Path: ...!Xl.tags.giganews.com!local-1.nntp.ord.giganews.com!nntp.supernews.com!news.supernews.com.POSTED!not-for-mail NNTP-Posting-Date: Fri, 26 Apr 2024 23:47:37 +0000 Subject: Re: PI3USB14-A dual 4>1 USB switch for analog Newsgroups: sci.electronics.design References: From: Phil Hobbs Message-ID: Date: Fri, 26 Apr 2024 19:47:37 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Lines: 98 X-Trace: sv3-yQ0QWZ+HDYw9n0OuLhb9KxUKc/ozhKiDQHiS+XxOtCk6YrXbOQWx5U3F7FGpUHeV6WdycoZsr85D7Zg!F6OfTcmS6H4HS1nfKHM3Xo5mVoIaQkRoG4JzAcGAzDCmW9pVjObGpAmkASqt0ZMXmx2rrszC72zx!rY9csq1InQSGk6vl2+kbEbEk X-Complaints-To: www.supernews.com/docs/abuse.html X-DMCA-Complaints-To: www.supernews.com/docs/dmca.html X-Abuse-and-DMCA-Info: Please be sure to forward a copy of ALL headers X-Abuse-and-DMCA-Info: Otherwise we will be unable to process your complaint properly X-Postfilter: 1.3.40 Bytes: 5234 On 2024-04-26 11:12, Chris Jones wrote: > On 27/04/2024 1:01 am, Chris Jones wrote: >> On 26/04/2024 2:28 am, Phil Hobbs wrote: >>> Hi, all, >>> >>> I'm in a bit of a rush, doing revs to a testing board for a prototype >>> lidar ASIC that turns out to be..., um..., well....  Let's just say >>> it's a _tiny_bit_delicate_ electrically.  Yeah, just a little >>> delicate, that's it. (Not our design or our nickel, fortunately.) >>> >>> It runs on a single supply, so the only sequencing issues are with >>> the signal pins. Thus the test board is going to bring up V_DD >>> smoothly and not too slowly: a monotonic edge of about 100 us.  While >>> that's going on, all the inputs and outputs will see 100k ohms to >>> ground. (All the enables are positive-true, which makes this easier.) >>> >>> The digital pins are all inputs, so they're just buffered with >>> SN74LVC1G125s (tri-state Schmitt noninverting), with the >>> aforementioned 100k to ground on their outputs. >>> >>> The analog outputs don't have much drive, but can go pretty fast if >>> you keep the capacitance down.  There are a fair few of them, so I >>> want to mux them down before the op amp buffers >>> (OPA2626es--surprisingly nice chips). >>> >>> Because we're not made of money, and the test boards aren't ITAR, >>> we're getting them made and stuffed at JLCPCB.  Thus we care a lot >>> about what their tame distributor LCSC has in stock. >>> >>> They're fairly light on analog muxes, it turns out.  Going through >>> what they do have, I was reminded just how awful the capacitance of >>> old-timey mux parts is--way over 100 pF at the common pin of an 8-1, >>> blech. >>> >>> They do have these very nice-looking USB switches, type PI3USB14-A, >>> for pretty cheap ($0.45 @ 1ea) and in plentiful stock. >>> >>> They look beautiful--6 ohms R_on, 4 pF C_off typical--but apart from >>> one crosstalk number, the datasheet gives only sketchy hints about >>> their actual analog performance--bandwidth and crosstalk, but nothing >>> about charge injection, switching times, make-before-break, >>> resistance linearity, or distortion. >>> >>> Has any of you used them for actual analog stuff? >>> >>> Thanks >>> >>> Phil Hobbs >>> >> >> As long as you don't need the signals to swing near the positive rail, >> I suggest you google "H-mode mixer" as a lot of amateur radio >> enthusiasts have built mixers using bus switches, basically just nmos >> fets with a gate driver. For example: >> https://martein.home.xs4all.nl/pa3ake/hmode/switches.html >> >> They may not be better than the PI3USB14-A but I have a better guess >> of what is in them. Thanks. The CMOS mux mixer was AFAICT first popularized by Ed Oxner of Siliconix back in the early '80s. They make nice strong mixers, for sure. > > Ah, I see they also tried ones with both NMOS and PMOS devices in the > switches. The FSA3157 seems to have somewhat more complete > specifications than the one you mentioned. Wouldn't be hard. ;) > If you don't happen to need it to swing near the positive rail, you > might get less capacitance with a NMOS-only type like the venerable > FST3125. Obsolete, unfortunately. > > I hope the ones that also have PMOS devices in the switches keep the > backgates of the PMOS at the rail (like a 4016), and don't do the > horrible backgate switching trickery to reduce Ron like in the old 4066. > I think some of those might have been so bad that they could momentarily > short your signal to the rail, and even if not, they certainly had > another big charge injection mechanism. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com