# LM324 output voltage gets low in difference amplifier configuration when UPS supply is turned on

I am using lm324 as difference amplifier to measure the string voltages of lithium ion battery pack ( four cells in series). I am using all single package of LM324 that have 4 Op-amp. The Lithium ion battery pack is being charged by the UPS ( uninterruptible Power Supply). The UPS voltage have some noise and you can see in the attached picture. The supply voltage of LM324 is also connected in parallel with UPS and Battery terminal.

The problem is that , whenever the ups start charging battery, the output voltage of LM324 is disturbed. Without charging of UPS, the voltage of each cell in the string is 3.3V as nominal. But when the UPS start charging, the same voltage becomes low as 1.6V.

We have connected decoupling capacitor with LM324 but it didn't work. Can you please recommend any suggestion.

UPS voltage

difference amplifier circuit configuration.

Complete circuit of LM324 as difference amplifier.

Output Voltage of LM324 with Oscilloscope.

• Please show the exact circuit you are using (including power supply to the OpAmp), not just a general outline.
– Curd
Jan 9, 2020 at 8:19
• The values and tolerance rating for R1 to R4 are also very much needed to make a proper circuit examination. And, what does this mean: Without charging of UPS, the voltage of each cell in the string is 3.3V as nominal. But when the UPS start charging, the same voltage becomes low as 1.6V.? Is this a real change in voltage or something inferred from the LM324 output. Show your circuit as mentioned by @Curd Jan 9, 2020 at 8:52
• Thank you so much @Curd, I have added picture of complete circuit diagram for difference amplifier with LMS324. Jan 9, 2020 at 9:06
• @Andyaka , the voltage when output when UPS voltage is applied to circuit for charging to battery is 3.3 , but we apply UPS voltage to battery for charging the lm324 output becomes 1.6 V Jan 9, 2020 at 9:10
• Please also explain where the nodes "GROUND SENSOR" and "+VE SENSOR" connect in your circuit. And, why does U1A connect to +12 volts whereas the others don't? Also you can't have independent pin 4s for a package connecting to different nodes. Please explain this too. Jan 9, 2020 at 9:38

The tolerance of resistor is 10%

If that is the case then THAT is a fundamental problem in your design. On the left (below) is a model of your op-amp circuit using perfectly exact values. On the right I've increased some resistors by 10% and decreased others by 10% to make a worst case tolerance scenario.

The good circuit produces 3.3 volts at the output and the bad circuit produces 1.1 volts at the output.

Just in case you are wondering about "my op-amp", I've used a VCVS with a gain of 100k to mimic an op-amp.

• Dear@Andy aka, I have used resistor with 100% accuracy. but the issue is still there. I have uploaded the picture of output voltage of LM324 . It has noise when there is a noise in UPS output voltage. Jan 10, 2020 at 14:36
• What are the DC voltages? Your o-scope picture doesn't give much info on which is input and what settings you have. If your UPS produces output ripple and surges this is a seperate problem to one of using the right tolerance of resistors. Jan 10, 2020 at 14:51
• The DC voltage is 12V of UPS. the first picture is the UPS votlage, and second picture is the output voltage of LM324. Jan 10, 2020 at 15:24
• But I don’t know what that output voltage is. I can’t properly tell from the picture. It looks like 3V3 so where’s the problem? Jan 10, 2020 at 16:19