Help, differential amplifier circuit analysis, temperature compensation - Page 1
Forum section: electronics / projects

author: daisizhou
author_rank: Super Contributor
timestamp: May 28, 2025, 12:38:15 pm
content: Hallo everyoneThis is a differential amplifier circuit connected to a Wheatstone bridge pressure sensor.The pressure sensor cannot obtain the data sheet,At present, we only know that the input impedance is 5.2k and the output impedance is 3.3k,Working voltage 9.3v。The circuit schematic is shown in the attachment below.I would like to know whether adjusting the R1109 (TC) adjustable resistor can compensate for the temperature drift caused by the pressure sensor, resulting in HI-LO voltage fluctuationsIf anyone can read the schematic, please tell me what zero position R1116 (ZERO) refers to? Because I measured U1103.1-lm307 (P2, P3), adjusted R1116 (ZERO), and did not show the effect of zeroing.Thanks

author: Terry Bites
author_rank: Super Contributor
timestamp: May 28, 2025, 05:48:55 pm
content: Its a non-invering amplifier driven from "LO". Is not clear where that signal originates but it seems like its a fixed reference.The pot is marked as "TC" though but there don't seem to be any temperature dependent signal sources to drive it.A sane man would do all the trims in software.That is an appalling schematic, it's utter chaos.It seems very like the designer wasn't very experienced in real world things.Can you identfy the pressure sensor? A photo might help. You might be able to take a good guess at the internal circuit of the sensor.Assume all the bridge elements are identical and there are pair of identical excitation resistors. I meant R5=R6 of course! Easy.If value your time and know its working range buy an off the shelf self compensated one and modify the machine.Knock yourself out.https://www.digikey.co.uk/en/products/filter/pressure-sensors-transducers/512?s=N4IgTCBcDaIMYHsC2AHApgOwM4EMAuaAJiALoC%2BQA

author: daisizhou
author_rank: Super Contributor
timestamp: May 28, 2025, 09:45:05 pm
content: Quote from: Terry Bites on May 28, 2025, 05:48:55 pmSorry.Makes it difficult to read schematics.The schematic diagram comes from the actual PCB. In order to make it clear, I drew the circuit schematic diagram based on the actual object.Please read further here--https://www.eevblog.com/forum/repair/micro-switch-pressure-sensor-help-needed/This is a pressure sensor manufactured in 1989.I haven't found any datasheet about it yet.The internal circuit of the sensor is shown in the figure belowThe pressure sensor (S+/S-) always has a temperature drift voltage of 0.31mv.(It lasts about 20-30 minutes from the time the power is turned on).Temperature compensation "TC" I guess should be to balance the voltage here.I have compared the same model of pressure sensors, please see Posts on 31st and 33rd floors--https://www.eevblog.com/forum/repair/micro-switch-pressure-sensor-help-needed/25/ The possibility of pressure sensor failure has been ruled outQuote from: Terry Bites on May 28, 2025, 05:48:55 pmDo you mean using software to simulate adjustments?I am not good at using this kind of simulation analysis software,If you can help me, it will be much appreciated.I uploaded the schematic file, I don't know if it will help with the software analysisQuote from: Terry Bites on May 28, 2025, 05:48:55 pmI have tested the linearity of this pressure sensor. Please see the attached EXCEL data sheet.From the data analysis, the linearity of the pressure sensor is normal, and the temperature drift of 0.31mv always exists.If the 0.31mv temperature drift can be offset, perhaps this pressure sensor can still be used normally

author: daisizhou
author_rank: Super Contributor
timestamp: May 29, 2025, 02:11:37 am
content: I have obtained a relatively easy to read schematic, hoping it can be helpful

author: daisizhou
author_rank: Super Contributor
timestamp: June 02, 2025, 10:09:18 pm
content: I tried adjusting R1109 (TC), R1102 (NULL), R1111 (XD BAL), R1116 (ZERO), R1123 (VB SET) to make the output HI/LO voltage meet the requirements, but the result was not satisfactory.It looked like the pressure sensor temperature was drifting beyond the set value, so I decided to try replacing the pressure sensor.But the new pressure sensor sensitivity is higher than the old pressure sensor sensitivity，How can I adjust the circuit so that the output HI/LO meets the requirements?

author: MathWizard
author_rank: Super Contributor
timestamp: June 07, 2025, 03:50:54 am
content: I'm guessing in the Schematic.jpg , the U11031 output is sinking from that R on the trim pin, it's connected to that GND, but I saw another thread about your sensor and you had 2 GND's so IDK, it looks wrong.

author: daisizhou
author_rank: Super Contributor
timestamp: June 07, 2025, 03:56:17 am
content: Quote from: MathWizard on June 07, 2025, 03:50:54 amPlease see the schematic here. It is based on the actual PCB, but the wiring is a bit messy, but the connection is correct.