Suggestions for resistor calibration - Page 1
Forum section: electronics / projects

author: martinjaymckee
author_rank: Contributor
timestamp: July 16, 2021, 10:52:55 pm
content: Okay, so I'm working on a project that does both resistance and current measurement on four independent channels. Due to cost and space constraints, I'm using hacks like using the MOSFET Rds_on for current measurement. As such, the whole system needs to be calibrated. Enter my current problem.To calibrate the measurements, I need reference resistances that can also handle up to 15A when calibrating the current sense. So, I've designed some boards using parallel Ohmite TKH45P series resistors. I've also mounted a TI TMP117 temperature sensor (overkill, but it was a good place to test it), and an I2C EEPROM to store the calibration data. The Ohmite resistors are 1% accurate with a temperature coefficient of 100ppm/C. Ideally I could calibrate them to 0.1%.My original - totally manual - idea was to take a temperature measurement with the TMP117, take a resistance measurement with a ~5 1/2 digit meter, and manually add them to a spreadsheet. With three different boards, each having four resistors, and needing to be measured at a minimum of three temperatures, that sounds like a lot of work... and quite error prone.If I get a DMM with serial control, it would be easy to automate the measurements. Similarly, I could fairly easily build a multiplexer to automatically switch between resistors. Finally, if the board is in an environmental chamber, I can track the temperature and take measurements at appropriate times.The question is, am I overcomplicating this? Is there an easier - or cheaper - way to do this? I don't currently have any of the test equipment to do measurements at 0.025% (which are needed to achieve the TAR of 4:1 and a final accuracy of 0.1%). As such, I'd rather limit what I need to purchase and build to the bare minimum. I am not, however, in a great hurry, so I can wait to find used equipment at a good price, and so forth.Thanks for any ideas you all may have.Cheers,Martin Jay McKee

author: viperidae
author_rank: Frequent Contributor
timestamp: July 17, 2021, 01:38:19 am
content: I think the first thing to do before trying to get a 0.025% calibration is to look at the data sheet for your MOSFET at the rds graph and Vds vs Ids graphs. There will be temperature graphs too, which is junction temp, not case temp, so pretty hard to accurately compensate for. It will be pretty bad, maybe around 20% for a 60c change in junction temp.It's also going to depend greatly on Vgs.The effective resistance will also change with the current, so it won't be linear.

author: martinjaymckee
author_rank: Contributor
timestamp: July 17, 2021, 03:19:07 am
content: My apologies for being unclear. I'm not trying to get to 0.025% accuracy on the MOSFET, or even on the resistors that I'll be using as a transfer standard for the final product. The 0.025% is the estimated measurement accuracy I need to have a valid calibration of the transfer standard. Best case, that would allow me to get to around 0.1% for the transfer standard and 0.4% for the resistance calibration in the final device. My ultimate limit, on the final device is about 1%. So that feels like I would have some reasonable margin. The current measurement -- using the resistance of the MOSFET -- is primarily a safety feature, and need not be particularly accurate (my accuracy "goal" is 5% only because the working temperature range is limited, but I don't really care if I hit it or not). I do, however, need to be able to test the system at high current, hence the need for the test resistors to handle high currents. As a result, I can't use the simple solution of buying four 0.01% Vishay resistors of each value I need (of course, at $30 each, that gets expensive pretty quickly in any case).The TLDR:I need to calibrate four resistors, on three boards, at multiple temperatures to around 0.1% accuracy. I'm looking at possible automated methods that are reasonably cost effective and are not a whole project in themselves.Cheers,Martin Jay McKee