Interfacing a NTC thermistor to a PIC

You could use a Platinum Resistance Thermometer (PRT) instead. They are very linear and come in 100Ω or less commonly 1KΩ resistances. These quoted values are for when the element is at 0 deg C. They are a bit more expensive than your proposed NTC's or PTC's but, would not require any complex and time consuming linearization in either hardware or software saving a lot of time.

 

If you are unable to use a more linear sensing element, you might be able to translate your readings with a non-linear formula (if one is available for your device) or a look up table.

One additional thought. You may wish to select a thermistor that can give you a wide resistance change over your target temperature so you get greater resolution. For example, if you select a part that has a 100 ohm change in series with a 10k resistor to form a voltage divider, you may not see as much of a voltage change as you would like for the microprocessor to measure.

 

What temperature range do you want to monitor?

 

No. An analogue circuit to do that would be complex. Much easier to do linearisation in software.

 

Using a PRT, it would be very straight forward. As they are linear over the temp range you quote, it would just need a bit of straightforward scaling with an opamp to provide an output compatible with you chosen pic.

 

I agree; for the temperature range you're wanting to measure over, using a platinum RTD would be a much better choice, as it wouldn't require a lot of math to linearize it. The circuit doesn't have to be very complex; here's a very simple design I used on one project to interface a 1000Ω RTD to a PIC's ADC:


VREF was +5V, which also powered the op amp. For your use, resistors R14 and R15 will need to be changed to get the offset & scaling you need.

 

Yes, they're a bit more expensive-- but a lot less trouble to work with because they're much more linear.

A diode (or a transistor with the base shorted to the collector) can be used, but the problem is the extremely large variation in forward voltage from unit to unit. Unlike RTDs and accurate thermistors, each unit has to be calibrated at a known temperature.

The feedback resistor R15 is used to adjust the gain, and the R13/R14 ratio adjusts the offset. However, the two adjustments interact somewhat.

 

I like your follow up to my suggestion for using a PRT. It would be possible with a small mod to remove the influence of the offsetting on the gain.

 

True. It's also possible, by adding a large-ish resistor between the op amp output and its non-inverting input, to correct for the slight nonlinearity that results from feeding the RTD from a resistor instead of a constant-current source. With a small tweak, that same resistor can also cancel out the negative second-order term in the RTD's resistance vs temperature characteristic.

But for the application of the circuit I posted above, perfect linearity was not important, and calibration was done in software so neither gain nor offset needed to be done by trimming resistor values. The most important design requirement was low component count, hence the circuit's lack of "nice" features.

 

Absolutely right, a constant current source would indeed improve linearity, it just depends on how accurate Rajinder needs it to be. Hopefully he will tell us.

 

No, it cannot. You can measure temperature deviations from some reference temperature, but you cannot directly measure temperature itself because at any particular forward current and temperature, the forward voltage drop will differ from one diode to the next.

Moreover, that 2 mV/C is only approximate; it can vary from diode to diode and depends somewhat on forward current.

So "as good as a diode" doesn't tell us much.

The foregoing posts pretty much lay out the options available to you. In my opinion, your best course of action would be to use a simple thermistor/resistor voltage divider, feed the resulting voltage divider output into your PIC's ADC, and do the linearization in software.