Noise of analog switch ICs - Page 1
Forum section: electronics / projects

author: ricko_uk
author_rank: Super Contributor
timestamp: February 02, 2020, 03:48:36 am
content: Hi,I have two coils at the bottom of an AC-excited Wheatstone bridge and the variation/measurement between those points (classic Wheatstone bridge measurement) is only uV.So they are fed into an instrumentation amplifier. The application is clearly very low noise.I am considering multiplexing various Wheatstone bridges into a single Instrumentation Amplifier through analog multiplexers/switches. But I cannot find details of the noise contribution of such switches.Does anybody have any information about this? Any info much appreciated!Thank you

author: moffy
author_rank: Super Contributor
timestamp: February 02, 2020, 04:31:54 am
content: I would think crosstalk would be more of the issue, but if you lower the input impedance they feed into it can be reduced. The article:https://www.analog.com/media/en/training-seminars/tutorials/MT-088.pdftalks about switch basics.

author: OwO
author_rank: Super Contributor
timestamp: February 02, 2020, 04:55:07 am
content: Usually negligible and is determined by the insertion loss of the switch. I recommend using RF switches instead of analog switches because they will have far lower parasitics. I've tested the MXD8641 and MXD8625C to work down to DC and can pass -1.5V to +1.5V.

author: SilverSolder
author_rank: Super Contributor
timestamp: February 02, 2020, 01:59:17 pm
content: Quote from: OwO on February 02, 2020, 04:55:07 amLooks like an interesting part.Did you check charge injection and other misbehaviours, just out of interest?

author: ricko_uk
author_rank: Super Contributor
timestamp: February 02, 2020, 11:15:52 pm
content: Thank you all for the feedback!

author: OwO
author_rank: Super Contributor
timestamp: February 03, 2020, 05:05:31 am
content: Haven't tested charge injection, but the unselected ports do get shorted to ground until they are selected, which seems to be the case on all RF switches I've tested.

author: sam[PS]
author_rank: Contributor
timestamp: February 28, 2020, 07:26:52 am
content: Quote from: OwO on February 03, 2020, 05:05:31 amAn even better design is to have a T arrangement with 3 cmos switch for each input. First two cmos switch are used in invert connecting to either the input or ground, then the third switch connect to the output and is switched with a slight delay, i think they call that make before break switching. That ensure to limit parasitic effect to minimum. Some IC use exactly this but you can also do it using two switch, then using switches from different chip solve the crosstalk issue.As it comes to the internal noise of a cmos switch it's basically 2x noise of a transistor, so it's pretty low.Just my 2cts...

author: splin
author_rank: Frequent Contributor
timestamp: March 01, 2020, 03:35:49 am
content: An interesting question which it seems it's hard to find any good answers. Analog switch noise may be negligable foir most applications but inevitably there comes a point where it becomes an issue.The most obvious noise source is the thermal noise of the channel resistance of the switch, so the lower resistance the switch the better. But lower resistance switches wil be physically bigger and suffer higher leakage currents which I assume will contribute shot noise. I have no idea if this is significant but this paper might help:https://www.nikhef.nl/~jds/vlsi/noise/sansen.pdfAnother important factor is flicker noise. This might not be a problem for your application, depending on your operating frequency, but generally the 1/f noise of a resistor is:1) A function of the resistor type - wirewound/bulk metal foil have almost no flicker noise, unlike silicon diffused resistors.2) Inversely proportional to the physical size of the resistor3) Proportional (approximately) to the square root of the resistor value (according to Doug Self)4) Proportional to the applied voltage.Unfortunately almost no analog switch datasheets have anything to say about noise; I did once find one which did, but maddeningly I can't find it now. It was a pretty mundane cmos switch, something like a 4061.This might be of some (small) interest:https://ez.analog.com/switches_multiplexers/f/q-a/77163/is-spectral-noise-density-nv-rthz-information-available-for-adi-analog-switches-muxesIn particular the bit:QuoteI guess you're essentially on your own here. It would be great if you would share any noise measurements you make should you investigate the issue further.

author: splin
author_rank: Frequent Contributor
timestamp: March 03, 2020, 12:33:30 am
content: I suggest you have a look the Linear Technology LTC201A/LTC202/LTC203 datasheet which shows a low noise application:https://www.analog.com/media/en/technical-documentation/data-sheets/201a23fb.pdfI'm not suggesting it's the best part for your application but it gives an idea of what noise levels can be achieved - 40nVpp of which the LT1028's spec is 30nVpp.Another noise contribution may be power supply noise coupled through to the signal via gate - drain/source capacitance and other parasitics. The lower the switch resistance, the bigger the switches, the higher the capacitance. Probably not significant in most cases, especially for low impedance signals.