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November 2015

Controller Quandary

I’m stuck with the limitations of my controller which has eight analog inputs each which can sense 0.5V changes between 0-10V. I need to measure temperature between 50-160°F within one or two degrees.

My idea to get the accuracy needed is to divide the thermistor output across three inputs where input one would resolve the 100’s, input two would resolve the 10’s, and three would be the 1’s. Example: temp 143 divided into three would produce a 1V signal on input one, a 4V signal on input two, and a 3V signal on input three. Then, in software in the controller, recombine the values back into a single temperature.

Does this concept seem doable and if yes, what would be the easiest way to create such a circuit? Thanks in advance for any assistance you can provide.

Harvey Mushman
Los Angeles, CA


I think an couple of quad op amps would enable you to scale the temperature range you want to 0 to 5 volts. That way, the resolution would be around 0.5degrees on each a/d input.

John McCullough
La Habra, CA

I suggest you consider the Maxim MAX31820 temperature sensor. Its accuracy is +- 3,6 Degrees F over the range of -67 F to 257 F. It has a digital output (rather than analog) with a "1-wire" interface using only 1 controller port pin. A large number of these sensors can be paralleled on the one pin so you could even put several together and average the readings. For a datasheet, see: [url=https://datasheets.maximintegrated.com/en/ds/MAX31820.pdf]https://datasheets.maximintegrated.com/en/ds/MAX31820.pdf[/url]

Mark Peterson
Plymouth, MN

Have you considered a digital sensor, such as the DS18B20? They are inexpensive (about $4), readily available, is accurate to +/- 0.5 C (so, about 1 degree F), wide temperature range, and then you only need 1 digital pin. I used one in a little oven controller that used a PIC, a while back. Programming is a little more work, and it can be a little tricky to use with longer cables.

Jay Jaeger
Madison, WI

Your proposal will not work. It reminds me of students who try to connect two 8-bit DACs to make a 16-bit DAC. It just doesn't work. But I can't offer a solution without more information. What's the resolution of the analog-to-digital converter(s)? Do you mean you can set any of the analog inputs for a 0-to-0.5V range anywhere within the 10-volt span, say, from 1.2 to 1.7 volts? What is the voltage output from the thermistor circuit? More information, please.

Jon Titus
Herriman, UT

You don't say what kind of controller you are using but if it has SPI capability, why not use a DS18B20 serial sensor. It is accurate to .5°C in the temperature range you specified and software routines for most MCUs are readily available.

Gene Sellier
Fairhope, AL