Relay / fet matrices for low current / voltage switching? - Page 1
Forum section: electronics / general

author: alanambrose
author_rank: Frequent Contributor
timestamp: August 14, 2018, 08:34:29 am
content: Hi,I have three related questions:(a) What are your go-to switching matrices for switching with better than 10uV thermal EMF?(b) Does anyone have any leads on the documented behaviour of relay contacts / contact materials when switching low voltages / currents?(c) Failing off-the-shelf solutions, are there any go-to designs for low voltage switching (i.e. uV with max 5V) into high-ish impedances (i.e. 10M ohms)?I'm looking for 25, 50 and 64 channel matrices - but only the 64 channel is critical. All leads welcomeTIA, Alan

author: Zucca
author_rank: Unknown
timestamp: August 14, 2018, 08:38:34 am
content: What are your low current spec?

author: alanambrose
author_rank: Frequent Contributor
timestamp: August 14, 2018, 09:09:10 am
content: >>> What are your low current spec?Good questionand we need do to some tests - this is an opt101 photo-diode but with v. low light levels on particular IR wavelengths.Alan

author: Alex Nikitin
author_rank: Super Contributor
timestamp: August 14, 2018, 09:32:18 am
content: https://www.pickeringrelay.com/applications/low-thermal-emf/CheersAlex

author: alanambrose
author_rank: Frequent Contributor
timestamp: August 14, 2018, 11:00:58 am
content: Thanks Alex.FYI, for anyone else looking at this in the future, there's a doc here which gives a bit more analysis of relay contacts re low thermal:https://www.mouser.com/pdfDocs/switching-low-offset-voltages-training.pdfAlan

author: Kleinstein
author_rank: Super Contributor
timestamp: August 14, 2018, 11:54:46 am
content: With so many channels an a rather low voltage, I would consider CMOS switches in IC form (like chips for a 8:1 MUX or even 16:1 MUX). The big down-side is possible leakage current. Due to usually very low power consumption and a compact design temperature differences can be kept low.Switching between photodiodes usually is not that critical with respect to voltage, it's more like a current sensitive case.

author: Echo88
author_rank: Frequent Contributor
timestamp: August 14, 2018, 12:40:22 pm
content: Thats the most comprehensive document on low-thermal-relay-construction ive ever seen and it even states the recommended manufacturer test procedure. Thanks.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: August 14, 2018, 02:27:29 pm
content: Quote from: alanambrose on August 14, 2018, 09:09:10 amFrom the datasheet that photo-diode + amplifier combination is rather noisy, with 300uV RMS noise in 20kHz BW (for a comparison the 1M feedback resistor thermal noise in the same bandwidth is ~18uV at room temperature). If you switch the amp outputs you will have more problems with the amps noise, offset and drift than with thermal voltages IMHO.CheersAlex

author: rhb
author_rank: Super Contributor
timestamp: August 14, 2018, 03:34:21 pm
content: Has anyone documented the thermal EMF of the 44421A and 44422A relay boards from the 3497A?

author: martinr33
author_rank: Frequent Contributor
timestamp: August 15, 2018, 12:55:38 am
content: The quad CMOS switches that seem to populate precision meters might help:https://www.maximintegrated.com/en/products/analog/analog-switches-multiplexers/MAX333A.htmlI'd be more worried about thermals around the connections.Keithley scanners /meters with scan options are reasonably inexpensive. Might be possible to switch out the relays with CMOS switches (maybe a little patch PCB). Then, you'll have a manageable package. 10uV isn't too hard to deal with.There are not so many of them on eBay right now, but they do show up.The bigger scanners have a lot more channels. The thermal offset is kind of in range as well.https://www.conres.com/test-equipment/wp-content/uploads/sites/4/2014/04/selector_guide_7001_7002.pdf

author: feedback.loop
author_rank: Frequent Contributor
timestamp: August 15, 2018, 04:09:54 am
content: How about Agilent 3499A/B/C + some multiplexing module like 44470A?3499B is a small unit with two slots for modules. 3499A is a bigger unit with five slots.They can be found for under $100. Modules are also cheap enough.Typical thermal EMF of 44470A relay module is specified as < 3uVBe warned that many modules are sold on ebay with no terminal blocks.

author: alanambrose
author_rank: Frequent Contributor
timestamp: August 15, 2018, 04:14:25 pm
content: As ever, some quality insight and experiencemany thanks to everyone who gave advice.For speed, I think we'll try to pull in some off-the-shelf matrices and then develop our own custom ones as and when we've been able to characterise a bit better.Probably 2 off 34972A plus 4 off 34908A 40-channel (3uV) or something similar.Good point Alex about whether we're being too ambitious on the scanning given these particular photodiodes.I'll keep you posted.Alan

author: splin
author_rank: Frequent Contributor
timestamp: August 16, 2018, 12:27:28 am
content: Quote from: martinr33 on August 15, 2018, 12:55:38 amThat won't cut the mustard - the spec shows a typical ON leakage of 200pA; combined with the 10M source impedance requirement that gives a 2mV offset! And that will vary drastically with temperature.The best such part I know of is the TI MUX36S08 with a typical ON leakage of 8pA. Still too high for this application but the leakage is a function of the input voltage relative to the supply voltages so actual performance may be better in this case. The scales of the leakage v temperature diagram in the datasheet are not helpful for room temperature applications but a more expanded graph is shown in figure 5 here:http://e2e.ti.com/blogs_/archives/b/precisionhub/archive/2016/02/19/does-a-low-leakage-multiplexer-really-matter-in-a-high-impedance-plc-systemThe 2N/PN/SST4117A Jfet (< 1pA leakage) might just do it but they are expensive, hard to get and require a lot of support circuitry. The MBM4117 is cheap but has a worse leakage spec - but possibly only because it isn't production tested.

author: feedback.loop
author_rank: Frequent Contributor
timestamp: August 16, 2018, 03:53:43 am
content: Check this out:https://www.ebay.com/itm/Agilent-3499A-Switch-Control-System-with-N2261A-40-Channel-Relay-Plugin-Card-DIM/352018520458

author: EmmanuelFaure
author_rank: Regular Contributor
timestamp: August 16, 2018, 04:39:28 pm
content: Quote from: splin on August 16, 2018, 12:27:28 amMAX326/327 : Max leakage 10pA, typical < 1pA.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 16, 2018, 08:44:04 pm
content: The max326/327 are low leakage, but also relatively high resistance. It dependents on the application if this is ok. The mux36S08 looks good - low leakage and much lower on resistance.