How to interface Thermistor(103AT) with PIC24-ADC - Page 1
Forum section: electronics / general

author: muthukural001
author_rank: Regular Contributor
timestamp: February 02, 2018, 08:53:36 am
content: Hello All,Previously, I have worked with external ADc to measure RTD in Ratio metric measuremnt arrangement.But, I am wondering that how to interface My 103AT to PIC24-ADC. As accuracy is matter, I am doubting whether I can directly interface or designing filters at ADC pins.There are lot of confusing factors on this chapter in the online and also no guide to design ADC frontend in the PIC24 datasheet. That's why I am here. It would be helpful if anybody suggest their views and circuit.103AT has maximum resistance of 329.5K at -50C and minimum resistance of 757 ohms at +110C.Regards,Muthu

author: picandmix
author_rank: Frequent Contributor
timestamp: February 02, 2018, 01:08:49 pm
content: Hi,The simplest form is as shown, a direct input to an ADC pin using Vdd.A potential divider with a multiturn 5k trimmer to calibrate.Depends on what temperate range you want to measure and what accuracy you need ?Many different hardware circuits can improve on its accuracy , as can your software code .Have used this circuit for many years on a narrow range heating system and its accurate to 0.1cWhile NTCs are good and cheap, you might find the DS18B20 digital sensor which can be purchased with a long lead and metal housing a better option ?

author: muthukural001
author_rank: Regular Contributor
timestamp: February 02, 2018, 02:01:19 pm
content: Hi picandmix,Thank you for your prompt feedback.In our case, temp range is -10C where resistance is 42K to 70C where resistance is 2.2K and accuracy we need is 0.1C.Unfortunately, We have no choice to go to Digital sensor.Q1:In the shared circuit, what is the exictation current for thermistor and have you taken any additonal effort to keep this current constant in the case of non Ratiometric measurement.Q2:Are you using 5K trimmer to decide Bias resistor value?Q3:How do you select Bias resistor for the interested temperature range (in my case -10C to 70C) in the thermistor ?Q4: Have you implemented any RC filter at the ADC pin of Microcontroller?Please suggestPlease suggest

author: picandmix
author_rank: Frequent Contributor
timestamp: February 02, 2018, 02:46:33 pm
content: Hi,As I said it was the simplest form of NTC circuit, as there are lots of different and sometimes complex circuity and software you can add to any ADC design.Sounds like you want someone to provide some very detailed explanation of such circuity, a college project ?Sure others will be better able to do so than me.

author: muthukural001
author_rank: Regular Contributor
timestamp: February 03, 2018, 04:29:40 am
content: Hi Picandmix,Sorry.But, I just want to know the things. I have done the following things. Please let me know if this ok.1.My interested range on thermistor is -10C where thermistor resistnace is 42.47K to +70C where thermistor resistance is 2.228K and accuracy needed is 0.1C. For Resistor R1 selection, I am taking Geometric mean for minimum (2.228K) and maximum resistance (42.47K) which results in 9.70K.So, I take R1 as 10K.If I am right, current flowing on the volatge divider changes with respect to the Thermistor resistance which is not fixed as it depends on temperature. By giving constant current, I think that we can give some linearity to the output. If yes, plaese give me tips to bulid constant current source circuit with less components. Pllease suggest.Regards,Muthu

author: picandmix
author_rank: Frequent Contributor
timestamp: February 03, 2018, 12:07:48 pm
content: If you search the web there are so many pdfs and examples of thermistor design from the manufacturers and others, so there is little I can add to them.You will find the odd reference to constant current, but a thermistor is a resistance so it does generate heat and as some datasheets explain using a constant current source can lead to self heating of the thermistor.Apart from that technical info, have you considered the more practical problems of your application with the Pic24 which has a 10bit ADC at 3v3 ?First, you have 1024 ADC steps which you can divide the thermistors voltage into.So if you are measuring with a resolution of 0.1c it would seem you can cover a wide range, but in practice you might find the result too inconsistant so you might need to use a lower resolution, which limits your range.Also consider that at your VDD 3v3 / 1024 steps means its trying to detect a change in 0.0032v per step, that means your supply must be very smooth.However your thermistors voltage output swing is far less than VDD so the voltage per step is even less.Use your simulator with some voltmeters and see what voltage range the 103AT can produce .And of course the Thermistors output is not linear so thats another thing to consider.So as you can see you need to do a lot of simulation or use an actual circuit to test and evaluate your thermistor and its practicality.If your tests show its not giving good results direct to a Pic24 ADC you might find moving to one of the 24 bit ADC chips a more practical input device.