Multiplexing oscilloscope inputs? - Page 1
Forum section: electronics / projects

author: mph7
author_rank: Newbie
timestamp: January 11, 2022, 05:06:14 am
content: Hi all,I'm in a bit of a bind and could use some tips. I'm trying to create an analog signal multiplexer. The idea is to take 16 inputs, have one output, and be able to route any input to the output. Ideally I'd have more output channels, but I'll start with one. The output will be connected to an oscilloscope input (or other test equipment).I designed a board around the MAX14661, however I'm getting an insane amount of crosstalk between input channels, even with the chip depopulated. So, something wrong with the layout. The traces are all spaced at least 3x the line width apart. The signal lines are microstrips on the top layer, and there is a ground plane on the 3rd layer. And then, do I need to match the output impedance to 50 ohms to match the oscilloscope?Besides moving the ground plane up to the 2nd layer, I don't know what to do. The signal traces are about as far apart as they can be. I've read that guard traces shorted to ground are actually counter-productive with microstrips in terms of crosstalk, however I see this commonly in RF circuits. Furthermore, I'm looking at pre-existing analog mux PCBs like Sparkfun's CD74HC4067 breakout, and they don't seem to be using proper trace spacing at all.My bandwidth target is 20MHz but I'm seeing almost 1Vpp crosstalk on a trace adjacent to a 5Vpp square wave at 10KHz.Any ideas for what I could be doing better?

author: mazurov
author_rank: Frequent Contributor
timestamp: January 11, 2022, 05:38:23 am
content: I use old analog scope(s) for this. Tektronix 7000 series (as well as many others) will happily output all their channels to the signal out connector simultaneously. I can then route the output to the input of my 2ch Rigol and have 4 traces. The trace color will be the same for all 4.

author: Haenk
author_rank: Super Contributor
timestamp: January 11, 2022, 08:10:44 am
content: Here is the reference design:https://www.maximintegrated.com/en/products/analog/analog-switches-multiplexers/MAX14661PMB1.htmlI'd say it is not great for anything much higher than audio frequencies, certainly not really suitable in itself for MHz signals:https://datasheets.maximintegrated.com/en/ds/MAX14661.pdf

author: CaptDon
author_rank: Super Contributor
timestamp: January 11, 2022, 04:01:05 pm
content: 20Mhz bandwidth while chopping 16 channels, not likely with parts you can obtain that cost less than a used car. And what about the unused unterminated input channels. They are just hi impedance antennas floating in air picking up everything they can. You can't load them down to reduce noise and you can't filter them without reducing bandwidth. As one poster mentioned, you are not likely to succeed with anything above audio/ultrasonics. There are already tons of good quality although expensive multiplexors on the market. Mostly used for instrumentation recording. We use them when characterizing new locomotive designs.

author: Terry Bites
author_rank: Super Contributor
timestamp: January 11, 2022, 04:16:31 pm
content: Alt?

author: MarkL
author_rank: Unknown
timestamp: January 11, 2022, 04:36:55 pm
content: HP used to sell the 54300A which is a dual 8:1 probe multiplexer:https://xdevs.com/doc/HP_Agilent_Keysight/HP%2054300A%20Operating%20%26%20Programming.pdfThe unit has a 2x 50ohm outputs. You can get three different input modules for it, including the 50003A which handles regular 1Mohm probes and gives you +/-20Vpp @ 300MHz at the probe tip for a 10:1 probe.Looks like there's at least one complete unit on ebay right now with 16x 50003A modules for $200. These cost a small fortune new ($6800 base + 16x $650 for the modules = $17200 in 1987 dollars). Poking around there's also some of the other modules available which includes miniature 1GHz active probes 54001A modules (6x for $100) if that fits your purpose, or the 54002A which is a plain 50ohm input module.Building such a thing is not a trivial project. It depends what your time is worth.EDIT: One further thought... If you need 16:1 multiplexing as per your original post, you could place a 2:1 RF switch on the output of this unit to select the A or B side. 50ohm SPDT RF switches are easy to get.

author: jonpaul
author_rank: Super Contributor
timestamp: January 11, 2022, 08:06:58 pm
content: Schematic? PCB layout? Unused inputs are shorted, 50 Ohm term or open circuit?Jon

author: snarkysparky
author_rank: Frequent Contributor
timestamp: January 11, 2022, 08:15:15 pm
content: I would think you need to buffer the signals where they enter the board so they match the stripline impedances. Before the Analog switch.But that introduces a lot of extra alligators.

author: ogden
author_rank: Super Contributor
timestamp: January 11, 2022, 08:58:54 pm
content: Unless you are designing mass-product, better get existing solution. No offense, but having expertise you demonstrated I would not even try such a project. You will burn lot of time and what's more important - money. While ago I needed 1:4 mux for RF equipment (SA). I solved it in a snap - ordered SP4T SMA coax switch and made simple controller out of SBC and some coil drivers. You may take same route. Get something like BNC SP8T switch, add controller and consider it done.https://www.radio741.com/57755-sp8t-bnc-coaxial-switch-l8-213d18-dynatech.htmlPic from same site.