RTD ADC help needed - Page 1
Forum section: electronics / projects

author: monoxide
author_rank: Newbie
timestamp: November 05, 2021, 08:22:01 pm
content: I'm working on a temp controller, and I was having problems getting the correct voltage to come out of the Op-Amp (INA326). Traced it back, and measuring voltage at the output of the constant current supply (ref200) I'm getting some weird stuff! With the RTD I'm not getting the calculated voltage, but if I use a fixed resistor (1/4 watt) everything works fine. I don't have another PT100 to test with, but I've heated it with a hot air gun and the temp/resistance are accurate with the published datasheet.I've replaced the ref200 twice, thinking it HAD to be a problem with that. I feel like using a fixed resistor (I've tried using different values, all work) and getting the expected output eliminates circuit board problems.This is the RTDhttps://www.auberins.com/index.php?main_page=product_info&cPath=20_15&products_id=21I've also tried removing the ADC from the board thinking it must have fried, and I'm still getting same voltage results.I drew up the two scenarios, and note the mV readings before the Op-Amp. What's going on?! I don't understand why the RTD which is a resistive device would cause the circuit to work differently than a fixed resistor. Also note the offset voltage measure at the Op-Amp. All measurements are reference to GNDA.I designed this circuit based of off the TI documentation here...https://www.ti.com/lit/ug/tidu969/tidu969.pdf?ts=1636134123537Any help or ideas are much appreciated!

author: Kleinstein
author_rank: Super Contributor
timestamp: November 05, 2021, 10:33:26 pm
content: That circuit is not working well, even of the bugs are eliminated. I would not even bother trying to fix it - just restart from scratch.The usual way to measure an RTD is to do it ratiometric. So there is no need for a real voltage or current reference. The relevant reference should be a resistor.To day the easy way is with an SD ADC that has differential input and maybe a differential input for the reference. So the circuit is mainly a suitable ADC a ref resistor and the RTD and than some protection again ESD. The protection part is what could make the circuit a bit challanging.The fixed resistor may have less capacitance and this way get a different oscillation frequency.

author: monoxide
author_rank: Newbie
timestamp: November 06, 2021, 04:14:56 am
content: Capacitance! @Kleinstein you were right! Put on a capacitor in series with the RTD, and works great now. Thanks!

author: KaneTW
author_rank: Frequent Contributor
timestamp: November 06, 2021, 04:37:21 am
content: RTDs should be used in at least a 3- or ideally a 4- wire configuration to eliminate wiring effects. Use ratiometric measurement, anything else is wasted effort and worse quality (e.g. ADS1248, but it's been a few years since i used it so there's probably better stuff out by now)

author: monoxide
author_rank: Newbie
timestamp: November 06, 2021, 04:40:25 am
content: Agreed with 3 or 4 wire, I knew that going in. Fortunately some quality math can at least get us close. I've got it accurate to within half a degree, which is plenty close enough just to brew some espresso.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 06, 2021, 08:24:06 am
content: With a 100 Ohms RTD with longer cables a 4 wire measurements is a good idea. Without it, the accuracy is limited, especially with a longer cable. With a 1000 Ohms RTD one can get away with 2 wires quite well.A capacitor in series to the RTD only works with a good AC bridge and even there it is not a good idea.One can get away with less than the relatively expensive ADS1248. An MCP3550 or similar is good enough - the main point is to have an external ref and differential input.One can also still build a simple bridge with a single OP and than use a µC internal ADC, especially if one does not need 4 wire connection.