low cost, low rdson Analog Switch - Page 1
Forum section: electronics / projects

author: delphes123
author_rank: Contributor
timestamp: July 05, 2023, 08:13:37 pm
content: Dear,I am curretly using this switch ADG1436YCPZ-REEL7 which is too much expensive for my application, by quantity 10k it cost 2,6 USD . it is a SPDT switch (very important) and can have up to 16,5 V passing ( I need 12 V). My question is how I can replace with transistor / mosfet with keeping very low On resistance ?Rgds

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 05, 2023, 10:02:12 pm
content: The combination of 1.8 Ohms and wide supply bumps you into a niche that is high price.If you really need the low Rds ON, one alternative would be to consider solid state relays / photoMOS - 60V versions come in low RDS and you may need 4 to replace your part,- which helps you see why they have set that high priceDiscrete mosfets like BSS138 could meet RDS, but at higher capacitance and leakage, and poorer RDS vs voltage. That may be ok for your application ?You need to level shift the control voltage plus have margin above the peak analog voltage to keep the FET low enough resistance. (but still be inside VGS max)The BOM is much larger, with 4 FET switches and 4 level shifters, but the piece price is lower.Dual packages can help reduce the PCB area, and digital transistors/RET/prebiased parts as PNP/NPN pairs, could help simplify the level shifters.

author: ArdWar
author_rank: Frequent Contributor
timestamp: July 06, 2023, 01:29:34 am
content: Is the signal bipolar?It's not that complex actually if you're okay with generating 20V supply

author: delphes123
author_rank: Contributor
timestamp: July 06, 2023, 07:54:05 am
content: I need to have one more things to help (or not !) , signal are coming from very sensitive low signal (PH meter sensor, CL sensor etc)

author: Kleinstein
author_rank: Super Contributor
timestamp: July 06, 2023, 09:01:05 am
content: The use if single transistors is more complicated and needs quites some efffort. It may still be OK if the application limits the conditions under which the switch is used (e.g. switch with a limited voltage range, no need for speed). For a more general use the CMOS switches are not easy to substitute with discrete transistor.A PH probe is usually very high impedance. In this case one does not need a low R_on and higher resistance switches should be OK (even the rather high resistance ones like max326).

author: ArdWar
author_rank: Frequent Contributor
timestamp: July 06, 2023, 11:12:19 am
content: Is it 4-20mA sensor? You do need reasonably low impedance if you want to pass the signals as is. Although anything below ~50 ohm should still sufficient.It's better to convert/scale the signals to voltage first however. That way you don't need low resistance/high voltage switches.

author: Warhawk
author_rank: Frequent Contributor
timestamp: July 06, 2023, 11:27:32 am
content: Quote from: delphes123 on July 05, 2023, 08:13:37 pmWhat about CD4053B and level shifters using bipolars ?Or TMUX4827 ?

author: delphes123
author_rank: Contributor
timestamp: July 06, 2023, 01:23:56 pm
content: After some research I made this schematics with complementary mosfet N + P, could you tell me if you see any limitations here ?

author: Kleinstein
author_rank: Super Contributor
timestamp: July 06, 2023, 01:30:42 pm
content: The normal MOSFETs and those in the chip shown have parasitic diode between drain and source. So one direction is blocking only for low voltages of some 200 mV or so. The other problem is the limitation on the gate voltage and possibly gate protection : for the DMC2004 a zener diode that starts to conduct at some +-8 V.

author: Zero999
author_rank: Super Contributor
timestamp: July 06, 2023, 02:28:50 pm
content: The problem with the body diode can be avoided, by connecting two MOSFETs back-to-back, but it doubles the on resistance and number of MOSFETs.

author: delphes123
author_rank: Contributor
timestamp: July 06, 2023, 02:33:07 pm
content: The parasitic diode is clearly a problem, that I need to solve, @zero999 could you give me more detail about changes to do ?

author: Zero999
author_rank: Super Contributor
timestamp: July 06, 2023, 09:15:45 pm
content: It requires four MOSFETs. Why not use an analogue switch IC? It'll probably work out cheaper.

author: SiliconWizard
author_rank: Super Contributor
timestamp: July 06, 2023, 09:18:33 pm
content: Quote from: Zero999 on July 06, 2023, 02:28:50 pmYep! Paralleling back-to-back MOSFETs can mitigate the increased on resistance, can't it? BUt then that's even more MOSFETs.

author: Marco
author_rank: Super Contributor
timestamp: July 07, 2023, 02:15:18 am
content: The 4-20 mA question from ArdWar is rather important, most of this will be besides the point for 4-20 mA sensors.

author: delphes123
author_rank: Contributor
timestamp: July 07, 2023, 07:31:34 am
content: Sorry it is not 4-20 mA signal, it is very low signal coming from electrodes (PH etc) and is goes only in one way from Electrode to ADC;

author: Kleinstein
author_rank: Super Contributor
timestamp: July 07, 2023, 07:57:44 am
content: As written before the PH electrode signal does not need a low R_on switch. The ouput impedance in likely in the > 10 K, maybe 1 M range. The point that may be critical (if used for extended time and a small probe) is the leakage current. The question with the switches here is more about getting some with lower leakage than the ADG1436. The other signals may also matter, e.g. if there is AC on top to cause capacitive coupling and rectification at the probe. Most ADCs would have too much input bias to directly work with a PH electrode and would need a suitable low bias amplifier.Depending on the other signals and needs, I would consider a separate buffer amplifier for the PH probes, so that than more normal switches (e.g. DG211 or CD4052 or similar) could work. Low bais OP-amps (e.g. TLC272 or OPA377 for higher performance) are relatively cheap and usually well good enough for a PH probe that has some temperature effect anyway.

author: DavidAlfa
author_rank: Super Contributor
timestamp: July 07, 2023, 03:17:31 pm
content: A simple 4066 has 125ohm (ON) and 10pA leakage current (OFF).