Analog switch beginners questions - Page 1
Forum section: electronics / beginners

author: Chris42
author_rank: Contributor
timestamp: December 09, 2021, 12:16:20 pm
content: HelloI'm building analog multiplexer and demultiplexer circuit and since I have never used analog switches before, I want to make sure that I understand the operation correctly before I order 50 switches from mouser.What I want to have is a single pole double throw switch that can conduct in both directions with minimal distortions. I have chosen NX3L4053 "Triple low-ohmic single-pole double-throw analog switch" (https://www.nxp.com/docs/en/data-sheet/NX3L4053.pdf).My understanding is that as long as voltage on Z, Y0 and Y1 will be in range from 0V to VCC, switch will act like a regular mechanical single pole double throw switch and I will be able to apply signal to Z and get it on selected Y and other way around, apply signal to selected Y and get it on Z. Of course there will be nonlinear transfer function but that is not a problem in my case.My understanding is also that switch will not conduct when signal voltage is below 0V. About that I'm not sure. Datasheet does not specify that explicitly.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 09, 2021, 01:09:43 pm
content: The CMOS switches would also conduct with a slightly negative voltage. With most types (and very likely also the given type) there are diodes from all pins to the supplies. So below some -400 mV the diode to GND will start to conduct too.

author: Zero999
author_rank: Super Contributor
timestamp: December 09, 2021, 01:38:56 pm
content: The general rule is that with analogue switches, the signal must fall within the power supply rails. There are a couple of exceptions: some devices have a built-in charge pump, to generate an internal negative voltage, which allows it to switch signals below the negative rail and quite often you can get away with a couple of hundred mV above and below the positive and negative rails, although leakage and distortion will increase significantly, if you push it too far.Another thing to bear in mind is the on resistance varies, depending on the supply voltage and the voltage on the switch, with respect to the supply voltage. The higher the supply voltage, the lower the resistance. Maximum on resistance occurs, at the lowest supply voltage and when the signal is half way between the supply rails. This is because the switch consists of two MOSFETs and the greater the gate-source/drain voltage, the lower the on resistance. If the load resistance is significant, this can cause distortion, as the switch forms part of a potential divider, which varies with the signal level. Refer to pages 8 and 9 of the data sheet.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 09, 2021, 02:07:50 pm
content: Also note charge injection, which is something not present in mechanical switches.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: December 09, 2021, 02:33:28 pm
content: Quote from: Chris42 on December 09, 2021, 12:16:20 pmQuote from: Chris42 on December 09, 2021, 12:16:20 pmStrictly speaking these two statements are mutually exclusive.Also you might need to pay attention to the capacitive non-linearities which would introduce an increase in distortion at higher frequencies. The switches you have chosen are low resistance but high capacitance so suitable for relatively low impedance circuits (well below 1K source/load impedance).CheersAlex

author: Terry Bites
author_rank: Super Contributor
timestamp: December 09, 2021, 05:42:16 pm
content: What amplitude are the signals and what power supplies will be used?