I have no answer for this strange opamp behavior. The circuit shown is from a typical nonisolated ,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 i.e 3.1V saturated voltage. whereas the DC Bus voltage opamp works well in all condition. Upon inspection of voltages i observe below voltage, a) Following are the reading 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 question :
- 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.
- 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 its non isolated H bridge inverter)