Pressure sensor device amplification - Page 1
Forum section: electronics / projects

author: jts1705
author_rank: Newbie
timestamp: April 04, 2019, 04:09:33 pm
content: Hello everybody,I am a student who is developing a pressure sensor device for a medical application. I need to measure pressures between 0 and 50mmHg. Therefore, I am using arterial pressure sensors that suit my purposes (http://www.icumed.com/products/critical-care/blood-pressure-monitoring/transpac-it.aspx).I need to amplify, filter and condition the signal before Arduino UNO can read it.I've designed an schematic that I think can work, but now I am trying everything in a protoboard and it does not work. I have tested with a voltmeter partial voltages and the DCDC converter seems to be not working good. It does not give any output signal. I have checked again its datasheet and seems like it has a CTRL pin... but I do not really think it is the reason why it does not work. (https://eu.mouser.com/datasheet/2/468/RS-S_D_Z-958418.pdf). I do not know if the DCDC converter is broken or if I have performed a mistake before...I would be very very very thankful if any of you could give me his/her opinion.I attach the schematic and two photos of the protoboard.Thank you very much!!

author: jbb
author_rank: Super Contributor
timestamp: April 04, 2019, 06:32:40 pm
content: I suggest you divide and conquer. You can gently pull out the RS1212D and test it separately.Note that the way your schematic is drawn, the RS1212D is not providing any isolation; the grounds are connected. Also something’s weird with the pin numbers - I’m pretty sure the datasheet doesn’t go up to 23 pins on it.I also have a concern about the instrumentation amplifier. I may be wrong but the high resistance on AR1 pin 5 could lead to poor results. Normally an INA needs a low resistance on the ‘ground’ connection (a few Ohms or less) to avoid disturbing the differential amp.

author: babysitter
author_rank: Frequent Contributor
timestamp: April 04, 2019, 07:09:30 pm
content: I don't know the icumed from the top of my mind but IBP transducers usually have a pinout where the excitation and signal lines are "intertwined" like supply+ signal- supply- signal+ or similar, but the supply together and signal together looks very wrong to me.Between the suppy in pins you should measure a higher (>1k-ish) resistance than between the output pins.When applying power supply, the signal lines should be close to 1/2 supply voltage relative to ground.Consider adding at least some capacitor between the Instrument Amplifier reference and ground, they like lower impedance here. That nice schematic in the datasheet, where the offset compensation voltage is buffered with a OpAmp, is there for a reason.Good luck!Hendrik

author: jts1705
author_rank: Newbie
timestamp: April 05, 2019, 09:36:47 am
content: Thanks to all. I followed all your instructions and I considered to remake the schematic and correct its mistakes. (I do not have Altium license anymore so I did it as I could).-->I have corrected the pins in the DCDC converter according to the datasheet and also the pins in TL062 (although they were wrong in the schematic I think I was doing it right in the protoboard).I do not really know why, but now the DCDC converter seems to be working but it gives almost +14V in Out+ and -10.10V in Out-. Instead of +12V and -12V that is what I was expecting. However, I think this should be enough to supply to the INA128 and TL062. But stills not working.I will definitely lower the impedance in AR1 to not disturb INA128. Thanks for that!!

author: aldi
author_rank: Contributor
timestamp: April 05, 2019, 10:23:48 am
content: Check section 8.2.2.4 in the INA128 datasheet, depending on the output structure of the pressure sensor, you may need to add a bias current return path. If the pressure sensor is just a wheatstone bridge this is not needed.

author: babysitter
author_rank: Frequent Contributor
timestamp: April 05, 2019, 10:47:49 am
content: Quote from: aldi on April 05, 2019, 10:23:48 amThey are.

author: jts1705
author_rank: Newbie
timestamp: April 08, 2019, 12:36:03 pm
content: Hello Everybody,I manage to solve all the problems I had. There was a problem with the wires of the transducers which gave me strange negative values all the time... Once I realized about that everything was perfect later.I share with you my final schematic for if there is anybody interested in it.Thank youBest wishes