Myself and a few others have built a custom device (PCB & components) which is designed to run while plugged into an OBD2 connector in a vehicle and capture GPS location data and transmit it via cellular to a cloud server. General functionality works well. However, we have noticed that we are consistently seeing poor GPS quality while the device is plugged in (e.g. a common scenario is the location is shifted to one side or another by about 3-4 meters or more and just generally drifts away from the true location by several meters in random ways). This phenomenon does not occur if we power the device directly from a separate battery (the reported GPS locations in this case form a much tighter group within about 1-2m of the true location). We've been back and forth with the GPS module & antenna vendor on the GPS reception aspect and believe that is all configured and tuned correctly (and indeed when we provide nice smooth power to the device we generally get great GPS reception).
When the power enters the device it goes through:
- an eFUSE/protection module: TPS26620DRCR
- a switching regulator: AP64352SP-13
- and it has appropriate decoupling caps near each of the relevant modules (MCU, GPS module)
We're looking for ideas at this point of how to narrow this down. We're not sure if the problem is possibly related to ground bounce (and btw, we're using chassis ground pin 4 on the OBD connector, and when we tried pin 5 signal ground the problem oddly enough gets worse). Or if it could just be ripple on the main input power somehow still making its way into the system, or both.
Any ideas of things to test, or common ways of protecting or isolating against this type of thing would be greatly welcome.
