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We have a display and a sensor that are installed in vehicles. The sensor is in a metal TO Can, and the manufacturer shorts the ground pin to the TO Can body for shielding. We mount that can in a metal probe, which of course touches the TO Can and so is also grounded. The metal probe is mounted to a plastic cover in the vehicle. However we have found some vehicles have metal covers now, which have electrical continuity to the chassis. The result is a ground loop which can cause intermittent failures of communication:

Block diagram of ground loop in vehicle through chassis and sensor probe

I have seen circuits where they separate chassis and signal grounds using resistors and capacitors. Could that mitigate the ground loop in this case, if one or both of the PCBs isolated their digital grounds from the chassis ground with a resistor and capacitor in parallel? I have also heard that resistors/capacitors to isolate grounds is a bad practice, so I'm confused why I see it on some automotive circuits.

I also thought of using opto-isolators so that grounds need not be shared between sensor and display. However, the problem with that is that if the sensor gets mounted in a plastic cover, then it has no connection to the chassis ground anymore. In that case the sensor would need to use the display's ground as a return path.

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    \$\begingroup\$ You have to provide the can sensor data. and what circuit. Some can sensors are differential output (balanced connection) (signal + is a positive going signal, and signal - is a negative going signal) vs single wire output (unbalanced) (just a signal + out and ground). They are distinctly different, and in how you would interchange them. \$\endgroup\$ Apr 23, 2021 at 21:28
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    \$\begingroup\$ You show 2 twisted pairs (4 wires) in the cable. Can another one be added? Where is the shield connected? Is the entire circuit (including display etc.) independent of other circuits in the vehicle? \$\endgroup\$ Apr 23, 2021 at 22:00
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    \$\begingroup\$ My guess is that you are not connecting the sensor board with I2C (as seems to be indicated in the rough schematic) but with some other communication medium (CANbus?). Could you please edit the question to add which medium you are using for communication? Or am I wrong in my guess? (I2C is really not meant for "remote" communication) \$\endgroup\$ Apr 23, 2021 at 22:00
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    \$\begingroup\$ How are you connecting the chassis to ground? Are you running on steel wheels without rubber tires or do you have one of those dangly straps (never were very effective either)? \$\endgroup\$
    – Solar Mike
    Apr 23, 2021 at 22:12
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    \$\begingroup\$ Series inductor may work depending on other issues. \$\endgroup\$
    – Russell McMahon
    Apr 24, 2021 at 0:51

2 Answers 2

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If it's automotive, consider EMI for your solution as you likely need it qualified.

You have to prevent a large loop between battery positive, display PCB, LDO, sensor PCB, sensor, chassis, back to battery negative.

This happens if you cut the chassis bond of the display board, although it will solve the ground loop. The large loop will remain and cause a lot of stray radiation pickup and voltage spikes in your circuit.

I can think of two solutions instead:

Option 1:

Use an isolated 12->5V converter to create power for the sensor and run the power and data lines each through common mode chokes over to the sensor. Ground the isolated sensor power negative on the sensor board and leave it float on the display board (cap-attach it to the display ground as per dc/dc converter design rules). Bond the chassis, to both boards and the the cable shield on both sides.

To power the display board, you must not use the isolated sensor power. You can use a local non-isolated psu for that (e.g. your LDO).

Option 2:

This is without an extra PSU, so potentially lower cost. It is also a variation of Bruce's answer. Remove the 12 V connection from the Display Board, and feed 12V power into the sensor board instead. Pass this 12V and a return wire (with Common mode choke) through the cable to the display board's LDO.

In this case, too, keep both boards' end of the shield attached to local chassis. But the Display Board's ground must not be DC-bonded to chassis. Only RF-bonded.

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You need to get the display PCB and sensor PCB grounded at a single point, and that can only be at the sensor itself. So disconnect battery ground from the display PCB (LDO etc.), feed it through a separate wire to the sensor PCB, and 'ground' it as close as possible to the sensor. Then run that ground back to the display PCB through the existing ground wire and/or shield.

This will eliminate voltage dropped across the ground wire between PCBs due to the ground loop, but there could now be some ground noise caused by operation of the display circuit. However with good MCU bypassing it should be OK because I2C has relatively high noise immunity.

Another way to do it is insert a resistor in the ground wire from the battery to the display PCB. This will increase the ground loop resistance and reduce the current flowing through the ground wire between PCBs. The resistor value needs to be low enough that the LDO still gets enough voltage to work, so it cannot eliminate the ground loop current between PCBs - but might reduce it enough to solve your problem.

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    \$\begingroup\$ This will form a huge loop through 12V, LDO, display PCB, sensor PCB, sensor back to battery.. Difficult EMI situation. It will be much better to consider a solution in which both sides remain chassis bonded. E.g. an isolated 5v psu in the display board. \$\endgroup\$
    – tobalt
    Apr 24, 2021 at 6:00

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