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I'm having a problem when I measure a voltage from a lead battery. I provide my circuit so you can understand better whats happening.

For example I measure a 24 V battery, goes to a resistor divider and then to an operational amplifier (+ terminal). On the other side (- terminal) there is a stable voltage through a TL431. The output of this goes to my PIC.

If I connect a variable supply instead of a battery and "mimic" how it would be, all function perfectly. The problems appear when I connect a real battery. I think that the problem is the ripple because while I'm measuring the battery the voltage does not exceed the reference voltage in the negative terminal of the AO but it activates the input of the PIC, therefore the output of the AO.

Summing up, what its the best option? An LC filter? An active filter with an AO? An AO as integrator?

Circuit

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    \$\begingroup\$ According to your schematics, you have swapped positive and negative from the battery. Is that the case, or did you just make a mistake in the drawing? \$\endgroup\$
    – Klas-Kenny
    May 13 at 6:15
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    \$\begingroup\$ Also, your schematic does not include any values of any of the resistors. And what is the 12 pin "BAT" thing in the bottom right corner? \$\endgroup\$
    – Klas-Kenny
    May 13 at 6:18
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    \$\begingroup\$ A battery shouldn't have any ripple. \$\endgroup\$
    – JRE
    May 13 at 6:28
  • \$\begingroup\$ Simplest causes, i think, are absence of decoupling capacitor for PIC (should be ceramic, electrolytic doesn't count), and absense of capacitors in the negative feedback loop of OPAMP. It should work as low pass filter with lowest acceptable frequency. Looks like long wires causes instability and oscillation. \$\endgroup\$
    – Vladimir
    May 13 at 8:16
  • \$\begingroup\$ @Vladimir there is a capacitor in the pic between GND and VCC... In the negative feedback of the opamp i think that you refer to put capacitors between R5 and GND, R7, and GND, R9 and GND. Im correct? But im pretty sure that will not solve my problem, because my problem comes in the positive feedback of the OPAMP... \$\endgroup\$ May 14 at 8:26

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