The device works reliably during testing with a bench supply and it works reading motor current while low side sensing, but when the motors switch direction the device becomes a high-side sensor and the resulting 24 V PWM into IN+ and IN- creates 20 V noise spikes on the USB5V line, so the USB devices reset/shutdown.
I've measured the PWM signal amplitude (24 V), frequency (~15 kHz), rise (94 ns) and fall (150 ns) times and I don't understand why noise spikes are being coupled into the DC bus. From what I understand this kind of coupling should occur if:
- the traces/wires are carrying high current
- trace length is long enough to act as an antenna
Case #1 shouldn't happen because the INA226 is a high impedance voltage sensor, case #2 shouldn't happen because the sense resistor wire is only ~15cm, and the trace length on the pcb is only another few cm, which is much to short for the 15 kHz PWM, or even MHz range noise from the 94-150 ns rise and fall times.
I'm looking for advice on:
- Any incorrect assumptions or mistakes in analysis
- How the noise is coupling into the DC power bus.
- Filters I can use to reduce the voltage spikes ( I've been considering a differential ferrite choke from Rs, or implementing something like this TI app note )
Attached are the schematics and block diagrams. I also created an imgur album with some 'scope readings