DIY precision thermistor/NTC temperature measurement - Page 1
Forum section: electronics / general

author: MiDi
author_rank: Frequent Contributor
timestamp: April 10, 2021, 11:49:18 pm
content: Goal was to design a quite cheap dual precision temperature measurement circuit with 10kΩ NTC.Target key specs:Range: 0-60°CAccuracy: <100mK (after calibration)TC: <1mK/KShort term noise: ~1mKpp/minMid term noise & drift: ~10mKpp/hrLong term drift: <100mK/yearSelf-heating: ~10mKCost: ~20$ (~10$/channel - single quantity w/o mcu)Chosen topology is simple ratiometric measurement, consisting of resistive divider (reference resistor & NTC), ADC and voltage reference.This needs only one precision resistor, a precision ADC and a jelly bean voltage reference.As ADC a MCP3911 was chosen, cheap dual channel 24bit with integrated 1.2V reference.Measured specs:Range: 0-60°C (with some margin)Accuracy: TBDTC: ~0.2mK/KShort term noise: ~2mKpp/min (estimated by eye)Mid term noise & drift: ~4mKpp/hr (estimated by eye), -4°C: ~6mKpp/9h, 25°C: ~6mKpp/13h, 87°C: ~18mKpp/11h (measured with dummy resistors instead of NTC)Long term drift: <100mK/year (~4mK/month w/o NTC - estimated by eye)Self-heating: TBD (calculated 0°C: ~12mK, 25°C: ~8mK, 60°C: ~3mK)Cost: <30$ (<15$/channel - single quantity w/o mcu) - with some tweaks and optimizations ~20$ should be possibleQuite impressive performance for such a cheap ADC that is not specifically designed to be used at DC...Wanted to share this not quite finished project in addition to threadMeasuring mK with small RTDs.The measurements done so far are attached.

author: MiDi
author_rank: Frequent Contributor
timestamp: April 10, 2021, 11:50:27 pm
content: reserved for future

author: Anders Petersson
author_rank: Regular Contributor
timestamp: April 11, 2021, 10:16:43 pm
content: Thanks for sharing, it looks good!The self-heating will depend on the sensor, of course. Also, since thermistors have such a non-linear response over the temperature range, all temperature performance has to be calculated across the range. (I'm sure you're already aware.)I suppose you don't connect the NTCs with long cables, given that it's a 2-wire connection? Your graphs say the resistance is 40 kOhm at -4 C and 1 kOhm at 87 C. Not too difficult resistances to work with, but without running the numbers I guess the tempco of the leads still is important at high resolution.Do you use tantalum capacitors as the 3D hints? If so, why?The C1, C2 caps are marked "MKS" and look orange in 3D. Looks like these:https://www.wima.de/en/our-product-range/metallized-capacitors/mks-4/Do they have any advantage over regular MLCC?R4 and C2 are the anti-aliasing filter that "attenuates the frequency content around DMCLK and keeps the desired accuracy over the baseband of the converter" (quoting the MCP3911 datasheet p 35). Is your placement of this filter before the buffer better than after? Can't the buffer introduce new noise?Is R6/C17 a low-pass filter for the buffer? What's such a filter called?What's the purpose of D1, D2? Won't they short out the signal?

author: Kleinstein
author_rank: Super Contributor
timestamp: April 12, 2021, 07:14:15 am
content: At 1 µF it is hard to get MLCC with C0G dielectric. The cheap X7R and similar types have higher leakage, significant dielectric absorption and may be pressure and temperature sensitive. So they are not the right choice for a low level signal.One may argue that 1 µF is not absolutely needed. So one might get away with 100 nF, which is available as C0G variant at reasonably, though slightly higher cost. This is especially true if fast response is wanted and not 50 Hz suppression. The MCP3911 is a rather fast ADC.The buffer may indeed add new noise and interference - especially chopper ripple and spikes. So some extra filter between the buffer and ADC may be a good idea.The diodes see a relatively low voltage, so they won't conduct much. It depends one the type used (schottky or 1N5401 may be bad, 1N4148 or similar should be OK)

author: Dr. Frank
author_rank: Super Contributor
timestamp: April 12, 2021, 10:54:31 am
content: MiDi,thanks in advance for your contribution.. I'm curious, which precision NTC you have used, and which reference thermometer, and how you did the physical calibration and characterization.What I also find great is your methodology, i.e. by first defining the requirements and tentative specification, then make the design-to-spec, and in the end to characterize the whole work product.Often engineers tend to make the design into Thin Air, meaning 'to put the cart before the horse'Frank

author: MiDi
author_rank: Frequent Contributor
timestamp: April 12, 2021, 02:28:00 pm
content: Quote from: Anders Petersson on April 11, 2021, 10:16:43 pmTarget sensor is GA10K3A1IB TT/Measurement Specialties with a dissipation factor of 0.85mW/K.There are much cheaper ones with good enough stability, so option to cut cost -NTC long-term drift measurement p.58ff.The sensor will be calibrated anyway, but you are right, with rising temperatures the required precision rises - especially in this not ideal configuration with high reference resistor (tradeoff for lower self heating).Quote from: Anders Petersson on April 11, 2021, 10:16:43 pmFor my primary purpose max lead length will be 1/2m.NTC has ~2.5kΩ@60°C with ~90Ω/K, e.g. a 30AWG wire has around +-41mΩ/m in +-30K span.This translates to absolute max +-1mK due to tempco of 1/2m wire - so even longer and smaller diameter wire is not an issue.Quote from: Anders Petersson on April 11, 2021, 10:16:43 pmYes, yellow cases are tantalum, but could be replaced with MLCC -followed reference design which uses tantalum (p.20).Kleinstein already gave the answers, in addition the cheap MLCCs (X, Y, Z) are microphonic.Quote from: Anders Petersson on April 11, 2021, 10:16:43 pmC1/C2 MKS 1µF are for anti aliasing and reducing noise (emi) in conjunction with NTC & R1/R2, but are not effective at line frequency or VHF.1µF increases response time of sensor only by some ms, which is insignificant as sensor has ~1s response time.Would be worth checking if an anti aliasing filter between opamp and ADC input is better - particularly if a more noisy opamp is used.R3/R4 give some input protection.R5/C16 & R6/C17 in FB: R for input protection, C for compensation of input/stray capacitance to GND (keep gain close to one at high frequencies).Most opamps have input protection clamping diodes between inputs and inputs to rails, which can handle only a couple of mA continuous.D1&D2 are low current jelly bean diodes (e.g. BAS316) for input protection of ADC, they are only slightly biased when measuring low temperatures @high PCB temperature.Quote from: Dr. Frank on April 12, 2021, 10:54:31 amTarget sensor is GA10K3A1IB TT/Measurement Specialties, but all (released) measurements were made with dummy resistors (PTF56 5ppm/K @21+-1°C).Calibration will be against a SHT35 in thermal chamber, which were used for TC characterization already.Have 3xSHT35, which were calibrated against each other, ice-bath and boiling point of water and were in accordance to each other mostly within noise (+-10mK), ~+20mK@0°C and ~+100mK@100°C.Overall the current version was designed to get the performance of the ADC, final version is targeted to get lowest cost/channel without sacrificing performance significantly.Full project files will be released soonTM

author: Anders Petersson
author_rank: Regular Contributor
timestamp: April 13, 2021, 09:32:32 pm
content: Thanks for answering my questions!It all makes sense. The only thing I question is the emphasis of low capacitor drift, as I don't think the filters and decoupling depend on exact values.Looking forward to hearing more results. As Dr. Frank said, you have a good methodology.

author: MiDi
author_rank: Frequent Contributor
timestamp: April 13, 2021, 11:33:45 pm
content: Quote from: Anders Petersson on April 13, 2021, 09:32:32 pmNot sure what you are referring to, capacitor drift or exact values are not relevant, "noise" is more a concern.

author: Anders Petersson
author_rank: Regular Contributor
timestamp: April 13, 2021, 11:53:37 pm
content: I assumed C0G were chosen for the low temperature dependence, but I suppose I was wrong.