Thermistor Buffer Signal Integrity - Page 1
Forum section: electronics / projects

author: geo_leeman
author_rank: Regular Contributor
timestamp: June 19, 2015, 01:30:25 am
content: Hi all,I'm going to need to measure temperature very accurately at a few depths in a steel block that is heated on one end. I am planning on embedding a few thermistors in the metal. My question is about the buffering and getting the signal to the ADC. The plan is to use a simple precision resistor + thermistor divider, then buffer that. Currently I've got a MAX4239 breadboarded up. I've been running things single ended with a 5 VDC supply. I have a simple RC filter on the input to the amp as well. This is a nice simple setup, but it seems a little too simple.My concern comes with the physical setup. The thermistors are in the steel block, but about 6-8 feet from the ADC. The ADC is a National Instruments 24-bit +/- 10VDC PXI card running at 10kHz (recording other things that change fast). I'd like to record temperature on that system since I have spare channels. Would it be best to place the amp/supply right by the steel block, then run the long cables out of the amp to the ADC? I can use shielded cable to help keep the signal clean, but again it seems like there is probably a better solution.I'm running the numbers to see if I need to worry about amplifying/offsetting the signal or if the resolution of the ADC over 0-5VDC will be satisfactory.

author: moffy
author_rank: Super Contributor
timestamp: June 19, 2015, 01:46:11 am
content: Use twisted shielded pair for your cable. It is better to buffer near the sensor so that the signal driving the long cable is low impedance, therefore less prone to noise. But you want to keep the buffer away from the heated steel to minimise drift etc.

author: geo_leeman
author_rank: Regular Contributor
timestamp: June 19, 2015, 02:03:16 am
content: Quote from: EmmanuelFaure on June 19, 2015, 01:49:01 amI was thinking a resolution of 0.1 degrees C at worst. 0.01-0.05 degrees C would be better.We're basically measuring heat generated from frictional sliding of the plate against another with some power in-between. Some back of the envelope numbers say we're looking at 2-3 degrees total change in temperature with a dT/dt of about 10-50 mK/sec. Good news is we can beat down ADC noise by averaging the 10k SPS down to a single temperature value every second or so. Thermistors seemed better for the job than thermocouples and since I can do the reduction to temperature from voltage in post-processing the non-linearity isn't a huge problem.

author: geo_leeman
author_rank: Regular Contributor
timestamp: June 19, 2015, 05:54:55 pm
content: Quote from: evb149 on June 19, 2015, 05:43:16 amI agree,we'll have the holes in the block filled with thermal compound and the thermistors down them on very small wires. We'll see some time lag from the friction heating of the surface since we are a few mm into the block. That's there a quick little 1D diffusion model will come in. I'll have thermistors at 3 distances from the face of the block and fit the model to them to project the surface temperature. There will of course be some loss to radiation, but for now that's probably okay if we insulate the setup.Quote from: EmmanuelFaure on June 19, 2015, 12:20:48 pmI agree with all of this! The divider seemed like the most straightforward way to do this. I'll probably but the buffer and divider resistor on a tiny PCB. I'll have 4 sensors in the end. The power wires are somewhat long, so I added some decoupling and the RC filter with a corner at 4.8kHz. The ADC is 1.192µV/bit, so assuming a "noisy" ADC, you're correct that we'll still have pretty great resolution. Power dissipation will also be tiny, so just a fraction of a degree. Since we're mostly concerned about dT, should be fine I think. Quick hand-schematic attached. Maybe it really is that simple? Geez. Here I was expecting a huge rabbit hole.

author: geo_leeman
author_rank: Regular Contributor
timestamp: June 19, 2015, 09:38:42 pm
content: EEVBlog ReplyQuote from: EmmanuelFaure on June 19, 2015, 06:35:21 pmThe power will come from a Rigol DP832, we don't have the NI gear setup as a supply. The ADC is operating in differential mode for all channels.On the buffer. I tested with an Arduino and got the following result (attached). The buffer looks to degrade the signal a little... on the breadboard anyway. Maybe just 4 of the 0.1% resistors on perfboard near the thermistors and it's good to go?

author: Andreas
author_rank: Super Contributor
timestamp: June 20, 2015, 05:54:57 am
content: Quote from: geo_leeman on June 19, 2015, 01:30:25 amThe largest problem will be that the 5V supply for the Sensor voltage divider is not synchronous to the 10V Range of the PXI card reference voltage.I would use either the reference voltage of the converter to supply (buffered) the voltage divider (for a single ended measurement)Or: Use 2 differential channels and measure the voltage over the 2 resistors (fixed + thermistor) and calculate the resistor ratio.In this case the external reference only has to be stable during the measurement.With best regardsAndreas

author: geo_leeman
author_rank: Regular Contributor
timestamp: June 20, 2015, 04:12:45 pm
content: Well, since this is a 16-channel ADC unit and I'm not using near all of them for these tests, I can always record the reference voltage as well.Something like a 0.1µF across the inputs should do the trick?Thank you for all of the suggestions! I'll update with some results. I think forum members may be mentioned in the acknowledgments of the eventual publication as well!

author: Andreas
author_rank: Super Contributor
timestamp: June 20, 2015, 07:42:49 pm
content: Quote from: geo_leeman on June 20, 2015, 04:12:45 pmYes thats usually sufficient. (for both channels: reference and thermistor)If you have very high demands you can look for low leakage current types (usually foil capacitors, low inductive)With your high measurement frequency you have nearly no mains frequency supression.So if you average the measurements you should chose the number of measurements to average as a multiple of mains frequency period time; e.g. evenly distributed within 100 ms (or 16.67 or 20ms dependand on where you are living)With best regardsAndreas

author: geo_leeman
author_rank: Regular Contributor
timestamp: June 20, 2015, 09:00:56 pm
content: Great! I'll rig a test up this week and check out the results, then we'll run the friction tests in the next month or so. Thanks!

author: geo_leeman
author_rank: Regular Contributor
timestamp: July 03, 2015, 02:57:54 am
content: A quick update. I ran a test with a thermistor in open air at 100Hz recording rate and got the following. Looks pretty goodOnce we get the rest of the setup rigged, I'll post that up.