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I'm developing an STM32-based device, one of whose functions is to periodically monitor the main power 12V battery voltage.

I created a simple switching circuit with a voltage divider. I have added a pull-down resistor (R7) to the MOSFET gate, to make sure the switch is always off. On the PCB, I placed these components on the 'dirty' ground corner, near the 12V source (and the switching DC-DC converter). It has a ground plane which connects with the MCU's ground plane at single point.

So the question is: Will switching the VBAT_SENSE_EN pin to low by the MCU, cause a ground loop to the 'dirty' ground? Will this effect be significant? Or should I eliminate R7 and use the internal MCU pull-down?

schematic

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  • \$\begingroup\$ 'Dirty' ground? What is that? \$\endgroup\$ – MCG Jul 20 '18 at 14:14
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    \$\begingroup\$ Um, you might have to read this \$\endgroup\$ – Long Pham Jul 20 '18 at 14:16
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    \$\begingroup\$ @MCG I'm not sure about English-speaking people terminology, that what I call a ground plane with covers part of PCB with input voltage connector, switching DC-DC converter, 3.3V LDO and some other 12V devices. MCU has own ground plane which connected to that plane in a single point. \$\endgroup\$ – asavartsov Jul 20 '18 at 14:18
  • \$\begingroup\$ I don't see where a ground loop is.... It is all part of the same ground yes? Connected on the same PCB? I think you may need to provide a screenshot of the PCB layout too if you are worried about a ground loop \$\endgroup\$ – MCG Jul 20 '18 at 14:24
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No, I don't think you have ground currents so huge to create volt levels to turn the MOSFET on.

If you really think so than moving the resistor will not solve the issue. The noise will be added instead of subtracted. I don't think is the case here.

I would worry about the VBAT_SENSE signal if it's going to an analog input.

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  • \$\begingroup\$ It's going to an internal STM32 Op-Amp in follower mode which output is internally connected to analog input. \$\endgroup\$ – asavartsov Jul 20 '18 at 15:19
  • \$\begingroup\$ @asavartsov Depends on what you need. I guess that you check the battery while on low power mode with low ground currents then the gnd voltage drop should be negligible. If you do something else like powering a high current transmitter to send some data then you can do the reading before to avoid having error sources. If not possible then a differential amplifier might be used but reading an analog input in the presence of noise is a different question. \$\endgroup\$ – Dorian Jul 20 '18 at 15:38
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100 ohm series from a low R source with 47k to ground will not significantly affect any gate drive function and helps if open circuit tested.

I assume C values were selected to minimize overshoot , noise and latency.

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