Multiple capacitive sensor reading issue (NOT TOUCH Sensors) - Page 1
Forum section: electronics / projects

author: onelectron
author_rank: Newbie
timestamp: November 08, 2021, 12:08:54 pm
content: Hello everyone,I am working on a project where I am using 16 custom-madepassivecapacitive-based pressure sensors. The idea is to monitor the real-time pressure change in different points of a system. The capacitance range of each sensor is around 100pf to 250pf. I am having difficulties measuring the capacitance value or change in capacitance of multiple sensors.Point to be noted it's not a capacitive touch sensor.I am using CAV424 (https://www.analog-micro.com/products/ics/cuconverter/cav424/cav424-datasheet.pdf) capacitance to voltage converter IC. If I directly connect a single capacitor to the IC it can measure the capacitance and can spit out the analogous voltage at the output and I can use an equation to convert the voltage into capacitance value/pressure level.My idea is to multiplex (Analog MUX) all 16 of the sensor so that I can use only one CAV424 IC to measure the capacitance. But so far I have failed to do that. What I have realized is that as the CAV424 IC charges and discharges the sensor to measure the capacitance when a particular MUX channel is activated it's actually activated using a MOSFET and the gate charge of the FET also charges up the capacitor hence when the CAV424 tried to charge and discharge the capacitor it gets the wrong value but I could be wrong. I have tested the setup with CM067B and ADG1606BCPZ-REEL7 MUX.https://ibb.co/kyW8CKJI also have used an analog switch (XS3A1T3157) but the same result (https://www.nxp.com/docs/en/data-sheet/NX3L1G66.pdf).Even though the idea is to make a very small device, I tested the idea of a mechanical relay but it also did not work (https://www.digikey.de/product-detail/de/kemet/EC2-5NU/399-11052-5-ND/4291118). Maybe due to the same reason.I was considering using something like Analog Device’s ADGM1004JCPZ-RL7 IC (https://www.analog.com/media/en/technical-documentation/data-sheets/adgm1004.pdf) but I suspect that it will have the same result, and it's also very expensive.Any idea on how to solve this issue? I am not married to the concept to use the CAV424 IC but I'm am stuck with the sensors. Here is a couple of napkin-style diagrams.https://ibb.co/VDND78rhttps://ibb.co/Vq8KRXHI would really appreciate it if someone could suggest an alternative solution. Surely this is not the first time someone using multiple capacitive sensors but could not locate any solution online.Thank you all for your time and consideration.Best Wishes.

author: onelectron
author_rank: Newbie
timestamp: November 08, 2021, 12:20:08 pm
content: Quick update. The mechanical relay concept is working. But still, the relay-based solution would be quite bulky. If the Mechanical relay concept works do you think that MEMS relays will also work? My concern is that as the MEMS relays are also FET-based so they will have the same issue as MUX or analog CMOS switches.

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: November 08, 2021, 05:21:35 pm
content: Do you know about charge injection characteristics of analog switches?This is probably messing up your sensor.A low charge injection switch will have a higher Ron. Since you have a working relay system you could try putting in a 120ohm resistor and see if that messes up your capacitance readings. If itdoes notthen you have a good chance that a low charge injection switch will work for you.https://www.analog.com/en/analog-dialogue/articles/ask-the-applications-engineer-26.html#:~:text=Charge%20injection%20in%20analog%20switches,make%20up%20the%20analog%20switch.&text=When%20the%20switch%20is%20turned,the%20gate%20of%20the%20PMOS.

author: Renate
author_rank: Super Contributor
timestamp: November 09, 2021, 06:27:05 pm
content: Why not use something with the mux built in?I use a NXP MPR121 (discontinued?) which has 12 inputs as a tank level detector with 12 copper tape stripes.I tried out the Azoteq IQS316 with 16 inputs but the silly thing is free-running and stupid.

author: RoGeorge
author_rank: Super Contributor
timestamp: November 09, 2021, 06:38:10 pm
content: Try a bridge like this type of sensor, it's 4 diodes and it outputs a DC. One capacitor is a fixed value, the other is the pressure sensor. Or maybe you can turn the pressure sensor into a differential capacitor for the bridge.https://www.docdroid.net/enKb2ed/nasaames-simpletiltmeter-pdfThere's abig threadabout something else, and somewhere in between the thread settled to the capacitive method shown in the above PDF.

author: onelectron
author_rank: Newbie
timestamp: November 11, 2021, 11:19:29 am
content: Thank you @RandallMcRee, @Renate, and @RoGeorge. I'll try all of your methods and come back to you. Thanks to you guys, now I have some other experimental method to try on.