MUX for different voltage levels - Page 1
Forum section: electronics / projects

author: okw
author_rank: Frequent Contributor
timestamp: September 02, 2023, 12:03:08 pm
content: I have a project which has two inputs. Based on what signal the user inputs, a MUX (or something else (not relay - it needs to me ultra small and very low power)) directs it to the appropriate pins of the MCU or to a conversion chip. The directing is configured by the user before connecting. The three choices of signals are:-differential signal (two lines): +2.5 idle and 0V/5V (high). Signal must be unaltered through the switch.-0V low, 12V high. (only one signal line)-0V low, 7V high. (only one signal line)I have 12V, 5V, 4.2V and 3.3V rails available in the project.The 7V and 12V signals are conditioned and protected before entering the MCU.How can I have only 2 inputs, then MUX such various signals? I can't divide any signals before MUX because of the differential signal. And most MUX seems to only accept signals lower than their VCC.

author: Alti
author_rank: Frequent Contributor
timestamp: September 02, 2023, 12:46:25 pm
content: If you have 12V available, why not ordinary cmos mux?Even CD4053 is capable of switching 15V no problem.

author: Zero999
author_rank: Super Contributor
timestamp: September 02, 2023, 12:47:44 pm
content: Sorry, your post doesn't make any sense.What input and output voltages are required?An analogue multiplexer such as the 4053 will work up to 12V, as mentioned above, can be used to select between different analogue signals and will work up to 12V.

author: okw
author_rank: Frequent Contributor
timestamp: September 02, 2023, 12:50:54 pm
content: Quote from: Zero999 on September 02, 2023, 12:47:44 pmThe signals must be unaltered, as said.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 02, 2023, 12:54:24 pm
content: If the signal goes to the µC, it much be less than some 5 V anyway. So the 12 V and 7 V signal should first be reduced to some 4-5 V and only than a MUX can chooise. As the reduction from 12 to 5 V is easy (a divider) one can do this before the MUX. The MUX can than be something simple like 4051.

author: okw
author_rank: Frequent Contributor
timestamp: September 02, 2023, 01:05:08 pm
content: Quote from: Kleinstein on September 02, 2023, 12:54:24 pmI know. I reduce it on the other side of the MUX. If I divide before the MUX, the differential signal will be destroyed. This cannot be altered.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 02, 2023, 03:34:03 pm
content: It is true that essentially all MUX can only work with signals within there supply, but the control signal can be smaller and a 12 V supply seems to be available.If the divider can be behind the MUX than it can also be in front of the mux - one just needs more of them, like 1 per input. A divider behind the mux is actually more of a problem as the MUX has some voltage dependent resistance.

author: okw
author_rank: Frequent Contributor
timestamp: September 04, 2023, 08:39:22 am
content: Quote from: Kleinstein on September 02, 2023, 03:34:03 pmThen you have to explain me how to implement the divider, and what happens if I feed a differential signal (idle/low 2.5V, 0/5V high) to it, which should be more of less unaltered through the switching.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: September 04, 2023, 09:43:24 am
content: Quote from: okw on September 02, 2023, 12:03:08 pmSo you mean the MCU has differential drivers inbuilt ?You need to specify the MHz and pF and ohms this can tolerate. What is the termination and drive current ? : There is no such thing as "unaltered through the switch"Quote from: okw on September 02, 2023, 12:03:08 pmAgain you need to specify a source impedance, load impedance, speed and desired threshold voltage, and any hysteresis.Do these signal spend any significant time between their limits ?If they are not fast, but slew slowly, the V-I curve of the buffer matters.

author: okw
author_rank: Frequent Contributor
timestamp: September 04, 2023, 09:54:40 am
content: Quote from: PCB.Wiz on September 04, 2023, 09:43:24 amNo, I use a driver than handles the communication for the diff signal. It can handle 40MHz, but I only use it at 1MHz now. Termination is 120 ohms, not sure about drive current. I know, not theoretical, that's why I wrote "more of less". The other posters idea of voltage dividing the 12V with the same divider as the 2.5V diff signal would completely ruin the signal.Quote from: PCB.Wiz on September 04, 2023, 09:43:24 am12V and 7V are both UART (19200 baud). I don't have any other specs at the moment.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 04, 2023, 10:00:52 am
content: Many of the MUX chips are not that robust to latch-up. So I would prefer to have the digital transceivers / drivers before the MUX and use a mux for the logoc level signals only.Also driving a 120 ohm terminated signal (e.g. ISDN, RS485) could be effected by the switch resistance. So one would need a rather low resistance MUX.It is not about using the same divider for all signals, but about an individual divider or tranceiver before the MUX. With different signals one may have to do that anyway.

author: okw
author_rank: Frequent Contributor
timestamp: September 04, 2023, 10:06:45 am
content: Quote from: Kleinstein on September 04, 2023, 10:00:52 amHow can I have individual dividers when I only have to lines in and out of the box, and any of the three signal types can come over these two lines (one line will be only used for one of the differential signal lines, the other line will carry any of the three (diff.signal/7V uart/12V uart).

author: David Hess
author_rank: Super Contributor
timestamp: September 04, 2023, 03:30:52 pm
content: There is no need to multiplex the signals before conditioning and level shifting. Do all of the conditioning and level shifting in parallel, and then multiplex the signals afterwards immediately before the digital inputs which will only require a digital multiplexor.This means the lower voltage signal conditioning must be tolerant of higher voltages, but this is not difficult.

author: tooki
author_rank: Super Contributor
timestamp: September 04, 2023, 03:47:47 pm
content: Quote from: okw on September 04, 2023, 10:06:45 amCould you draw a block diagram or something? And give a proper description of what you are trying to accomplish? (That is, explain your goals and requirements, not your attempted solution. Cf. "X-Y problem".)

author: EPAIII
author_rank: Super Contributor
timestamp: September 04, 2023, 10:05:25 pm
content: How about the CD4066?https://www.ti.com/lit/ds/symlink/cd4066b-mil.pdf?HQS=dis-mous-null-mousermode-dsf-pf-null-wwe&ts=1693864668285&ref_url=https%253A%252F%252Fwww.mouser.com%252FThey come with a variety of supply Voltages, on resistance, and frequency ratings. And they can be used for both analog and digital switching (mux).https://www.mouser.com/c/semiconductors/switch-ics/analog-switch-ics/?q=4066&supply%20voltage%20-%20max=12%20V~~18%20V&rp=semiconductors%2Fswitch-ics%2Fanalog-switch-ics%7C~Supply%20Voltage%20-%20Max

author: PCB.Wiz
author_rank: Super Contributor
timestamp: September 04, 2023, 10:06:27 pm
content: Quote from: okw on September 04, 2023, 09:54:40 amSince these are COMs links, you likely also need to also cover common mode tolerance and ESD tolerance.It may be easier and simpler to just choose suitable BUS drivers. These are made in large volumes and already solve the comms link needs.eg LIN drivers can manage the 12V/7V uart and some can tolerate -27V ~ 40V on the pins.CAN/RS485 drivers have +/- 12V common mode range and also lightly load the pins when off.some exampleshttps://www.lumissil.com/products/interfaceThere are also dual RS232/RS485 transceivers, but RS232 is +/- drive, unlike LIN which is nominally 0~12VIf your remote can tolerate a negative drive in, that could be another choice.eg a SP330E RS232-in switches at +1.5~1.8V so it can tolerate single-ended LIN style drive.

author: okw
author_rank: Frequent Contributor
timestamp: September 05, 2023, 10:37:03 am
content: This is the basic idea. All comms are duplex (both diff driver and uart converter support this).

author: DavidAlfa
author_rank: Super Contributor
timestamp: September 05, 2023, 11:06:49 am
content: 12V with 120ohm termination? Not for a 4066... Use a proper transceiver.

author: okw
author_rank: Frequent Contributor
timestamp: September 05, 2023, 11:14:22 am
content: Quote from: DavidAlfa on September 05, 2023, 11:06:49 amNo, 12V is not terminated. 12V and 7V signals are uart.The 2.5V differential signal is 120ohms terminated.

author: ledtester
author_rank: Super Contributor
timestamp: September 05, 2023, 06:37:13 pm
content: I have a question ... does the transmitting system and MCU share a common ground? For UART traffic I'm curious what is signal A is relative to, B or a common ground?

author: okw
author_rank: Frequent Contributor
timestamp: September 05, 2023, 06:40:56 pm
content: All share common ground.

author: okw
author_rank: Frequent Contributor
timestamp: September 20, 2023, 09:15:52 am
content: *bump*