# Differential Opamp Strange Behavior

I have no answer for this strange opamp behavior. The circuit shown is from a typical non-isolated, non grid-tie inverter product (imagine H bridge inverter), which has a common -Ve high power circuit and isolated low-voltage 24V and 3.3V. DC bus and battery share the same common -Ve.

The battery voltage sensing opamp shows wrong output e.g. 3.1V saturated voltage, whereas the DC Bus voltage opamp works well in all conditions. Upon inspection of voltages, I observe below voltage:

Following are the readings across various nodes of OPAMP (opamp LMV931MGX/NOPB):

• BAT- R104 to GND = -122
• BAT+ R108 to GND = -16.9
• Battery voltage 105.3
• BAT U25 -Ve IP (3) (R105) = 0.372V (note +Ve sign)
• BAT U25 +Ve IP (1) (R110) = -0.385V (note -Ve sign)
• DC link sensing U23 gain 0.004799
• DC link sensing U23 pin 3 (R95) = 1.622
• DC link sensing U23 pin 1 (R102) = 1.622

My questions:

1. Why is the voltage on inverting and non-inverting pin are not the same? How can I make it work properly? Also note that I assume opamp is saturating because it is seeing (-Ve) -0.0.385V at its +Ve non-inverting pin. How can I fix this? Note the 3.3V is isolated from HV (battery and DC bus) voltage it is reading differentially.

2. Both circuits are same in nature DC Bus sensing opamp has no issue and Battery sensing opamp throws wrong reading. I have tried replacing opamp with equivalent part # but still, the issue is the same. I don't think this is noise related issue as I didn't see any sign of noise.

Edit: We have AC sensing in the same way as this. Now noticed that when the AC sensing is in circuit we see this DC battery wrong reading issue, when we have AC sensing out of circuit. Battery sensing circuit works fine (remember it's non-isolated H-bridge inverter.)

• What is bandwidth of your scope? Also, make a 1 inch by 1 inch loop, and go looking for magnetic fields. Nov 16, 2018 at 4:10
• Your op-amp (+) input needs a ref voltage equal to 1/2 Vcc. Your input common mode range is centered around that ref voltage.
– user105652
Nov 16, 2018 at 4:15
• You just miss ground connection on R110 lower pin ;) Nov 16, 2018 at 11:33