SPDT analog switch for passing negative voltages - Page 1
Forum section: electronics / projects

author: microvia
author_rank: Regular Contributor
timestamp: July 18, 2025, 01:15:56 pm
content: Hi,I'm looking for a single-channel SPDT analog switch running at 3.3V and capable of passing negative voltages of up to -12V (-25V would be even better, yet not mandatory).The TMUX4827 fits the needs but is a bit expensive (2.50 euros / 100pcs, which I can afford). Isn't there a cheaper similar device ? I don't mind using a 2-channel type if necessary.However I will avoid devices like CPC1017/1117 which take too much place on my existing pcb.Thanks.

author: Benta
author_rank: Super Contributor
timestamp: July 18, 2025, 03:27:16 pm
content: Vishay/Siliconix DG419 should do it.

author: voltsandjolts
author_rank: Unknown
timestamp: July 18, 2025, 03:42:29 pm
content: Quote from: microvia on July 18, 2025, 01:15:56 pmTMUX4827 is quite schmick.1.56 euros / 100pcs at LCSC ...ah, only 74 in stock

author: microvia
author_rank: Regular Contributor
timestamp: July 19, 2025, 07:58:53 am
content: Quote from: Benta on July 18, 2025, 03:27:16 pmInteresting. A few variants are indeed cheaper, but most are significantly more expensive.Will have to check the differences.All I need is to let one of two negative voltages pass, both under 1mA, to drive a vaccuum tube grid.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 19, 2025, 08:27:45 am
content: Quote from: microvia on July 18, 2025, 01:15:56 pmTry TMUX4157 , NLHV4157, SGM3716 - those have +ve control signals and VEE is -4 to -12V, intended for depletion mode FET control I think.Quote from: microvia on July 19, 2025, 07:58:53 amYou could derive VEE from a couple of diodes to the -ve supplies ?

author: voltsandjolts
author_rank: Unknown
timestamp: July 19, 2025, 09:48:56 am
content: Maybe a couple of opto-isolators, or ssr's would do. They can be had for like 40 cents each.

author: microvia
author_rank: Regular Contributor
timestamp: July 19, 2025, 11:09:52 am
content: Quote from: PCB.Wiz on July 19, 2025, 08:27:45 amI could, but I can only power the switch under 3.3V.

author: Zero999
author_rank: Super Contributor
timestamp: July 19, 2025, 12:05:46 pm
content: I recommend the TMUX4827 in the original post.The DG419 is no good because it won't work down to 3.3V and can't handle voltages outside the supply range.TMUX4827 might work, but not with positive voltage. That might or might not be any good. The original post didn't mention positive.A dual photovoltaic opto-coupler, with the output connected to depletion MOSFETs for normally closed and a enhancement MOSFETs for normally open, but that probably takes up too much room on the PCB

author: microvia
author_rank: Regular Contributor
timestamp: July 19, 2025, 01:19:33 pm
content: Quote from: Zero999 on July 19, 2025, 12:05:46 pmIt's a good option which requires additional room. Same problem if I use opto-triacs.

author: voltsandjolts
author_rank: Unknown
timestamp: July 19, 2025, 05:26:07 pm
content: Quote from: Zero999 on July 19, 2025, 12:05:46 pmCouldn't the optocoupler alone do the job, since it's just 1mA?NPN output will conduct in one direction, which is all that's needed IIUC.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 19, 2025, 07:52:19 pm
content: Quote from: microvia on July 19, 2025, 11:09:52 amI was meaning derive the required VEE from your most negative gate voltage, the control line is positive logic, above GND.

author: microvia
author_rank: Regular Contributor
timestamp: July 20, 2025, 01:03:03 pm
content: How about using a NC7SB3157P6X ? Datasheet is athttps://www.onsemi.com/pub/Collateral/NC7SB3157-D.PDF.I need to connect one of two negative voltage sources to the grid of a vacuum tube. One of these two voltages won't exceed -11V. In other cases the other won't, but some circuits can deliver up to -25V.Current passing through the grid is really negligible. I said 1mA by mistake. It's around 1µA, so much that I even maneged to drive the grid using the voltage generated by a APV1121S photovoltaic optocoupler.

author: Zero999
author_rank: Super Contributor
timestamp: July 20, 2025, 01:11:03 pm
content: Quote from: microvia on July 20, 2025, 01:03:03 pmIf you have a negative input voltage, then that will work, otherwise no, it won't.I'm confused about what your requirements are.What power supply voltages are available?What's the voltage range of the signal being switched?How much current does the switch have to conduct?

author: microvia
author_rank: Regular Contributor
timestamp: July 20, 2025, 01:14:26 pm
content: I updated my above post.More precisely, the tube to drive is a magic eye, which grid will not pass more than 1µA with a resistor before the anode of at least 470K (which is always the case anyway).Without resistor (for the sake of testing the worst case), the maximum current I measured didn't exceed 2mA.In most cases the grid voltage will vary from 0V to -10V but there may be exceptions where up to -25V may have to pass through.The device I'm considering is to be powered at 3.3V.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 20, 2025, 02:08:39 pm
content: Maybe you can use one of the "photomos" relays. These have a LED on the primary side, and a bunch of photocells and two "back to back" MOS fets on the "secondary side".Another option could be to switch a "normal" positive voltage, and amplify it with an opamp in an inverting configuration. I guess there are a lot of ways to design around (or within) the limitations of (cheaply available) semiconductors.I quite often see this happening. Some nice IC that replaces a handful of other parts, but when you look up the price, then that single IC is often much more (4x or more) expensive then the other / discrete solution. Just recently I bumped into the L99BM1T It provides galvanic isolation for an SPI bus. A quick check showed it costs EUR 5 in low quantities. while optocouplers are between 10ct and EUR1, depending on their speed and such.Maxim has become notorious for this. They made a lot of nice IC's, often started with somewhat reasonable prices, and after a few years prices become significantly more expensive and the parts get discontinued altogether.Another example is the TPIC6B595. It's a nice IC to get some (higher current) drivers on a uC. but it's much more expensive then a 74hc595 (20ct) with a bunch of "digital transistors" (with built in resistors, these cost around 2ct each). I am thinking about designing in an STP16CP05 as a relay driver. It costs around EUR1.5, but you get 16 outputs for that. BOM cost is still 3x that of the solution with the 2xHC595 + 16 BJT's but it saves PCB area and assembly costs.

author: microvia
author_rank: Regular Contributor
timestamp: July 20, 2025, 02:36:32 pm
content: Quote from: Doctorandus_P on July 20, 2025, 02:08:39 pmPhotomos is too expensive. OPAs will take too much space on the pcb which is already tight and exclusively powered at 3.3V.Hence why I considered the NC7SB3157P6X as a SPDT switch to pass negative voltages as described above.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 20, 2025, 07:31:57 pm
content: Quote from: microvia on July 20, 2025, 02:36:32 pmNot even close, that is a +1.65 to 5.5V part !If you really need to tolerate -25v the -13v parts I gave above could have clamp zeners.Otherwise, a pair of digital PNP parts driving P-JFET x2 could switch -25v

author: microvia
author_rank: Regular Contributor
timestamp: July 22, 2025, 11:07:22 am
content: As there's no IC that can handle the requirements, and since I hardly use mosfet transistors, I took the opportunity of asking flux.ai to design it.Well, I'm quite impressed by the output even though it took quite some time to obtain a readable schematic, which in turn had to be manually entered in KiCAD.From what I understood, bootstraps / charge pumps had to be added in order to let negative voltages pass using a GPIO command.I don't yet understand how this works but here's the overall schematic:What do you think ? Thanks !

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 23, 2025, 08:25:39 am
content: Quote from: microvia on July 22, 2025, 11:07:22 amThat's because it is AI ideas-in-a-blender, and simply will not work.You can sort of see what they were trying to do, create a series FET SW, a la SSR's but for that FETS need to be common source, not diodes in series as drawn.FETS also need level shifters, but negative rails are not common so AI blender missed that detail.You can do this with a single fet to each rail, and a level shifter to each gate. (PNP or PMOS)To avoid cross conduction spikes during handover, you can either use 2 diodes, or 2 resistors may be enough at low currents.What range of actual grid voltages and currents does this need to handle ?If you need to go to 0V, a bit more care is needed on the FET gate level shifters.

author: microvia
author_rank: Regular Contributor
timestamp: July 23, 2025, 09:14:30 am
content: Thanks for your feedback.The grid current is in the µA range. In all cases, magic eye tubes have a series resistor at the anode of its triode section (which grid receives the negative voltage from the SPDT).Removing this resistor would cause the grid current to reach almost 2mA.Grid voltage can be anything between -12V and 0V.

author: voltsandjolts
author_rank: Unknown
timestamp: July 23, 2025, 09:24:48 am
content: With such low currents, simply two opto-isolators is worth trying.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 23, 2025, 09:26:13 am
content: Quote from: microvia on July 23, 2025, 09:14:30 amTo get to 0V, you need to drive the FET gate above 0V with the level shifter, choose a low threshold FET - generic 7002's are a bit high on vgs.Something like this can drive the gates all the way up to close to +3V from a 3.3V CMOS out MUX signal, while allowing pull down to -25V when off.L = NGB and H = NGAI just picked a round number, ~ 100uA could even be high for a MUX FET gate driver.Or use a mute transistor for switch, they avoid the S-D diode of a nmos part.eg ROHM IMX25 gets a dual mute transistor.

author: microvia
author_rank: Regular Contributor
timestamp: July 23, 2025, 10:39:46 am
content: It looks like the additional parts will take quite some space and I'm not familiar with FET design (hence the flux.ai experiment).I guess I'd rather use opto-couplers. Photovoltaic devices aren't suitable: the grid current of the magic eye is so low that they can directly drive it ! I sent a square wave of variable frequency to the APV1121S such that the magic eye could deflect from min to max ! But I had to opt for a diode/cap charge pump for cost reasons. So... I'm a bit stuck for now.Edit: I found the CPC2317, a dual optocoupler with one SPST in NO and the other SPST in NC position by default, which allows making a SPDT driven by a single GPIO without using an inverter.However, this part takes quite some place, and I don't like enlarging the whole pcb. The image below looks like if some space could be easily saved to include the optocoupler, but that space is either taken by silkscreen detailing the pads, or is simply required to protect nearby components from being too close to soldering pads.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 23, 2025, 07:43:29 pm
content: Quote from: microvia on July 23, 2025, 10:39:46 amThe parts come in sot363, but if you only need -12v, why not use one of the VEE switches I already mentioned?Ie Ti TMUX4157N , or onsemi NLHV4157, or SGM3716Addit: maybe also TI TMUX4053, has 1v8 control, slightly out of spec at 3.3v,-12v but that may affect only RON above 0VIt’s not clear what your signals are, but another approach is to mux in the 0~3v3 domain, and use a single sot25 opamp to amplify and invert to 0~-12v ?

author: azone
author_rank: Newbie
timestamp: July 24, 2025, 12:55:18 am
content: Good answer was the first reply....DG-series. DG419 etc... many of them have VL for logic (3.3v) and can do +/-15v

author: thm_w
author_rank: Super Contributor
timestamp: July 24, 2025, 01:16:20 am
content: If you really care about space, put components on the other side of the PCB.IMO its already so tight to the point that its going to cause you problems when you are debugging or assembling it.You can also switch to a through hole cap and bend it above the ICs, unless there are height limits.

author: microvia
author_rank: Regular Contributor
timestamp: July 24, 2025, 10:15:00 am
content: Quote from: PCB.Wiz on July 23, 2025, 07:43:29 pmHere's a schematic to show how I want to switch the two negative voltages to drive the magic eye.

author: microvia
author_rank: Regular Contributor
timestamp: July 24, 2025, 10:27:13 am
content: Quote from: azone on July 24, 2025, 12:55:18 amIn my case I only have a 3.3V supply and need to allow 0V...-12V passing as shown in the above schematic.The 3V version of the DG419 won't allow passing negative voltages as far as I understand.

author: Zero999
author_rank: Super Contributor
timestamp: July 24, 2025, 11:07:41 am
content: You gave the right answer yourself, right at the start of the thread: the TMUX4827. It might not be the cheapest, but it's certainly the most suited and you're not going to save much by increasing complexity.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: July 24, 2025, 03:01:33 pm
content: Here is a somewhat cheaper option, might be good enough in this case. V1, V2 are the input voltages, V3, V4 are control 3.3V inputs (0 - ON, 1 - OFF). C1 imitates the load capacitance.CheersAlex

author: Zero999
author_rank: Super Contributor
timestamp: July 24, 2025, 06:06:12 pm
content: Quote from: microvia on July 19, 2025, 01:19:33 pmThe TMUX4827 is tiny. Only 1.6mm×1.6mm.

author: microvia
author_rank: Regular Contributor
timestamp: July 24, 2025, 08:14:04 pm
content: Quote from: Alex Nikitin on July 24, 2025, 03:01:33 pmInteresting, and clever ! Dual BC557 exist in smd (BC857 series) and is very cheap. The 1N4148 may be replaced by shottky's to reduce the voltage drop (which isn't critical at all). So indeed this circuit will definitely cost less than the $2.5 TMUX (which is a potential problem as stocks are either limited, or you need to purchase a large number of these). And space won't be a problem.Thanks a lot for this, I'll prototype it and see how this behaves.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 24, 2025, 08:24:40 pm
content: Quote from: microvia on July 24, 2025, 08:14:04 pmWhile that's simple, it does have a load on the signals when OFF, and can only sink current, so it is not really an 'analog switch' but it may be 'good enough' - worth trying.You can also use digital transistors, to save more parts.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 24, 2025, 08:31:43 pm
content: Quote from: microvia on July 24, 2025, 10:27:13 amIt is not mandatory for Analog Switches to have balanced +/- rails, that's just a common spec/usage so the DG419 can operate with +3.3 -12 for example.Some rarer Switch parts like TMUX4157N aredesignedfor unequal supply voltages.If your signal only asks to use 0 to -12V the analog sw does not care.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: July 24, 2025, 08:41:42 pm
content: Quote from: microvia on July 24, 2025, 08:14:04 pmSchottky diodes are very leaky and in this circuit might create a problem, if the added voltage drop is not critical (as you say), better leave 1N4148 or an equivalent small dual diode to save space.CheersAlex

author: microvia
author_rank: Regular Contributor
timestamp: July 24, 2025, 08:49:13 pm
content: Quote from: Alex Nikitin on July 24, 2025, 08:41:42 pmThe current grid is around 1µA in most cases so the current leak is critical in this case.Voltage drop isn't critical with most magic eyes, which require 0V...-8V. Some older types work between 0V...-2V and 0V...-3V, hence the shottky idea. Fortunately I don't think I'll have to deal with these, but I like taking care of such cases.

author: microvia
author_rank: Regular Contributor
timestamp: July 24, 2025, 08:50:10 pm
content: Quote from: PCB.Wiz on July 24, 2025, 08:24:40 pmTell you what ? I've never used these.So no idea how they work nor what they can save.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 24, 2025, 11:57:11 pm
content: Quote from: microvia on July 24, 2025, 08:50:10 pmThey are just NPN/PNP with resistors inbuilt.Some call them RET ( resistor equipped transistor) some call them pre biased transistors

author: microvia
author_rank: Regular Contributor
timestamp: July 25, 2025, 07:48:03 am
content: Ok, but I don't see what parts I could save with them.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 25, 2025, 08:00:11 am
content: Quote from: microvia on July 25, 2025, 07:48:03 amFrom #29 r1 & r2 can go inside the SOT363 package, with a slight change.

author: microvia
author_rank: Regular Contributor
timestamp: July 25, 2025, 02:26:42 pm
content: Quote from: PCB.Wiz on July 25, 2025, 08:00:11 amSomething like Nexperia PIMP31 series ? They have a 1K base resistor.There's also the Rohm EMB2 / UMB2N / IMB2A which may allow doing an inverter, allowing to drive the SPDT with a single GPIO. But they have 47K resistors inside.

author: Zero999
author_rank: Super Contributor
timestamp: July 25, 2025, 04:36:49 pm
content: There are some pretty cheap and small solid state relays. I've just found the ISOM8600. It's much cheaper than the TMUX4827, even when the fact two of them is required.https://www.ti.com/lit/ds/symlink/isom8600.pdfhttps://eu.mouser.com/ProductDetail/Texas-Instruments/ISOM8600DFGR?qs=IKkN%2F947nfDtXT3NS%252B87%2Fg%3D%3D

author: microvia
author_rank: Regular Contributor
timestamp: July 25, 2025, 07:16:30 pm
content: Quote from: Zero999 on July 25, 2025, 04:36:49 pmI thought about using similar devices (CPC1017/1117) but they take too much space on the pcb. I didn't find smaller ones unfortunately.The circuit proposed by @PCB.Wiz will take little space as its transistors and diodes exist in dual form.My real limitation is the use of Spice to simulate circuits. That's something I have to get used to...

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 25, 2025, 11:40:52 pm
content: Quote from: microvia on July 25, 2025, 02:26:42 pmYes, here is the circuit of #29 redrawn for lowest part count, and simplest BOM, using dual digital transistors, with 47k/47k - you can also get 100k/100k and 47k/22k etc because this is low power low current, higher resistors mean less drain.You can get lowest part count and simplest BOM using a PNP/NPN pair, twice, eg LRC LMUN5313DW1T1G Dual_PNP_NPN_DigitalTRX_47k-47k_50V_50Vin_SOT363 1.9c

author: azone
author_rank: Newbie
timestamp: July 26, 2025, 04:09:57 am
content: Quote from: microvia on July 24, 2025, 10:27:13 amOk I see, I misunderstood your use case, was under impression you have -ve supply available. Apologies for adding noise to the thread. You may find yourself right back where you started at the TI part even w/ the added cost. They're $1.19/1000which is pretty good and if just for one off or small run $2.23ea is not too bad.

author: Zero999
author_rank: Super Contributor
timestamp: July 26, 2025, 10:46:10 am
content: Quote from: Alex Nikitin on July 24, 2025, 03:01:33 pmThat's good if the voltage drop isn't an issue.I would replace the BJTs with MOSFETs and connect one as common source and the other as common gate. Now only one output pin is required.

author: microvia
author_rank: Regular Contributor
timestamp: July 26, 2025, 10:51:57 am
content: Quote from: PCB.Wiz on July 25, 2025, 11:40:52 pmAmazing ! Indeed all that's needed is 1x MUN5313DW1T1G and 1x BAS40-06 (dual diode common anode). Its typical reverse current is 10nA at 10V. It will be interesting to see the incidence of each negative voltage input towards the other as the maxium grid current is 1µA or so.

author: Zero999
author_rank: Super Contributor
timestamp: July 26, 2025, 05:31:20 pm
content: How about the CD4007?It can be used to make a SPDT analogue switch.Refer to page 3 of the data sheet:.https://www.ti.com/lit/ds/symlink/cd4007ub-mil.pdfV1 & V2 are the voltages you want to switch between.M1 to M6 are inside the CD4007.M1 to M4 are the analogue switches. M4 & M6 are a CMOS inverter.M7 level shifter to convert the 3.3V signal, to -12V to 3.3V required for the analogue switch.Nodes dd and ss are pin 14 and pin 7, respectively on the CD4007. The negative supply to the CD4007 is derived from D1 and D2. The fact the voltage being switched is just outside the negative supply rail isn't an issue, so long as it's by not more than 400mV or so, which is the case, as it's below the voltage drop of a Schottky diode.This circuit will work down to -16V.The circuit can be redesigned to work down -20V, which is limited by the CD4007. M7 will need to be replaced with a P-JFET and the pp notes will have to be connected to 0V.

author: microvia
author_rank: Regular Contributor
timestamp: July 26, 2025, 05:37:03 pm
content: Thank for this interesting idea.But V1 and V2 both vary between -12V and 0V, so I'm not sure if the level shifter handles this.

author: Zero999
author_rank: Super Contributor
timestamp: July 26, 2025, 06:02:55 pm
content: Quote from: microvia on July 26, 2025, 05:37:03 pmIt doesn't matter. The circuit will draw its power from either V1 or V2, depending on which one is more negative. The level shifter's output will swing between V3 and again V1, or V2.

author: DavidAlfa
author_rank: Super Contributor
timestamp: July 26, 2025, 06:04:27 pm
content: Only one channel?Just do it with a fet?

author: microvia
author_rank: Regular Contributor
timestamp: July 26, 2025, 06:10:54 pm
content: Ok but there's still a problem: the CD4007 takes way too much space. It seems that there is no QFN / BGA version (unless under a different reference).All I can find at the moment is a TSSOP version (CD4007UBPWR) but still too large unfortunately.

author: Zero999
author_rank: Super Contributor
timestamp: July 26, 2025, 06:56:47 pm
content: Quote from: DavidAlfa on July 26, 2025, 06:04:27 pmNot quite. It's to switch between two different negative voltage, with a positive control voltage.Quote from: microvia on July 26, 2025, 06:10:54 pmThe basic idea will work with any SPDT analogue switch though.I had a quick look and found the TC4W53FU, which would do.https://www.farnell.com/datasheets/4001435.pdf?

author: Alex Nikitin
author_rank: Super Contributor
timestamp: July 26, 2025, 07:23:51 pm
content: OK, 2x SOT23 transistors, 3 x 0603 resistors (could be 2 if Q1 is "digital" = has an internal resistor for R1). R4 only imitates the internal resistance of the voltage source, not needed in real life. V3 is 0/+3.3V control voltage.J1 -MMBFJ177LT1G.CheersAlex

author: microvia
author_rank: Regular Contributor
timestamp: July 27, 2025, 07:46:06 am
content: Your output shows less switching noise than Zero999's schematic (post #45), but this noise is actually not a problem as the output is a magic eye tube, not an audio circuit.Room is really tight so even two more resistors will be a problem.

author: microvia
author_rank: Regular Contributor
timestamp: July 27, 2025, 07:56:09 am
content: Quote from: Zero999 on July 26, 2025, 06:56:47 pmThis device could fit, but I'm not sure if it can pass negative voltages below Vss (and Vee), and it seems to require +5V.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 27, 2025, 09:33:42 am
content: Quote from: microvia on July 27, 2025, 07:46:06 amYou can get smaller package transistors. Even Digital ones can be as small as DFN1006 for PNP + resistorSmaller PFETs are rare, but you can get SOT323F Muting transistors, which can be driven with a light base current, which the negative signal needs to tolerate.Base current = NPN on, and the special/rare high VEBO of these parts can tolerate reverse off bias.ROHM-DTC923TUBTL NPN_Mute_VEBO_40V_VCEO_20V_HFEm820_Rb2k2_SOT323FROHM-DTC914TUBTL NPN_Mute_VEBO_40V_VCEO_20V_HFEm820_Rb10k_SOT323FQuote from: microvia on July 27, 2025, 07:56:09 amIf you have no external VEE, You can either use the TC4W53FU substrate diodes to pull VEE to most-negative pin, or you can add an external double-diode in SOT323 or smaller to do the same.With +3.3V the high side on resistance will be higher, but still ok.It will need at least one pin to be negative enough to give good low side resistance.Bothpins at low values -1V or more positive would be a problem.

author: Zero999
author_rank: Super Contributor
timestamp: July 27, 2025, 09:45:02 am
content: Quote from: microvia on July 27, 2025, 07:56:09 amThe idea is to power it from either negative voltage, depending on which one is more negative, using diodes to switch between them.

author: microvia
author_rank: Regular Contributor
timestamp: July 27, 2025, 12:55:45 pm
content: I don't think this can work. Control input current is +/-10mA, which will alter the charge pump output.The idea is nice, but I think it's better to go with the dual BSS84 as suggested in post #45.

author: Zero999
author_rank: Super Contributor
timestamp: July 27, 2025, 01:25:26 pm
content: Quote from: microvia on July 27, 2025, 12:55:45 pmWhat gives you that idea? The control input has a high impedance, like any other input pin. The current through the ESD diodes is what's specified on the data sheet. You'll see a similar specification on most CMOS IC data sheets.

author: peter-h
author_rank: Super Contributor
timestamp: July 27, 2025, 01:28:03 pm
content: How can one design a straight analog switch whose supply is GND and +3.3V and which can switch -11V?Surely this is impossible.The only way is to generate a negative rail.Alternatively optoisolation will obviously work.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: July 27, 2025, 02:04:21 pm
content: Quote from: peter-h on July 27, 2025, 01:28:03 pmAs shown in this thread, it is possible as long as you use the signal input(s) as one of the control/supply voltages and prepare to compromise on some parameters (switching speed/resistance/max current/etc). P-ch JFET switches will work just fine providing the "OFF" control signal is more positive than the switched signals on both sides plus Vth of the JFET.CheersAlex

author: Zero999
author_rank: Super Contributor
timestamp: July 27, 2025, 04:52:13 pm
content: Quote from: microvia on July 26, 2025, 06:10:54 pmThere are some pretty small MOSFET packages.https://www.ti.com/lit/ds/symlink/csd83325l.pdfhttps://www.onsemi.com/download/data-sheet/pdf/ndc7002n-d.pdfhttps://toshiba.semicon-storage.com/info/SSM6P15FE_datasheet_en_20220224.pdf?did=22741&prodName=SSM6P15FEHere's another idea.M3 & M4 and M5 & M6 can be either connected either common drain or common source, which I've shown both configurations.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 27, 2025, 09:31:15 pm
content: Quote from: Zero999 on July 27, 2025, 04:52:13 pmYes, dual MOSETs are high volume, modern parts, you can get them down to DFN1010B-6

author: microvia
author_rank: Regular Contributor
timestamp: July 29, 2025, 03:29:21 pm
content: Quote from: PCB.Wiz on July 27, 2025, 09:31:15 pmThere's the 2N7002AKRA-Q which is a dual transistor in DFN1412-6 package. But it has two diodes in opposite direction between gate and source (internal ESD protection ?).The NDC7002N is in TSOT23, already too large for the existing space (unless I enlarge the pcb, which I want to avoid, or if I add parts close to soldering pads, which isn't an option).

author: microvia
author_rank: Regular Contributor
timestamp: July 29, 2025, 03:54:42 pm
content: Quote from: Zero999 on July 27, 2025, 04:52:13 pmVery nice ! How does the output look like if you replace the two negative voltages by one sine and one triangle ranging between -12V and 0V ?

author: Alex Nikitin
author_rank: Super Contributor
timestamp: July 29, 2025, 07:35:10 pm
content: Quote from: microvia on July 29, 2025, 03:54:42 pmEven my simple circuit manages quite well.CheersAlex

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 29, 2025, 07:42:22 pm
content: Quote from: microvia on July 29, 2025, 03:29:21 pmYes, ESD protection.You just check the gate source rating, for 20V, some ESD parts differ in g-s specs.There is also NX7002BKXB in DFN1010, 20V vgsFor P-FETs the PMDXB1200UPE is also DFN1010, lower VGS but that is ok for the PMOS part.The PMEG3001EEF Stky comes in DFN0603-2 package.

author: Zero999
author_rank: Super Contributor
timestamp: July 29, 2025, 08:11:41 pm
content: Quote from: Alex Nikitin on July 29, 2025, 07:35:10 pmYes, go with that, if the 1M output impedance isn't a problem.Use the MMBFJ177, the SMT version of the J177, to save space.Q1 could be replaced with another P-channel J-FET or a MOSFET, to save the additional resistor.Quote from: microvia on July 29, 2025, 03:54:42 pmIt should work. There might be slight glitches, when it switches due to coupling from the gate capacitance.I've provided the .asc files, so you can simulate it for yourself.I strongly recommend building a prototype of any circuit first, before doing a production run.

author: microvia
author_rank: Regular Contributor
timestamp: July 30, 2025, 02:44:16 pm
content: Quote from: PCB.Wiz on July 29, 2025, 07:42:22 pmWith a lower Vgs I guess it's better to go common source ?Thanks for the DFN1010 hint. I'm now on holiday so can't update the pcb until my return in August, but I'll eventually use these devices if required.The 1-FET circuit from Alex looks also interesting but not sure how it will behave. I'll prototype both to see. Glitches during switching are not an issue as the negative voltages are fixed most of the time (as in an old AM radio staying tuned on the same channel). It would be a problem if intermittent switches happened because of negative voltages being close to each other.

author: microvia
author_rank: Regular Contributor
timestamp: October 07, 2025, 09:44:53 am
content: Hi all,It's been some time since we discussed about this topic, and I finally got enough time to make and install a prototype of Alex's circuit.I realized that the V1 input needs to be protected against slightly positive voltages which will otherwise lower V2.In a nutshell, I don't only switch negative voltages using Alex's circuit. At the same time, a relay also changes the circuit generating V1, making it raise around to +1.8V when V2 is selected.A guess a simple BAT54 would do the job, unless something similar has a lower Vf. Any recommandation ? Thanks.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: October 07, 2025, 09:04:27 pm
content: Quote from: microvia on October 07, 2025, 09:44:53 amYes, a couple of BAT54 (or a BAT54C dual diode with a common anode) should be sufficient.CheersAlex

author: microvia
author_rank: Regular Contributor
timestamp: October 10, 2025, 08:50:44 am
content: Indeed it works much better with these. V2 is never above 0V, so I guess only one diode at V1 is sufficient.