Is it normal for the voltage fluctuation of REF01 reference circuit to be 150mv? - Page 1
Forum section: electronics / projects

author: daisizhou
author_rank: Super Contributor
timestamp: June 06, 2025, 03:46:35 am
content: Hallo everyone.This is a circuit composed of REF01 and LM307.The power supply on the left is 29.18v。On the right side of the VCC power supply output, I found that the voltage fluctuates by 150mv-200mv. Is this normal?

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: June 06, 2025, 03:50:12 am
content: Why do you have two different grounds?

author: daisizhou
author_rank: Super Contributor
timestamp: June 06, 2025, 03:57:46 am
content: Quote from: Alex Eisenhut on June 06, 2025, 03:50:12 amThe analog ground is the common terminal.Below is the initial schematic diagram, there is no standard accurate resistor value

author: daisizhou
author_rank: Super Contributor
timestamp: June 06, 2025, 04:27:45 am
content: This is a Wheatstone pressure sensor, differential amplifier circuit。I measured that the HI-LO output always has a voltage fluctuation of 200mv,I used 4 5K resistors to replace the pressure sensor, and the voltage fluctuations were still the same.So I measured the front stage composed of REF01 and LM307, which is the VCC-GND port for the pressure sensor power supply.I don't know where this 200mv fluctuation comes from

author: oz2cpu
author_rank: Frequent Contributor
timestamp: June 06, 2025, 09:43:52 am
content: lets try this first ? only the REF01 alone ?remove R1122 and 1121 now baby-step it from there ?

author: daisizhou
author_rank: Super Contributor
timestamp: June 06, 2025, 10:01:48 am
content: Quote from: oz2cpu on June 06, 2025, 09:43:52 amDo you mean to remove the R1122 and R1121 resistors and measure the voltage at the 6th output pin of REF01?There are also LM307J and LM346M in the circuit as well as R1118 and R1114, do these need to be removed?

author: Terry Bites
author_rank: Super Contributor
timestamp: June 06, 2025, 02:45:40 pm
content: U1102 is incorrectly connected. Pins 2 and 3 are reversed.The drift of the opamps is far worse than that of the REF01.Not enough to explain your drift issue but enough be throwing away some precsion.

author: Terry Bites
author_rank: Super Contributor
timestamp: June 06, 2025, 02:48:52 pm
content: Pins 2 and 3 of U1102 are reversed, whats going on there?Get rid of the trimmer and use a "zero drift" opamps.eg the great VFM OPA2182I, £3.29 from Mouser etc.You might want also to think about bandwidth limiting the system to keep circuit noise down.

author: daisizhou
author_rank: Super Contributor
timestamp: June 06, 2025, 10:10:17 pm
content: Quote from: Terry Bites on June 06, 2025, 02:48:52 pmIn the schematic diagram of U1102, the negative input terminal of pin 2 is at the top and the positive input terminal of pin 3 is at the bottom. I have made adjustments according to the original schematic diagram.Quote from: Terry Bites on June 06, 2025, 02:48:52 pmLet me describe what happened from the beginning.This is the mainboard(A3A3A2) inside the RT-203 airflow module of an RT-200 gas flow analyzer.Calculate flow rate by pressure difference.The error code "HELP 0" is occasionally displayed during use. I checked it according to the instructions on page 10-6 of themanualhttps://mega.nz/file/sMYVDYCS#Uezoi3J0ui5RHs7EFoCrBJ3jXdoJ0x2UB1VggLn92Lw.The manual describes the "HI-LO" voltage, which is the voltage between TP4 and TP2, as 0 ± .005 VDC, which is a range of 10 millivolts.However, actual tests found that the voltage fluctuation ranged from 150 to 200 millivolts.The V/F converter is also beyond the range of 500-250,To fix this problem, the only way is to reduce the fluctuation range and make it fit within the 10 millivolt range before normal use can be restored.I don't know where the problem is.For this I checked the pressure sensor, Found that the pressure sensor is normal,see here（https://www.eevblog.com/forum/repair/micro-switch-pressure-sensor-help-needed/）A circuit schematic was drawn for this purpose, in which a "1698-99-0" resistor network was found.(https://www.eevblog.com/forum/projects/what-model-is-this-chip/)I tried adjusting R1123 (VB SET), R1116 (ZERO), R1104 (XD BAL), R1102 (NULL), R1109 (TC), but there was no improvement.I don't know if I'm adjusting these calibration potentiometers incorrectly or if some other electronic component is malfunctioning causing the voltage to shift by 150-200 millivolts.Test video->pressure sensor vcc-gnd test video->

author: MathWizard
author_rank: Super Contributor
timestamp: June 07, 2025, 12:16:52 am
content: Have you looked at it with a scope? Is there anything higher frequency going on that a DMM would miss?

author: daisizhou
author_rank: Super Contributor
timestamp: June 07, 2025, 12:24:10 am
content: Quote from: MathWizard on June 07, 2025, 12:16:52 amDo you mean using a digital oscilloscope to view the HI-LO voltage signal?

author: daisizhou
author_rank: Super Contributor
timestamp: June 07, 2025, 02:49:02 am
content: Thanks MathWizard,Remind me of using the oscilloscope tool.I used four 5.1K resistors to simulate the pressure sensor access circuit and tested the HI-LO voltage waveform and the pressure sensor VCC (V+) and GND (V-) waveforms respectively. I hope it will be helpful for analysis.

author: daisizhou
author_rank: Super Contributor
timestamp: June 07, 2025, 10:54:11 pm
content: I have checked the power supply and it always stays between 29.17v and 29.18v, but I don't know why the HI-LO voltage keeps dropping by 150mv to 200mvI have checked these two capacitors and they were disassembled offline from the PCB for inspection. The capacitors are normal and there is no possibility of leakageI don't know what's wrong with this PCB that causes a voltage drop rate of 10 millivolts per second.

author: Terry Bites
author_rank: Super Contributor
timestamp: June 08, 2025, 05:45:45 pm
content: I tried to simulate this circuit and it didn't work.Eliminate Solder joint problems.Are the reference and the opamps genuine parts?It could be a dodgy opamp or reference ic.Have you tried replacing and/or upgrading them?If you value your time, I can commend to you a simple replacement tracking reference.An LT6658xxxxx-5 single chip solution is ideal.

author: daisizhou
author_rank: Super Contributor
timestamp: June 08, 2025, 10:12:38 pm
content: Quote from: Terry Bites on June 08, 2025, 05:45:45 pmThanks.If you wish to try simulating this circuit again.If you need more circuit information, please let me know and I will provide it to you.(The actual measured zero drift of the pressure sensor output signal (S+, S-) is about 0.38mv).I have attached the data sheet for the pressure sensor."RT-203 FLOW MODULE-SCH_Schematic1_2025-05-24" is the complete connection diagram, and the resistor values ​​are correctly marked.[I drew it based on the actual PCB, so it might be hard to read.]"allesCaput" is a simplified schematic that describes how the entire circuit works, but some resistors lack accurate values.Quote from: Terry Bites on June 08, 2025, 05:45:45 pmThe board looks very good quality, the solder joints are solid, I re-plugged the chip and chip connector to rule out the possibility of pin oxidation.The voltage source and operational amplifier are both in their original factory state. I have not replaced them.The five adjustable resistors R1123 (VB SET), R1116 (ZERO), R1104 (XD BAL), R1102 (NULL), and R1109 (TC) seem to have been adjusted or calibrated by the previous owner (Some of the red protective paint has been damaged).What I know so far is:R1123 (VB SET) is used to adjust the voltage or current of the pressure sensor VCC and GND.R1104 (XD BAL) is used to adjust the voltage difference of the pressure sensor (s+ and s-).R1102 (NULL) is used to adjust the output HI and LO voltage values.R1109 (TC) may be a temperature drift adjustment.[I didn't adjust it, the only one with red protective paint intact]R1116 (ZERO) I don't know how to adjust it correctly,In other words, I tried to adjust R1116, but I didn't find any signs of zeroing.Instead, the voltage of the pressure sensor (VCC-GND) changes from the initial 9.3v to 8.4v. I don't understand it.To sum up, I don't know whether the fault is caused by my improper adjustment of the potentiometer or by the aging of the electronic components as you said. I am confused.

author: daisizhou
author_rank: Super Contributor
timestamp: June 09, 2025, 01:17:01 pm
content: I tested the voltage drift of REF01 pin 4 and pin 6.

author: Terry Bites
author_rank: Super Contributor
timestamp: June 09, 2025, 02:45:44 pm
content: I'm still thinking that this is a poor connection at the opamp pins or a dodgy passive component lead.Say there was a bad, high impedance solder joint or a fractured trace.It might look like it's working but its actually forming an RC circuit with local capacitances.Most likely at the input pins. I had a chat with a friend who'd seen exactly this phenomenon.Check also for sources of RFI. Opamp inputs are usually great RF detectors, add a bit of capacitance and there you go.Experiment with some grounded shielding.

author: daisizhou
author_rank: Super Contributor
timestamp: June 10, 2025, 12:55:56 am
content: Quote from: Terry Bites on June 09, 2025, 02:45:44 pmFollowing your suggestion, I used the resistance range of a digital multimeter to measure the purple connections in the figure below and found that they are all normal.This is a single-sided PCB.The back is the wiring layer, and the welding quality seems to be very goodIs the correct procedure for adjusting R1116 (ZERO):Measure the output voltage of pin 6 of LM307,Then adjust R1116 to keep the voltage output of pin 6 of LM307 at zero for the best result?

author: daisizhou
author_rank: Super Contributor
timestamp: June 14, 2025, 07:37:49 am
content: I tested REF01 alone and it seems that REF01 does not seem to be faulty.I connected the circuit as shown in the figure, input 18vdc voltage, adjusted the 10k adjustable resistor, and the output voltage was between 9.74v and 9.96v.It just can't reach the minimum voltage of 9.7v and the maximum voltage of 10.05vIt looks like REF01 is carrying too much current, causing excessive voltage drop,Is it related to the incorrect settings of R1123 (VB SET) and R1116 (ZERO)?

author: iMo
author_rank: Super Contributor
timestamp: June 14, 2025, 07:59:13 am
content: Try to use a smaller value R1, like 100k, for example..1mA is not an excessive current for that chip, afaik (provided your resistors and the trimmer are ok). Do measure the current off your 18V supply and doublecheck your wiring.

author: daisizhou
author_rank: Super Contributor
timestamp: June 14, 2025, 08:52:17 am
content: Quote from: iMo on June 14, 2025, 07:59:13 amThanks.Please see C1105 in the original circuit，That is, there is a 150mv voltage fluctuation between the 4th and 6th pins of REF01. I don't know why.Testing REF01 is only intended to verify whether it is faulty.Now it seems that REF01 is normal

author: iMo
author_rank: Super Contributor
timestamp: June 14, 2025, 09:18:52 am
content: Well, in your first schematics above the REF01's output current is above 3mA.. Doublecheck the datasheet what is the max current (and the load characteristics).29V in and 10V out at say 3.5mA is 66mW power loss at the REF01.That heat may create an output voltage fluctuation after power on, but not 150mV, imho.PS: doublecheck the schematic's vs PCB's markings - for example the VBset trimmer on the pcb is marked R1123 and in your schematics R1117.

author: daisizhou
author_rank: Super Contributor
timestamp: June 14, 2025, 09:41:05 am
content: Quote from: iMo on June 14, 2025, 09:18:52 amAs you can see, the load is a pressure sensor (Wheatstone bridge pressure sensor).I have measured the VCC and GND of the pressure sensor to be 9.4v operating voltage,Input impedance is 5.2K,Output impedance is 3.3K，S+ and S- There is about 38mv zero offset (This is within the range of the sensor data sheet).I don't know why, but eventually, there will always be a 300mv voltage fluctuation on the HI-LO output, causing my host to display "HELP 0"

author: iMo
author_rank: Super Contributor
timestamp: June 14, 2025, 09:49:01 am
content: You have to go step by step, focusing on the first issue - the ~150mV difference at the REF01's output after power on.The ground of the REF01 is "floating" in your schematics (the "AGND - the virtual ground" comes from the LM307 opamp's output), afaik, so it could be the input voltage of the REF01 is changing somehow (against the fixed Vcc) after power on.Measure the input voltage of the REF01 after power on as well (between pin 2 and pin 4 of the REF01).

author: daisizhou
author_rank: Super Contributor
timestamp: June 14, 2025, 10:17:57 am
content: This is the complete connection diagram, which I drew based on the actual PCB diagram.The input voltage is always 29.18v.HI-LO is the output voltage signal, which is 300mv here.I have been trying to reduce this 300mv fluctuation, but the effect is minimal.R1123 (VB SET) :is used to adjust the working voltage of the pressure sensor.R1116(ZREO): I have never been able to confirm which zero position this "ZERO" refers to.I tried adjusting it but it didn't work.I don't know how to adjust it correctly.R1111(XD BAL):This is to adjust the balance of the pressure sensor output signal s+ and s-,That is, when the pressure sensor P1 and P2 ports are open, it is used to adjust the s+ and s- voltages to zero output.R1102(NULL):This is used to adjust the HI-LO voltage output, which is within the range of 500-250 as stated in the manual.The corresponding value is 385R1109(TC):I haven't adjusted it yet, it looks like it's temperature dependentFrankly speaking I'm a bit lost, I don't know how to check now

author: iMo
author_rank: Super Contributor
timestamp: June 14, 2025, 10:40:41 am
content: Quote from: daisizhou on June 14, 2025, 10:17:57 amThat is the powering of that entire module, but not of the REF01.The REF01 is powered from Vcc=29.18V and "AGND" which is a "virtual ground" created by the LM307 opamp.So AGND is not "zero volt" but some XY voltage against your "real ground".Could you measure the voltage between pins 2 (Vcc) and 4 (AGND) of the REF01 after power on?Quote from: iMo on June 14, 2025, 09:49:01 am

author: daisizhou
author_rank: Super Contributor
timestamp: June 14, 2025, 12:02:46 pm
content: Watch the video,It seems to be fluctuating.But the input power 29.18V is stable

author: iMo
author_rank: Super Contributor
timestamp: June 14, 2025, 12:55:00 pm
content: Ok, so the REF01 is powered by quite stable 19.7V, thus you have to see a stable output voltage between pin 6 and pin 4 of the REF01.The AGND is at 29.18-19.7=9.48V (output of the LM307) against "GND".The next step could be to measure all other voltages against the AGND (let us call it the "virtual ground") and write them down into the schematics. Afterwards people here may spot the problem easier.

author: daisizhou
author_rank: Super Contributor
timestamp: June 14, 2025, 01:12:57 pm
content: Quote from: iMo on June 14, 2025, 12:55:00 pmYou mean I should continue to connect the black negative probe of the digital multimeter to the 4th pin of ref01,Then use the red test lead to measure?Can you please mark the test points on the diagram?Please note that voltage may fluctuateFrankly speaking, this 19.7V voltage seems to be fluctuating a bit.I don't know if it will affect the results.

author: iMo
author_rank: Super Contributor
timestamp: June 15, 2025, 08:11:58 am
content: Yes, the black probe (GND of your meter) should be connected to the pin4 of the REF01 (the green AGND).That is the AGND - the virtual ground.You will see positive and negative voltages on various places of your schematics.Try to measure the voltages after power on on all places, if possible.The 19.7V between pin 2 and pin 4 changes a little bit, but not too much and it should be ok for the REF01 to produce pretty stable output (pin6 against AGND).If the pin6 voltage will move by 150mV after power on there is something wrong happening.There is only one way how to find the issue - to measure, measure, measure..

author: daisizhou
author_rank: Super Contributor
timestamp: June 15, 2025, 08:45:11 am
content: Quote from: iMo on June 15, 2025, 08:11:58 amI measured and recorded the voltage as you said. Please see if there are any additional test points that need to be added.

author: iMo
author_rank: Super Contributor
timestamp: June 15, 2025, 09:32:17 am
content: Also add if possible - see below to have it complete..Are those voltages measured when the module is wired to the other parts/modules of your system?

author: daisizhou
author_rank: Super Contributor
timestamp: June 15, 2025, 11:51:30 am
content: I connected the module to the mainboard for measurement.I have completed the parts that need to be supplemented

author: daisizhou
author_rank: Super Contributor
timestamp: June 16, 2025, 08:36:39 am
content: I found a LH0070-OH，It looks like it is also a 10v voltage reference,Is a replacement test needed?(It may be difficult to adjust the voltage output)

author: daisizhou
author_rank: Super Contributor
timestamp: June 17, 2025, 03:26:16 am
content: The voltage is still fluctuating.Does anyone have any better suggestions? Thank you

author: daisizhou
author_rank: Super Contributor
timestamp: June 19, 2025, 06:44:40 am
content: Can someone help me analyze this circuit?The HI-LO voltage offset still exists and I don't know how to check the circuit