Termination resistor in RTD analog front end - Page 1
Forum section: electronics / projects

author: ejht
author_rank: Newbie
timestamp: September 30, 2021, 03:42:37 pm
content: First of all sorry for my English, it is not my first language and sometime I cannot make myself completely clear.I am in a project in which I would like to use RTD in order to measure temperature in several points, as usual, when I have to do an analog design, since it is not my strongest point I try to find some application note to get ideas.In this case I have found an application note from TI (TIDU575–December 2014) which I have found very interesting, but I am struggling to understand why they are using 247R resistors at the end of the analog stage, after the filter and just before the ADC.It remainds me of the termination resistor that are usually used for impedance matching in transmission lines to avoid wave reflections, but I dont get it in this case, I hava also consulted the articles in the documentation section and neither of them use this resistor.I have also consulted the datasheet of the ADC (ADS1248) but I cannot find any reference to that.All the opinions and advices are more than welcome, thank you in advanceThe circuit I am talking about:

author: TimFox
author_rank: Super Contributor
timestamp: September 30, 2021, 05:00:51 pm
content: Do you intend to delete the components marked in red?I assume the input resistors are to limit the current into the protection networks (diodes in the red section) when an accidental unwanted connection to a large voltage happens at the RTD wires. Temperature probes are often installed in hazardous locations.

author: Dave
author_rank: Super Contributor
timestamp: September 30, 2021, 06:10:21 pm
content: The 249R resistors at the input form a low-pass filter (common mode as well as differential mode) in combination with the three capacitors. Section 4.1.2 of the document you referenced has a detailed explanation on this.Quote from: TimFox on September 30, 2021, 05:00:51 pmCrossing a component out is a common way of denoting that the component shall not be fitted during assembly.Quote from: TimFox on September 30, 2021, 05:00:51 pmThey wouldn't protect the circuit if it's connected to a mains line, but it might offer protection (in combination with those TVS diodes) in the event of ESD. Care must be taken when selecting those resistors (surge rating).

author: ejht
author_rank: Newbie
timestamp: September 30, 2021, 07:40:52 pm
content: Actually the capture is from the application note, and it means, as a colleague has stated, that they are not mounted.

author: ejht
author_rank: Newbie
timestamp: September 30, 2021, 07:47:54 pm
content: Quote from: Dave on September 30, 2021, 06:10:21 pmI thought that the low pass filter was formed by the 4.12k and 4700pF that arejust before the 249r, since they are at the input of the signal path, having in consideration that the conector JP6 is where the RTC is conected and the ports RTD_AINP4 and RTD_AINN4 are the differential inputs at the ADC (sorry I did not mention that before).That is why I dont see the filter, since following the signal path there is no capacitor to act as the other part of the impedance divider and to shortcut the highfrequency part of the signal after the 249r

author: Dave
author_rank: Super Contributor
timestamp: October 01, 2021, 04:20:10 pm
content: D'oh.Yes, I assumed the signal flow was from left to right and didn't stop to think, that it would make sense for it to be the other way.You're right, I don't see a good reason for putting those resistors in there. The only thing I can think of, is giving themselves a convenient way to access the chip's pins directly, if they decide to reuse the board for experimenting with some other circuit. But in that case, 0R jumpers would make more sense.

author: ejht
author_rank: Newbie
timestamp: October 04, 2021, 01:42:51 pm
content: That is waht i thought, I think i am going to put 0R.Thank you!!

author: peter-h
author_rank: Super Contributor
timestamp: October 04, 2021, 07:35:18 pm
content: One just has to laugh at the totally clueless use of 249R resistors

author: SeanB
author_rank: Super Contributor
timestamp: October 05, 2021, 05:19:43 am
content: The resistors are there to reduce input current pulses from the ADC affecting the measurement, by providing filtering from the ADC affecting the RTD current. 249R simply because those are E96 values, that are easy to get in 0.1% tolerance or better, so there is no asymmetry in the switching currents, because of small differences in resistance of the resistors.