Temperature probe output noise protection - Page 1
Forum section: electronics / projects

author: BorisRap
author_rank: Contributor
timestamp: May 08, 2023, 02:24:51 pm
content: Hello, I'm designing a circuit that has to measure temperature using a PT100. The device is a module meaning the user has to power it and connect the PT100 to two of the pins I provide.In ideal condition the cable of the PT100 would be rather short, twisted or ideally shielded. But in practice I have no Idea what the user is going to use so I'd like to protect the input against noise injection since it is a measuring device and I would like to keep everything as clean as possible.How would you recommend to protect the input? I figure to put a capacitor between the pins to dump high frequency noise.Is that enough?Should I put some configuration of inductor?Are common mode chokes (or coupled inductors) useful here?This question is just about noise injection, I know I have to protect against ESD also and the circuit is not ideal for very precise PT100 temperature measurements, but is good enough for what I need and I have only one op amp at my disposal.Thanks!

author: ajb
author_rank: Super Contributor
timestamp: May 08, 2023, 02:42:53 pm
content: Quote from: BorisRap on May 08, 2023, 02:24:51 pm"As ____ as possible" is a very expensive design requirement to meet! What is reasonable here really depends on what sort of noise you expect it to be exposed to. That depends on the environment and application.Adding a low-pass filter in the form of a capacitor is a good start, but that also slows the response of the system. Fortunately, temperature sensing tends to be something where the expected rate of change is very slow to begin with, so you can potentially make that capacitor quite large, which may make other steps unnecessary.

author: BorisRap
author_rank: Contributor
timestamp: May 08, 2023, 03:21:09 pm
content: Quote from: ajb on May 08, 2023, 02:42:53 pmHaha, I knew that would come back to bite me. I meant as clean as practically possible. I have no knowledge on how to measure the noise level, but I know that simple things can cause a big impact compared to not having anything.Yeah, the 100n cap there was just an example I can bump it to 10uF, its a fairly small circuit so I cannot use electrolytic caps, I'm using all SMD 0402 so big capacitance values are really not achievable. But taking advantage of the slow nature of the temperature measurement, I put that 10k/10uF filter at the output that should be a fairly good filter.The thing is the input, I don't quite understand how the noise gets in and how it affects my circuit that's why I'm asking if I should put some inductors or some other topology

author: mawyatt
author_rank: Super Contributor
timestamp: May 08, 2023, 04:31:18 pm
content: Please realize at RF and above most type caps are useless for filtering. Electrolytics, high capacity ceramics, and film types are in this category.You'll likely require a good decoupling network on the input, one that protects against ESD, maybe direct Mains ?, and the usual RF/MW interference.Small quality COG/NPO ceramic (1~10nF) chip caps work well at RF/MW, add some good lossy Ferrite Beads and this should provide adequate RF/MW filtering, and real series resistance in front of the shunt C also helps and will likely be required to protect against direct Mains voltage.Anyway, if you don't know who/what are going to be using/connecting your device inputs, you need to design for the worst case. The ESD and RF/MW are straight forward and small, the Mains protection will require some hefty components tho, and take up some PCB real estate!! A PT100 sensor is ~100 ohms, so the Mains protection may be a little difficult, large and somewhat costly.Some research into the various means to protect against direct Mains may prove worthwhile.Best,

author: geggi1
author_rank: Frequent Contributor
timestamp: May 08, 2023, 04:38:26 pm
content: You could design the circuit to use diffirencial measurement and screened cables.By using a diffirensial input the noise will be equal on each conductor and can be eliminated.

author: BorisRap
author_rank: Contributor
timestamp: May 09, 2023, 06:28:17 am
content: Quote from: mawyatt on May 08, 2023, 04:31:18 pmThanks for your answer, I'm gonna take it into consideration.I'm not going to protect against Mains, the whole module is 30mmx30mm and operates with 3.3v if someone connects it to Mains than it shall burn

author: Kleinstein
author_rank: Super Contributor
timestamp: May 09, 2023, 01:13:33 pm
content: For the filter capacitance more is not helping more for the high frequencies. The 100 nF as shown already do quite a bit for ESD. Additional protection should be at the pos input of the op-amp - how much depends on the environment. The minimum would be something like 1 ferrite bead and 1 capacitor, but maybe another RC filter could be reasonable.If some common mode filtering is needed or a good idea depends on what is connected at the output side. No need if isolated and battery powered. It may be a good idea is liked to USB - though than the question is if the fitler is better at the USB or at the sensor.For the sensor cicuit it would be better use a more assymetric bridge with R19 and R20 larger (e.g. 2.2 K) and R18 smaller and possibly left out alltogether. The bridge with 2x100 Ohm looses half the sensitivity due to loading from the fixed upper resistor.