have a Vref which is more suitable for the input voltage range
 

horace1 said:
once you have a digital value from the ADC you can multiply it but that will not increase the accuracy
if you want a greater range of digital data while retaining the 10bit ADC accuracy you need an amplifier between the sensor and ADC input. An alternative may be to have a Vref which is more suitable for the input voltage range
Click to expand...

Thank for your reply, I got the same idea
My RTD having 3 wires,when i gone though the datasheet i found that common 2 wires,
how to connect it with LM358 (I have this one)......
 

To use an RTD resistor, you need to do one of two thing. Either use a wheatstone bridge amp, or a stable current generator to create a fixed and stable current. This application note discuss the last variant: https://ww1.microchip.com/downloads/en/AppNotes/00687c.pdf.

The reason you have 3 wires, is to make it possible to compensate for the voltage drop in the wires to the RTD, since you always will run a current through the RTD element, and the wires.
 
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The microchip application note should give you an idea how a good RTD signal conditioning circuit can look like. Depending on the temperature range, you may want to shift the input voltage range by an additional offset to utilize the 10-bit ADC range better.

My personal favourite goes the opposite way: If you don't rely on a 3-wire circuit (no long cables), use a single reference resistor in a voltage divider with the RTD and a slow, high resolution (e.g. 16 bit) ADC and perform the linearization for the divider circuit in software.
 

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