Measurements on emf-error of switches - Page 1
Forum section: electronics / general

author: Echo88
author_rank: Frequent Contributor
timestamp: November 05, 2016, 12:58:47 am
content: Since a lot of people here in this forum have nanovoltmeters, id like to ask if anyone of you has ever done a systematic comparison of different switches/relays/solid-state-switches regarding emf-error, with numeric results. If it has been done, can you publish it here?Regarding relays: As far as i know suitable low-emf-relays are for example the G6AK/G5AK-series from Omron, Panasonics TXS-series or COTOs 3500/3600-series.Ive seen that a few times solid-state-switches were used for low emf in high-end-scanner-cards, but never a mention of the used solid-state-switch-component (manufacturer, device name). Maybe someone knows more about it?

author: ManateeMafia
author_rank: Frequent Contributor
timestamp: November 05, 2016, 02:42:22 am
content: On a Data Proof 160A with inputs shorted using copper tubing , I was able to measure between 50nV -100 nV on different combinations of switch connections. The data I have seen in their manual shows typical values under 50nV. My test conditions were far from perfect but were consistent. It was all automated with software and measured with a 34420A.I have plans on doing the same with some TX-S relays next.

author: Vgkid
author_rank: Super Contributor
timestamp: November 05, 2016, 02:51:59 am
content: In one of the voltage intercomparison from bipm/nist??/a bunch of others. Used a automated Electroswitch C4 switching system.

author: dacman
author_rank: Frequent Contributor
timestamp: November 05, 2016, 02:58:24 am
content: Latching relays, which are in scanners such as the Data Proof 160A and Measurements International 4220A, will have lower EMF values than non-latching relays.

author: Vgkid
author_rank: Super Contributor
timestamp: November 05, 2016, 03:19:45 am
content: Quote from: Vgkid on November 05, 2016, 02:51:59 amhttps://www.google.com/url?sa=t&source=web&cd=5&ved=0ahUKEwjIyK3n0pDQAhWESiYKHcvYCNkQFgglMAQ&url=http%3A%2F%2Fwww.bipm.org%2Futils%2Fcommon%2Fpdf%2Ffinal_reports%2FEM%2FS28%2FEUROMET.EM-S28.pdf&usg=AFQjCNGPtV70IXTjBpYG1zqa7BQNNLk-Pg&sig2=yfK3JPMqw-opK81A6LTzIQhttps://www.cenam.mx/memsimp06/Trabajos%20Aceptados%20para%20CD/Octubre%2025/Bloque%20A/A3-ELECTRICA%20I-Patrones%20Cuanticos/A3-3.pdf

author: ManateeMafia
author_rank: Frequent Contributor
timestamp: November 05, 2016, 04:02:39 am
content: I should have been more specific that I plan on using the TXS2-L2 dual coil latching relays. They appear to have good low thermal specs.

author: Andreas
author_rank: Super Contributor
timestamp: November 05, 2016, 06:21:57 am
content: According to datasheet it would be 0.3uVBut for which contact?The low level contact (option -1) or the standard contact?With best regardsAndreas

author: Echo88
author_rank: Frequent Contributor
timestamp: November 05, 2016, 11:18:24 pm
content: Ive found the AQV210 PhotoMOS-switch, which is stated to have 1µV emf-error....but it is only mentioned on the presentation page of 2 distributors, not in the data sheet:http://au.rs-online.com/web/p/solid-state-relays/1137210/@Andreas: Maybe Panasonic does emf-tests like COTO: across 2 antiserial connected contacts, so they cancel the emf-error slightly ->http://disti-assets.s3.amazonaws.com/testco-inc/files/datasheets/17016.pdf

author: Andreas
author_rank: Super Contributor
timestamp: November 06, 2016, 06:23:45 am
content: Hello,ok bad trick.Unfortunately this is not the way I plan to use with my new relay scanner for my LTZ-references.I want to be able to measure the difference between any of the 15 inputs.In the old 7 channel scanner I have used TQ2-L2 relays. (and I still have some of them specced with 2uV).But the question is if the thermal EMF plays a large role when latching relays are used.The whole scanner (including processor) is powered one week with a 9V NiMh battery.In the TXS2 datasheet if I interpret the EMF distribution diagram correctly they have used standard contacts for the test.But on the other side: how can they get a sum quantity of 20 results out of 6 relay samples?So also the relay type might be wrong.I guess that the EMF is lower with standard contacts (same material for moving and fixed contacts).With best regardsAndreas

author: VintageNut
author_rank: Frequent Contributor
timestamp: November 06, 2016, 10:59:47 am
content: The Keithley 7168 Mux card is specified to have contact potential of <30nV. The problem is that 10V is the maximum voltage.The 7035 Mux has <1uV contact potential.The 7011-S Mux card has <500nV contact potential.All of these cards use the 7001,7002 switch frame.For the 2700 family DMMs, here are some contact potentials7708 Mux card, 1uV max contact potential, <500nV typical7700 Mux card, 1uV max contact potential, <500nV typicalThe relays in my 7708 card are a NEC EE2-3SNUH

author: HighVoltage
author_rank: Super Contributor
timestamp: November 06, 2016, 12:20:36 pm
content: I am using the Keysight 34970A data aqusition unit with the 34901A 20 Channel Multiplexer CardsThese cards have relays and those contacts are rated as <3uV Thermal Offset.

author: Echo88
author_rank: Frequent Contributor
timestamp: November 06, 2016, 12:21:12 pm
content: The Keithley 7168 also uses "CONTACT TYPE: Solid state JFET switch"Now i really want someone to open such a card and expose which components are exactly used and how the layout is done.The mentioned NEC EE2-3SNUH-relays dont mention emf, but are quite cheap at 2,5€:http://www.mouser.com/ds/2/212/KEM_R7002_EC2_EE2-540906.pdf

author: doktor pyta
author_rank: Frequent Contributor
timestamp: November 06, 2016, 03:16:11 pm
content: Have You seen this: Tektronix 148-0140-00 Latching Relays ?Great stuff for DIY.http://www.ebay.com/itm/4-pcs-Tektronix-148-0140-00-Latching-Relays-2400-Serie-Oscilloscope-Attenuators-/381691204065?hash=item58de9191e1:g:U0UAAOSwkl5XeYdoIf only they were available in large quantity ...

author: David Hess
author_rank: Super Contributor
timestamp: November 06, 2016, 11:59:11 pm
content: For a while Tektronix was making their own relays and OEMing them for others. They pioneered the relay design where all of the contacts are at the bottom allowing good high frequency performance. In the 1970s, they were selling the relays for like $50 each. Now you can buy equivalents, but not with the superior Tektronix pinout, for $2.The low EMF relays are not suitable for switching while under load so rugged designs use two relays in parallel with the tough relay switching first to protect the contacts of the low EMF relay.

author: Echo88
author_rank: Frequent Contributor
timestamp: December 07, 2016, 02:07:11 pm
content: Infineon (formerly International Rectifier) has a bunch of Photo-FETs with stated Offset Voltage < 200nV:http://www.infineon.com/cms/en/product/power/solid-state-relay/photovoltaic-relays/channel.html?channel=5546d4624eeb2bc7014ef42feeed30d4#goto_producttablefor example:http://www.infineon.com/dgdl/pva33n.pdf?fileId=5546d462533600a401535683a68329287 Eur/piece at digikeyThe older PVA30-type even has a Offset Voltage-diagram:http://www.irf.com/product-info/datasheets/data/pva30.pdfon page 3.Thoughts or can anyone see pitfalls?

author: TiN
author_rank: Super Contributor
timestamp: December 07, 2016, 02:26:33 pm
content: There is also low leakageOmron G3VM-41GR6released recently, however En not specified

author: Kleinstein
author_rank: Super Contributor
timestamp: December 07, 2016, 07:23:29 pm
content: In all cases the actual thermal EMF depends on the temperature gradients. This is why latching coils are so much more attractive. For the Optomos and similar, there is always a sizeable power consumption driving the LED. So the thermal EMF depends on the layout - cooling.If used right they could be a good choice, though the on resistance might be a problem in some applications. For example the HP34420 uses quite a few of the Photomos in the input stage.For a low voltage scanner one could even consider standard CMOS switches - the low power consumption helps to keep temperature gradients down.

author: Echo88
author_rank: Frequent Contributor
timestamp: December 08, 2016, 11:27:37 am
content: Time to drill a hole a teeny weeny hole in those Photo-FETs, attaching a fiber and controlling it from far away to avoid thermal gradients.

author: dr.diesel
author_rank: Super Contributor
timestamp: December 14, 2016, 05:00:09 pm
content: So I included a few different relays on my last DK order:Omron G6AK-234P-ST-US-DC5Panasonic TQ2-L2-5VPotter & Brumfield V23079E1201B301My interest here are volt-nut scanner options, so at 10V with a >10G input impedance we're in the ~1nA range. Starting with the Omron relay I unsuccessfully attempted to measure the closed contact resistance in the 1nA range, but couldn't get reliable results with any of my equipment, k2450, k155.So then I setup a simple test, Fluke 731B > Omron relay > k7510, all fairly sloppily connected via Pomona Minigrabbers.731B measured directly = 9.999888, in circuit above latched = 9.999888,veryrepeatable unlatching/latching (about 20 cycles). (2uV noise/bobble)I've not played with the IO on the back of the k7510, but might be possible to utilize that for switching the relays in a smaller scanner setup.More testing required obviously, but a glimmer of hope maybe

author: 3roomlab
author_rank: Frequent Contributor
timestamp: December 15, 2016, 08:59:41 am
content: Quote from: dr.diesel on December 14, 2016, 05:00:09 pmi was reading the omron 6A pdfs, and then looking at 125VAC resistive specs, for 274P @ about 0.1A, its over 50million ops, did i see correctly? that much? (the 234P seems lower at 15million ops?)i am thinking, if to induce a fake sense of heating, what if a small 1w resistor is stuck to 1 side of the relay and some temperature diff is introduced? maybe then with this, a visible emf can be measured? then an actual uV per celcius can be found?another idea, what if the relay can be wired paralleled with 1 in reverse to self cancel its EMF? (assuming only SPST operation). parallel also halves the contact resistance.downside is 2 sets of coils to fire instead of 1. double the price for no EMF?

author: ap
author_rank: Frequent Contributor
timestamp: December 15, 2016, 11:47:56 am
content: Quote from: 3roomlab on December 15, 2016, 08:59:41 amIn order to minimize th. EMF (if it is really critical) good practice always is to use one relay with two contact pairs in series in such a way that the EMF of both contact pairs cancels each other out as good as possible. Also, one would try to make an isothermal design (e.g. avoid heat sources arround...).

author: Kleinstein
author_rank: Super Contributor
timestamp: December 15, 2016, 03:55:16 pm
content: Using two contacts in series is a two sides thing. It can reduce thermal EMF if the two contacts compensate, but if thermal gradients are not well defined the errors can also get larger. So it depends on the type of relay. With reed relays using two in series is often a good idea - with a latching relay, a single contact could be better.For measuring there should be no need to have a voltage applied. Just measure a nominal short with maybe a very small bias current (e.g. nA range) should be enough. The trouble could be that one might want to reverse the DMM / amplifier as well, but this adds some extra switches. So one could use a circuit with several switches in parallel an compare the voltage of using one switch at a time. So the result would be the difference in thermal EMF between different contacts. With several of the same type one should get a reasonable value - there will be a tendency to have different values anyway. I would expect that one might need a kind of nV meter - the normal 100 mV of a DMM might not be good enough.

author: janaf
author_rank: Frequent Contributor
timestamp: May 07, 2017, 12:48:59 pm
content: Revitalizing bump for this thread....Hardly any of the OP's questions where answered. No more input?Any measurements?Anything on IC MUX and emf?I'm about to build a MUX for my voltage references.....How measure?!!?

author: Andreas
author_rank: Super Contributor
timestamp: May 07, 2017, 01:34:16 pm
content: Hello Janaf,its a long time since I last heard something from you. (almost missing your comments).I also want to built some MUX (since around 1.5 years now).But this is still "work in progress"In the end I will use that what I have already in the drawer:TQ2-L2 latching signal relays which do not heat up since they are powered only some ms during switching.With best regardsAndreas

author: janaf
author_rank: Frequent Contributor
timestamp: May 07, 2017, 04:47:27 pm
content: Hi,Today Iv'e posted here for the first time in about two years. Time flies.I've got a bunch of TXS2SA-L2 that I was going to use. Some datasheet say 0.3uV emf while others just recommend the AgPd option, without specification. Mine are not the "-1" AgPd option.I was probably going to switch them directly with some PCA9539DWR. Simple.The attached pic is from the page:https://na.industrial.panasonic.com/blog/thermal-electromotive-force-emf"How To Offset EMF: Change in contact materials; Silver Paladium (AgPd) is an excellent material for contacts" Does not give any help really....This overview:http://www3.panasonic.biz/ac/e_download/control/relay/signal/catalog/mech_eng_chart_signal.pdf?f_cd=402060mention GN and GQ series have AgPd contacts but no specifics.High sensitivity seems to refer to low coil power only.Anyway, if you want some TXS2SA-L2 I can send some!But what do you mean by "almost missing"

author: Andreas
author_rank: Super Contributor
timestamp: May 07, 2017, 09:28:32 pm
content: Quote from: janaf on May 07, 2017, 04:47:27 pmWhen looking through the LTZ1000 thread some weeks ago,just thought: where is that guy who shares his systematic measurement results?Quote from: janaf on May 07, 2017, 04:47:27 pmI am using 5V types and put one of the coils (178 Ohms) over a capacitor directly to a 5V Port Pin (30 Ohms in my case) of a microcontroller.You can also put the 2 coils in series to save further current with slightly slower switching.On power up I activate the pull ups of the processor to charge the capacitors without switching.with best regardsAndreas

author: chuckb
author_rank: Frequent Contributor
timestamp: May 08, 2017, 04:29:06 am
content: The Keithley Model 2000-SCAN is a 10 channel scanner card for the 20XX DVMs. It's specified at less than 1uv of offset.The Service information specifies TQ2E-L2-5V relays.The card I used actually had NEC EA2-5TNJ Orange Relays, date code 1411.To test the card I shorted all the inputs at the terminal block with bare 22 ga copper wire from the same spool. The card was installed in a K2000 however, for better resolution, the relay offset voltage was measured with a K182 Nano Voltmeter. Each channel was selected for 1 minute and the offset voltage was recorded every 10 seconds.I verified that the different inputs on the card do have less than 1uV of offset. The offset repeatability for each channel seems to be less than 100nV. See the attached pdf for the strange offset pattern that this card provides. The numbers in the middle of the graph note which channel was selected at the time the voltage was recorded.

author: TiN
author_rank: Super Contributor
timestamp: May 08, 2017, 04:45:09 am
content: Thankschuckb, interesting data. How did you log the data. Perhaps I can do same test on 2001-TCSCAN card. I have 182-M as well.janaf, happy to see you back. Voltnut community worried about your disappearance a bit.

author: Andreas
author_rank: Super Contributor
timestamp: May 08, 2017, 05:13:32 am
content: Quote from: chuckb on May 08, 2017, 04:29:06 amThanks for sharing.Looks like the Relay 1 + 6 is located near a heat source.4 + 10 are far away from this.Do you have a plan of the physical location?With best regardsAndreas

author: chuckb
author_rank: Frequent Contributor
timestamp: May 08, 2017, 05:42:24 am
content: TiNThe K182 was logging data as part of a larger Agilent VEE data acq program. It collects voltages from 14 DVMs every 10 seconds. It runs 24/7. I did not want to interrupt the larger program so I had the K2000 automatically step through the channels with a 1 minute dwell on each channel. The data collection and the channel changes were not synchronized but it did not matter for this application. The program recorded 5 or 6 good readings for each channel setting. I did not use the slowest filter on the K182 because of the rapid (1 minute) change of input signal.During another set of tests I also used the the K1801 Nanovolt Preamp (less than 1nV noise) to record emf of a standalone Panasonic latching relay (TXS2) with the normal and alternate contact material. The relay was on a small pcb so the thermals would be realistic. The PCB was in a heavy aluminum case that was insulated with 2" foam. The PCB was laid out so the relay would reverse a signal or pass it straight through the two contacts. The performance was excellent. The relay had less than a 10nv step when reversed. Also the balanced EMF changed less than 5nV per deg C of ambient temperature change. The relay was arraigned so the individual contact thermal EMF was balanced with the other contact. So I don't know what each contact thermal EMF is. But they balance well. Basically the relay was stable within 15nV during 7 hours of testing. I will post that data when I dig it out of the large data file.The next project will be to test the Electroswitch C4 rotary switch later this month.

author: janaf
author_rank: Frequent Contributor
timestamp: May 08, 2017, 03:15:09 pm
content: Quote from: TiN on May 08, 2017, 04:45:09 amThanks!Frankly, I hit the wall after too much work. It took a while to recover...

author: janaf
author_rank: Frequent Contributor
timestamp: May 08, 2017, 03:28:56 pm
content: I have a couple of NI PXI-2503 scanners. Specified as "Thermal EMF (differential) <2 ?V".They use NAIS "TXS2SL-4.5 H11" The H11 may be some manufacturing code, not the -1 option as the scanners are rated 1A.

author: janaf
author_rank: Frequent Contributor
timestamp: May 08, 2017, 03:35:23 pm
content: From the Keithley Model 7168 JFET scanner data sheet:The Keithley Model 7168 is an 8-channel, 2-pole card with <30nV of thermal offset.MAX. SIGNAL LEVEL: 10V, 50mA peak (resistive load only).CONTACT RESISTANCE: <12W.CONTACT POTENTIAL (HI to LO) BETWEEN CHANNELS: <30nV when properly zeroed with supplied leads (see manual for recommended procedure). Typically <60nV withoutzeroing.CONTACT TYPE: Solid state JFET switch.INPUT LEAKAGE: <50pA per channel at 23°C.MAXIMUM VOLTAGE BETWEEN ANY TWO TERMINALS: 10V.CONTACT RESISTANCE: <1ohm.CONTACT POTENTIAL: <200µV.OFFSET CURRENT: <1pA (<30fA typical).Tnd there is a teardown here at EEVhttps://www.eevblog.com/forum/metrology/volt-nut-test-for-eevblog-readers/

author: janaf
author_rank: Frequent Contributor
timestamp: May 08, 2017, 03:54:13 pm
content: i understand you want to use yours to compare references? Same here. Would like to compare a bunch of LTZ boards. Would like to measure differentialy between them. Skipping absolute values for now. Mine are within a 40mV span, so I'd use an instrumentation amp with decent gain.My headache is how to measure difference with the low of the circuit is NOT=GND. They may differ by some (fraction of?) uV? I just may have to ignore that for now.

author: Echo88
author_rank: Frequent Contributor
timestamp: May 08, 2017, 05:38:24 pm
content: Thanks for the data chuckb! Wouldnt have thought that the TX2-relays are this good. I dont intend to change the content of this thread, but would like to ask (since both TiN and chuckB have access to a K182(-M): Do any of you know, if the -M-suffix on the Keithley 182 has any meaning (couldnt found anything about it)?

author: chuckb
author_rank: Frequent Contributor
timestamp: May 09, 2017, 10:09:12 pm
content: Quote from: chuckb on May 08, 2017, 04:29:06 amThe card I used actually had NEC EA2-5TNJ Orange Relays, date code 1411.

author: chuckb
author_rank: Frequent Contributor
timestamp: May 10, 2017, 01:14:43 am
content: Attached is a plot of 3 days of evaluating a Panasonic TXS2-L2-12V-1 relay. This is a small 12v latching relay (L2) with an alternate contact material (-1). I have tested both contact materials in the same setup. The normal contacts seemed to have 5nV random jumpsof the normal contact material. It was hours between the jumps. Either contact material is fine for Zener diode work when you have 1,000nV of noise anyway. The voltage measured is the difference in thermal EMF between the two contacts.You use pulses to drive the coil in a latching relay, so for a few tests I kept power on one coil for 10 seconds. It may have built up 5 nV of extra offset. So it looks like a well designed relay for low signal use.

author: Andreas
author_rank: Super Contributor
timestamp: May 29, 2017, 07:06:48 pm
content: Hello,I have tested my relay multiplexer with TQ2-L2 relays.Sorry no nanovolt meter available so I had to use the K2000 for this.Measurement values averaged over one minute to reduce noise.The multiplexer contains several daisy chained relay contacts.Plus additional one relay to switch the output off (both poles).Measurement is done at the output connector.The active input of channel 1-7 is shorted at the input connector.Channel 8 is a internal short to ground in the Multiplexer.Channel 0 is without multiplexer (simply using the shorting connector to the measurement cable).Result: no more than 0.6uV between the measured channels.with best regardsAndreas

author: chuckb
author_rank: Frequent Contributor
timestamp: June 02, 2017, 05:05:37 am
content: Quote from: chuckb on May 08, 2017, 05:42:24 amThermal EMF evaluation of an Electroswitch C4 rotary switch.The switch and A10 nanovolt preamp were located in an insulated enclosure. This enclosure reduced ambient temperature variations and the rate of temperature change. There was no active temperature control.I used 22 ga solid OFHC copper wire to short the switch poles together. Then the wiper contact was directly connected to the copper input terminal of the preamp.It looks like a single contact of the C4 switch had a 50nV / deg C thermal EMF.When the switch was wired differentially the thermal EMF was less than 5 nV / deg C.When the preamp input was shorted with the same OFHC copper wire the preamp drift with temperature was less than 1 nV / deg C.The Electroswitch C4 looks like a good low themal EMF solution when it’s wired differentially.Timeline for the attached pdf.0-12 hours, C4 switch in position 1, a single contact was measured, there seemed to be 50nV / deg C thermal EMF12-25 hours, the leads to the preamp were manually reversed to isolate the thermal EMF of the switch from a preamp offset. The offset voltage is generated by the switch and wiring.25-26 hours, the C4 switch was selected to position 2. An extra 10nV of offset was introduced.26-37 hours, Preamp input shorted to determine offset voltage and temperature coefficient. The room temperature was reduced 1.5 deg C with maybe a +1nV change in preamp offset.37-53 hours, The C4 switch was rewired to operate differentially. The offset voltage was less and the temperature coefficient was greatly improved. Note: Between hours 37 and 39 it took several hours for the switch contacts to equalize after being soldered.53-60 hours, The C4 switch was in Position 2.60-84 Hours, the C4 switch was in position 3.84-100 hours, the C4 switch was back in position 1. Overall the thermal EMF of the C4 switch was within a 10nV for 60 hours, when operated differentially, even with a 1.5 deg C temperature change.

author: doktor pyta
author_rank: Frequent Contributor
timestamp: June 02, 2017, 09:24:43 am
content: I hope You have seen this:https://doc.xdevs.com/doc/_Metrology/qian2016.pdf

author: Muxr
author_rank: Super Contributor
timestamp: July 11, 2017, 05:25:23 am
content: Forgive me if this is a wrong topic, but I think it's related. I am interested in building a scanner/switch, and I was wondering. Has anyone thought about the feasibility of using chopper amps like the LTC2057 as buffers in parallel and leveraging their ^SD (Shutdown Mode).. to make a "poor man's" scanner?From the datasheet:QuoteBasically only one of the parallel LTC2057 would be on at the time, while the rest would have their ^SD pins pulled down (shut down mode), so channel selection would work this way. ^SD pins can probably be driven with a shift register.I am interested in measuring drift of my various DC references ie. I am not even concerned about absolute accuracy. As long as I have linearity and resolution. Also the switching speed is not a concern since I would only measure a different channel every 5 seconds or so.

author: zhtoor
author_rank: Frequent Contributor
timestamp: July 11, 2017, 12:29:02 pm
content: Quote from: Muxr on July 11, 2017, 05:25:23 amHello,welcome to the "Poor Man's Club"maybe start a thread "Poor Man's Scanner".regards.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 11, 2017, 03:46:32 pm
content: Using the SD pin for a kind of scanner works, if the amplifiers a wired for a gain of 1. The input bias on the inverting inputs would add up to load the divider - but this is not a problem with a gain of 1. With a different gain setting here could be problems, as the input differential can be higher and this might cause higher input currents.However the OPs input is only one side of the signals, so the scanner would be just one pole, not differential. In addition there should be input protection (and maybe some filtering) for the inputs. So it is more than just the LTC2057.

author: Muxr
author_rank: Super Contributor
timestamp: July 11, 2017, 04:45:24 pm
content: Quote from: Kleinstein on July 11, 2017, 03:46:32 pmThanks Kleinstein,Yes I would use a buffer / gain of 1 configuration, I can definitely see the issue of using different gain settings because the inputs would then be coupled to the amplified bus across the amp even when they are shut down. Not to mention expensive resistors etc..For the sake of simplicity I think single pole is fine.I should study some DMM/Nanovolt Meter schematics to see how to implement a sensible protection.Quote from: zhtoor on July 11, 2017, 12:29:02 pmI think I will design and build a simple 8 channel scanner based on this idea and share my findings in a new thread. Thanks!

author: zhtoor
author_rank: Frequent Contributor
timestamp: July 11, 2017, 05:22:21 pm
content: Quote from: Muxr on July 11, 2017, 04:45:24 pmhow about a "flying capacitor" scanner based on some 8 channel differential multiplexer like MPC507A (or equivalent).regards.

author: Muxr
author_rank: Super Contributor
timestamp: July 11, 2017, 05:57:55 pm
content: Quote from: zhtoor on July 11, 2017, 05:22:21 pmI did consider analog multiplexers but I am worried about large voltage and current offsets.MCP507A says this in the datasheet:Although my source impedance will be much lower, I thought going with a tried and true chopper op amp was a known quantity. Worth of consideration though, thanks!

author: Kleinstein
author_rank: Super Contributor
timestamp: July 11, 2017, 06:54:42 pm
content: Unless the Bias currents are really high, the offset voltages due to the R_On are not that high: Typical R_on is more in the 100 Ohms range. it is only the protected types that have extra R_on.The leakage current is typically lower, 10 nA is over full temperature range.When measuring low voltages one usually does not have the full +-15 V across the MUX - this also helps to reduce leakage.A mux with only 8 Inputs would also have less leakage (e.g. half).If you are into very low voltages, one might not need the highest gain precision. So something like a MUX / preamplifier might be still very useful, even if the gain is not super stable. The LTC2057 noise level is lower than some of the DMMs - of causes this also due to not having much protection.So an protected MUX like the MPC 508 (8:1) and an LTC2057 with some optional gain might not be such a bad solution. It has it's limits though: gain drift and accuracy, bias currents, and limited maximum voltage.The advantage of relays is usually the higher voltage capability and less leakage - though thermal EMF can be a problem. Especially it is hard to get good data / estimate on the thermal EMF. Of cause the thermal EMF is not just depending on the relay, it also depends on the thermal setup. Another problem with many relays is the not so well defined performance at very low currents - no such problem with CMOS switches. Getting at least some data on switches / relays it the purpose if this thread.

author: Andreas
author_rank: Super Contributor
timestamp: July 11, 2017, 07:05:58 pm
content: Quote from: Kleinstein on July 11, 2017, 03:46:32 pmHello,be carefully!!!Like many precision OP-Amps the LTC2057 has input protection diodes between the +/- inputsSo you can get a very low impedant path between the output and the postive input even when the output is shut down in a unity gain configuration.Life is not easy, but you can easily kill your LTZ1000.I had only luck that I usually use a capacitive load protection cirquit on all my buffers.with best regardsAndreasThe next thing is: the output leakage is not specified.

author: Muxr
author_rank: Super Contributor
timestamp: July 12, 2017, 03:48:42 am
content: Thanks for the input and advice guys. When all is considered, a mux seems to be the right way to go in order to keep everything simple. Will see how it goes.

author: Andreas
author_rank: Super Contributor
timestamp: July 12, 2017, 05:28:14 am
content: Hello,on my LM399 ageing box (with floating supply voltage) I use MAX4051A multiplexers in a differential configuration.They are low ohmic and have relative low leakage. (but are limited to 16V total supply voltage)with best regardsAndreas

author: Muxr
author_rank: Super Contributor
timestamp: July 12, 2017, 05:32:41 am
content: Really cool Andreas! I was looking at TI's MUX506 or MUX507 as well which can handle 36V, but they don't mention much about protection, they also advertise low leakage. I don't really need anything above 16V.

author: Andreas
author_rank: Super Contributor
timestamp: July 12, 2017, 09:34:42 pm
content: What do you mean with protection?Most of the muxes have no protection against input voltages above supply voltage.I am now trying some FETs (BSS138) on my 2:1 dividers to protect the unbuffered LTZs if I have a flat battery/bad battery contact on my ADCs.On my relay mux I have high ohmic output resistors which can be shorted if the output voltage is within 1 mV of the expected value.With best regardsAndreas

author: Muxr
author_rank: Super Contributor
timestamp: July 12, 2017, 10:04:03 pm
content: Indeed, I am finding that most analog muxes don't have protection. The MCP508 zhtoor suggested does however:

author: Andreas
author_rank: Super Contributor
timestamp: July 13, 2017, 05:05:46 am
content: Hello,for a unbuffered LTZ a 1K resistor is relative low (compared to the 7V/4mA zener current).I want a cirquit that limits the current below 2mA.On the other side: a 1 K series resistor (additionally to the switch of 300-500 Ohms) is relative high when regarding leakage currents of up to 10 nA.with best regardsAndreas

author: Muxr
author_rank: Super Contributor
timestamp: July 13, 2017, 01:52:41 pm
content: Ahh, I see. All the references I will be measuring are buffered. Just the hodge podge of all the references I own and perhaps some more I make in the future.

author: doktor pyta
author_rank: Frequent Contributor
timestamp: December 05, 2017, 12:08:59 am
content: Just to have this in metrology section /if someone reads only metrology/:https://www.eevblog.com/forum/projects/diy-low-thermal-emf-switchscanner-for-comparisons-of-voltage-and-resistor-stand/msg1366769/#msg1366769

author: janaf
author_rank: Frequent Contributor
timestamp: February 06, 2019, 01:34:48 pm
content: How do you measure EMF of relays?Just zero the voltmeter, short the inputs, and then connect a (closed) relay and take a reading? Or what?I saw this method somewhere (picture): connect a relay/switch, open and close the relay, take readings, the difference would be due to switch EMF. What's do you think is a reasonable resistor value, low to keep noise down but high enough not to load down the EMF? 1K?(Otherwise, I kind of like that Japanese paper; it seems they measured a number of relays, selected pairs, two per channel, that more or less cancel each others EMF).

author: janaf
author_rank: Frequent Contributor
timestamp: February 06, 2019, 01:42:01 pm
content: ...and the picture.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 06, 2019, 05:22:35 pm
content: 1 K might be more on the high side - the switch resistance should be much lower than even 10 Ohms. I would not be worried so much about noise, more about bias current. 1 K and 100 pA would already be 100 nV. I would be less worried about the loading effect that might change a few percent of the voltage at most.The other point would be possible thermal EMF from the resistor, if not at a constant temperature.I would consider testing a few contacts at a time, by switching between the contacts.If temperature is really constant all over the voltage reading should be zero, even for poor contacts. So one would also get a kind of typical, not so good thermal condition. How this is defined is not clear to me. Anyway a few temperature readings would be needed. With a mono-stable relay the self heating of the could could be the relevant heat source - so maybe check with a voltage close to the upper limit to get more effect.

author: janaf
author_rank: Frequent Contributor
timestamp: February 06, 2019, 09:30:16 pm
content: I'd use a 7.5 digit DMM, don't think there'd be anything like 100pA there but would have to check what it looks like...Once I tested thermocouples and was surprised what load they could drive.