Omron relay contacts add 200uV error - Page 1
Forum section: electronics / projects

author: dobsonr741
author_rank: Frequent Contributor
timestamp: January 05, 2023, 01:59:14 am
content: I have an Omron G6S-2F relay in my analog frontend I designed. It turns out it's two contacts in series add a 170...200uV error. Is this cause completely lost, and mechanical relays will always mess in this measurement range? Or perhaps is it a bad relay? I'm also open to better relay recommendations close to it's footprint.This is the data sheet link:https://omronfs.omron.com/en_US/ecb/products/pdf/en-g6s.pdf

author: TimNJ
author_rank: Super Contributor
timestamp: January 05, 2023, 02:27:12 am
content: How much current through the relays? I assume the current is quite low since it’s for an analog front end as you say. Contact resistance related voltage drop is a possibility but if you still see that much voltage difference and your current is let’s say, oh, less than 100uA or so, then it’s probably not contact resistance.Maybe it’s a thermal EMF issue:https://cotorelay.com/product/3500-series-low-thermal-emf-reed-relays/

author: ejeffrey
author_rank: Super Contributor
timestamp: January 05, 2023, 03:18:33 am
content: 200 uV thermal emf is pretty high but possible. Type K thermocouples: made of materials chosen specifically to have a high thermal emf are 41 uV/ K. Normal materials will be considerably less than that. I'm not sure how your "two contacts in series" work, if possible you would arrange to have the + and - paths symmetric so that thermal emf would be cancelled to first order but that isn't always possible.One possible cause of thermal emf is heating from the coil. If this is the case you should see one value immediately after switching and then slowly drift as the coil heats the contacts up. You can reduce this effect dramatically by using a latching relay so you only briefly pulse the coil.

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 05, 2023, 03:18:59 am
content: I assume you already ruled out capacitive noise coupling and leakage currents? Does the offset happen if you use a neodymium magnet to make it switch and have the coil disconnected? If not, does it happen if you reconnect only one end of the coil?

author: dobsonr741
author_rank: Frequent Contributor
timestamp: January 05, 2023, 05:09:06 am
content: An IPA wash and rerouting a 4mA GND line to the reference improved the error from 200uV to 20uV. The relay is not worse than a solid wire replacement when testing. So Omron, sorry pointing the finger at you. Planning more tests and more IPA. In hindsight, I should have added the guard ring, which I considered but dropped as I only aimed at 5.5 digit accuracy.

author: SeanB
author_rank: Super Contributor
timestamp: January 05, 2023, 08:59:34 am
content: Would suggest you look at reed relays instead, with sealed reeds, as those are a lot more stable in low voltage and current switching, and will not degrade like normal open relays do from the contact material eroding through to expose different alloys.

author: Kalvin
author_rank: Super Contributor
timestamp: January 05, 2023, 09:33:51 am
content: OP may want to check the following discussion thread for potential solution / remedy:https://www.eevblog.com/forum/metrology/measurements-on-emf-error-of-switches/

author: Kleinstein
author_rank: Super Contributor
timestamp: January 05, 2023, 10:20:49 am
content: To avoid thermal EMF errors it helps to have a latching relay if the circuit allows.Reed relays are usually non latching and the need to use a magentic material makes thermal EMF an issue. Many (especially the cheaper ones) reed relays are not that great in the isolation resistance. The main advantage is the speed and low bounce.

author: TimNJ
author_rank: Super Contributor
timestamp: January 05, 2023, 02:53:31 pm
content: Quote from: Kleinstein on January 05, 2023, 10:20:49 amSo, why do so many high performance multimeters (e.g. Keithleys) use reed relays on their inputs? Is speed really that much of a concern in this application? Or does the low bounce let it auto range more accurately and more quickly?

author: Kleinstein
author_rank: Super Contributor
timestamp: January 05, 2023, 03:57:13 pm
content: In some of the higher end meters a relay is used as part of the protection and for this it needs to be fast. It is also convenient for use if the relay is quiet.The thermal EMF error from a reed relay is not so much a problem with 10 µV resolution. For higher demand there are special (relatively expensive) low thermal EMF reed relays with 2 contacts in series to get good symmetry. Compared to a non latching normal relay the reed relay can still get away with less power (especially a shielded version). A normal, non latching relay is thus often worse.In a dirty environment some normal relays can have problems with dust or oil / nikotin deposits.Latching relays are not that new, but they need more effort to drive and were not that popular some 30 years ago.From my feeling the reed relay were more popular back than and now getting less popular and in parts replaced by electronic switching (e.g. photomos).I am not so convinced the reed relays in some of the old Keithley meters were such a good choice. It may have been at that time and part could also be a traditions from even older days, when normal relays were not that good and compact. Newer meters (e.g. KS3446x, ...) seem to prefer latching relays.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: January 06, 2023, 01:15:21 am
content: Happy to report the last 20uV error has disappeared when I removed the mythical 1.5K PTC fuse from the signal path.For reference, the PTC was B59886C0120A051 from TDK:https://www.tdk-electronics.tdk.com/inf/55/db/PTC/PTC_OC_Leaded_380V_500V.pdf

author: Kleinstein
author_rank: Super Contributor
timestamp: January 06, 2023, 08:30:52 am
content: How did the PTC contribute to an offset ? Is it just from the resistance (high bias current), an effect on the thermal layout or is there thermal EMF from the PTC ?

author: jonpaul
author_rank: Super Contributor
timestamp: January 06, 2023, 01:14:16 pm
content: Poor choices of relay.use a gold plated bifuracated contact relay designed for " dry Signal" use.jon

author: dobsonr741
author_rank: Frequent Contributor
timestamp: January 15, 2023, 10:21:56 pm
content: This is how I solved it: moved all autozero and full scale calibration switching before the input protection and on reed relays. Solid max +/-1 1uV noise in the 150.000mV range. No more offset error. Auozero runs every 6.5 seconds, the ADS1263 is at 10 samples per second.Before this mod the autozero and full scale calibration was on two CD4053, each contributing 30uV probably due to the leakage currents. In a repair attempt replaced the 4053 with ISL84053, 2pA typical On leakage, costing $2.60 each - absolutely no change.I'll post an INL curve when I replaced the breadboard with a proper PCB.