do analog switches break alot? - Page 1
Forum section: electronics / projects

author: coppercone2
author_rank: Super Contributor
timestamp: April 27, 2025, 10:45:20 am
content: IIRC, people agressively prosecute analog switches in keithley meter repairs. Like the DG#xyz chips from Analog.. its almost the same recommendation as electrolytic capacitor replacement... possibly in other meters too (maybe HP 34401a and others... not sure)In the back of my head I seem to recall these being brought up ALOT for different repair projects too over the years.Is there as flaw in implementation with them? some bad chips going around? Or just coincidence? Hard to search because it just kind of dawned on me that I think I have seen alot of posts about replacing them over the years. It just stands out to me that I see it and think "oh its this fucking thing again..."my thinking almost goes to : is it precise and ranging? bet I am going to see someone mention its been fixed by replacing a analog switch.

author: moffy
author_rank: Super Contributor
timestamp: April 27, 2025, 10:57:02 am
content: Personal experience, DG switches are easy to break, things might have improved though.

author: coppercone2
author_rank: Super Contributor
timestamp: April 27, 2025, 10:59:49 am
content: they went to DG+ as a replacement. It just has a + on the end of it with same specs.It makes me think something was wrong with it.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 27, 2025, 07:14:20 pm
content: The early CMOS switches where a bit prone to latchup, e.g. from ESD or otherwise exceeding the supply. This can destroy the chip. The newer versions are supposedly less sensitive and may also have less leakage. E.g. the DG201 was replaced with a DG201B (and looks like later D and E) with better specs. The name details may be different from manufacturer to manufacturer. The old first generation parts may no longer be available, at least not so easy.In DMMs some of the DG... switches are somewhat exposed to the input and may fail for this reason.They can still be fine even if old, but they are candidates to check - though not not so much for replacement, just because the are old.

author: David Hess
author_rank: Super Contributor
timestamp: April 27, 2025, 09:09:52 pm
content: Analog switches are not inherently unreliable or fragile, but they are often exposed to ESD or adverse operating conditions.

author: coppercone2
author_rank: Super Contributor
timestamp: April 27, 2025, 10:16:57 pm
content: but is the DMM protection not typically first class?

author: David Hess
author_rank: Super Contributor
timestamp: April 28, 2025, 03:05:35 am
content: Quote from: coppercone2 on April 27, 2025, 10:16:57 pmNo, DMM protection is often "this is as good as we can do within the constraints", and there are some things which are easy to overlook. Individual parts are being operated outside of their recommended specifications so circuit design and physical layout have to be accompanied by parts qualification.For instance there will be an unbypassed series resistance (1) before the input multiplexor and shunt protection, however this is rarely a high voltage resistor and it is subject to voltage breakdown during an ESD event directly connecting the input to the input multiplexor. The shunt protection at the multiplexor may not be fast enough, or low impedance enough, to prevent damage to the multiplexor.Any shunt protection added before this series resistor can make things worse by adding capacitance, which increases the discharge energy into the multiplexor if the series resistor suffers breakdown.(1) Oscilloscopes also do this for input protection, but the series resistance must by bypassed with a capacitor, typically around 1000 picofarads, for good AC response.

author: coppercone2
author_rank: Super Contributor
timestamp: April 28, 2025, 03:14:17 am
content: Damn I knew it I often wondered about resistors in input circuit (current limiting). I know they might not break but if its arcing internally then thats no good. The only thing I do there is spec some rather HV caps (some people will throw in regular SMT caps for this). Supposedly you can get the ESD rated ones too.That really is something that people push under the rug. It might not be doing what you think its doing.So the "nothing but a small change in PPM value" that people say happens from ESD might make you feel good because its not a resistor that effects precision, but that also might mean that its not doing its job.Thats a really interesting thing about the resistorLmfao I checked high voltage rated options for TH and I can see that someone might be able to 'optimize' a BOM to get rid of that in a design if no one is looking!!

author: David Hess
author_rank: Super Contributor
timestamp: April 28, 2025, 03:29:50 am
content: Quote from: coppercone2 on April 28, 2025, 03:14:17 am"Nothing but a small change" is interesting because in applications like these, a small change can indicate complete but almost unnoticeable failure. I have a couple of DMMs waiting for repair that "only" have errors of a few counts, but that is outside of the expected worst case performance and I have verified failure in each case.If I have an ADC design and I calculate the expected input noise to be less than a count, but the reading is varying by 2 or 3 counts which seems like an insignificant "small change", then either my calculation is wrong, or something is broken in the design, like the ADC clock has too much jitter or the input protection circuit (or printed circuit board) has too much leakage. I am very gratified when the empirical measurement of performance matches my calculations.

author: coppercone2
author_rank: Super Contributor
timestamp: April 28, 2025, 03:34:05 am
content: What do you think a good voltage rating is for a part that is supposed to do this? Like if you are picking parts

author: David Hess
author_rank: Super Contributor
timestamp: April 28, 2025, 04:53:19 am
content: Quote from: coppercone2 on April 28, 2025, 03:34:05 amI would face the same problem that the designers did, and there is a gotcha I have seen before.Older designs used carbon composition resistors because of their higher than normal breakdown voltage ratings, their benign failure modes, and low parasitic elements. Ceramic composition resistors are available with the right values, but are pretty expensive. There are small high voltage metal film resistors like the HV and HVR series;I like these. And then there are very high voltage thick film resistors.The gotcha is that if a higher voltage resistor breaks down, then the energy stored in the parasitic capacitance before the resistor is greater, as in proportional to the voltage squared greater, so the ESD resistance can be *worse*. The higher breakdown voltage however allows protection elements before the resistor, like gas discharge tubes, more time to activate.In practice the circuit elements need to be carefully modeled, and variations in the design need to be tested to destruction to see which combination of qualified parts works best.

author: coppercone2
author_rank: Super Contributor
timestamp: April 28, 2025, 05:33:06 am
content: OK those are reasonable, some of the HV ones in reasonable sizes have a similar cost to ceramic composition, though I am thinking they might not be a bad choice anyway if its important

author: David Hess
author_rank: Super Contributor
timestamp: April 28, 2025, 12:25:11 pm
content: The most common solution might be to use several normal resistors in series to raise the total voltage rating.