Analog switch noise contribution - Page 1
Forum section: electronics / projects

author: grouchobyte
author_rank: Regular Contributor
timestamp: March 26, 2016, 06:00:40 pm
content: I have an ultra-low noise application coming up that requires some passive filters for bandshaping that must be switched in and out. There is no room for reed relays and so my only option is to use analog switches. The signal bandwidths are in the single megahertz range and there is literally no commutating going on so charge injection is moot.Since these filters are way up near the analog front end (optical), the noise floor is already at sub-nv per root Hz levels and the signal is in the range of 10's to 100's of microvolts? I am a liitle worried the the analog switch(s)might have a noise contribution level that would compromise performance. Obviously its not spec'd, nor would I expect it to beHas anyone characterized analog switches in this way or have some thoughts regarding this? Analog devices was stumped and TI had some interesting points. They both hinted that the analog switch would contribute shot and johnson noise and would have negligible impact to the noise floor.I could move the filters downstream after some gain stages resulting in a smaller affect, but would prefer to have them up front. Do you agree?

author: timb
author_rank: Super Contributor
timestamp: March 26, 2016, 06:44:24 pm
content: You might start by looking at the schematics for precision 5.5 and 6.5 digit multimeters and see what sort of analog switches they used. I know most of the time discrete JFETs were used versus monolithic switch ICs due to low leakage and noise. However, things have certainly changed in the intervening 20-30 years since that equipment was designed.

author: Marco
author_rank: Super Contributor
timestamp: March 26, 2016, 07:19:55 pm
content: I'd just select a couple to test based on the easy requirements which you can meet from datasheet specs, ie. <10 Ohm to keep Johnson noise deep below a nv/sqrtHz and whatever capacitance it needs to be to meet your isolation requirements.Intuitively I'd say shot noise and 1/f noise will be bugger all. JFETs especially should act very close to a pure resistor, but all analog multiplexers/switches nowadays are CMOS.

author: grouchobyte
author_rank: Regular Contributor
timestamp: March 26, 2016, 07:28:02 pm
content: Quote from: evb149 on March 26, 2016, 07:17:52 pmGood and very valid points! Thank you. I am certainly considering those things but it never hurts to hear them again. FYI: My power supplies are uber clean. I characterized the noise on a spectrum analyzer and it is down below -129 dbm across a very wide spectral range

author: Andy Watson
author_rank: Super Contributor
timestamp: March 26, 2016, 07:40:01 pm
content: Some years ago I designed a system that used switches to change the gain of at differential amplifier - so the switches were sat on the front end. Like you, I had a similar conversation with the manufacturers (Maxim ?) and we came to the same conclusion, i.e. that there would be at least an equivalent Johnson noise contribution relating to the "on" resistance of the switch. The switches were surrounded with circuit impedances of 100-1000 ohms. All I can say is that any contribution the switches made was insignificant/unnoticed compared to the resistance of our source - 1-4k.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 27, 2016, 06:32:43 am
content: What is system impedance (Zo) in the relevant locations?Can you afford a preamp first?You'll get best noise performance in both states when the on resistance (which will exhibit regular Johnson noise) is the same ratio from Zo as the leakage resistance equivalent is. Which is the noise figure, so you'd have to convert the leakage shot noise into an equivalent thermal resistance.FETs come in a variety of sizes, where a wider junction has lower resistance but more leakage; so you simply pick the one that's most evenly balanced around Zo.Or if you have a priority (expecting one switch to be on a majority of the time, or optimizing noise versus bandwidth options?), you can select different ratios based on that.The worst part will be that leakage is exponential with temperature, so things will be more problematic if you have to tolerate high ambient as well.Tim