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I want to interface a 103AT-2 thermistor directly with the PIC24 10-bit ADC. I am interested in the temperature range between -10°C (where thermistor resistance is 42.47kΩ) to +70°C (where thermistor resistance is 2.228kΩ) and accuracy needed is 0.1°C.

I have explored something in the online and even in this forum under "Selecting Bias resistor for thermistor". For Bias resistor Rb selection, if I take geometric mean of the minimum (2.228kΩ) and maximum resistance (42.47kΩ) values, Rb is 9.70kΩ. I rounded this to 10kΩ.

Below is my schematic which is without a constant current source. The circuit includes a 10kΩ pot in series with Rb for calibration.

schematic

My question is: How can I build a constant current source, so the current flowing through the voltage divider would be constant, irrespective of the thermistor resistance?

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  • \$\begingroup\$ We are not here to design a circuit for you. Have you searched for constant current sources, online or in a book? Also, in order to design such a circuit, you will want to define your requirements (max error, output voltage range, max power dissipation in the thermistor, etc). \$\endgroup\$ – uint128_t Feb 3 '18 at 5:20
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Before you run off and create a current source, do the math to see if you need one, and whether it will actually solve the problem if you do use one.

You want to measure 2.2 kΩ to 42.5 kΩ. The geometric mid point is therefore 9.7 kΩ as you say. So the first question should be whether just a 9.7 kΩ pullup is good enough.

Each limit will be 18.5% from its corresponding end. The unused ends use 37% of the range, leaving 63% available for measuring. That results in 645 usable values from your 10 bit A/D. You have a 80 °C span and need better than 0.1 °C resolution. That means you need 800 accurate values. In practice, the resolution probably needs to be twice that at least. This means the simple pullup method won't work.

If you amplify the result from the simple divider to nearly the full A/D range, then at least there are more available counts. The next question is then if the resolution at the ends of the range is good enough. It takes more of a relative change in resistance at the ends of the range to cause the same voltage difference as in the middle of the range. You can do the math, but it seems clear to me that covering the whole range out of a resistor divider into a 10 bit A/D isn't good enough.

Consider also that you want 1/800 of full range accuracy. It is unlikely any 10 bit A/D, particularly one built into a microcontroller, is good enough for that. With 10 bits, you're only starting with 1024 values, so with the usual individual and non-linearity errors, you aren't going to get 1/800 full scale accuracy over the whole range.

Consider also the accuracy of the thermistor. Look at the datasheet, and you'll probably see it's nowhere near 0.1 °C from -10 to +70 °C. You will have to do careful calibration of each individual unit. Note that this requires measuring the actual temperature to better than 0.1 °C. In short, your goals seem unrealistic.

One way you can at least get the resolution so that correction is possible if you do somehow manage calibration, is to use a external delta-sigma A/D. These are slow, but very high resolution. 20 bits, for example, is possible. Since temperatures don't change that fast, the slowness should be acceptable. With good calibration at a few known points, at least it is theoretically possible to do what you want. You will either end up running something like the Steinhart-Hart equation on the fly, or use a lookup table of a few thousand points and interpolate in-between.

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  • \$\begingroup\$ So true. Just one point: Many PICs allow to define upper and lower bound of the ADC by an external voltage. This maps the given temperature range to the entire ADC range, though, as you said, precision is not the best. \$\endgroup\$ – sweber Feb 3 '18 at 15:05
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One way is to use a single component current source instead of building a complete current source from scratch.

The REF200 is a precision current source that can be "programmed" to 50, 100, 200, 300 and 400 microampere by pin strapping but also other currents given some resistors. It has an accuracy of 0.5 % and a temperature drift in the order of 25 ppm/K.

http://www.ti.com/lit/ds/symlink/ref200.pdf

Another single component is a constant current diode, there is a great variety available with different precision and currents. https://eu.mouser.com/Semiconductors/Discrete-Semiconductors/Diodes-Rectifiers/Current-Regulator-Diodes/_/N-ax1ml

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A better way if the wires to the thermistor are short-ish is to use a PIC with a built in "555" type of RC oscillator and switch between the thermistor and a precision reference resistor. This method (sans PIC) is used in virtually all consumer contact or probe thermometers including the body temperature type.

By using a counter to measure the resulting frequency you can get a large dynamic range, which is necessitated by the thermistor characteristics. If you do the maths, to a first order, everything (capacitor value, divider values, reference/supply voltage) cancels out to first order except the value of the reference resistors (and a 0.1% resistor is cheap). A constant current source that drifts relative to the ADC reference will add errors and you have that pesky dynamic range to deal with (meaning lots of ADC bits for little resolution at the extremes).

Of course not every application is suited for having AC across the thermistor and limitations on wire capacitance.

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