I have a system that's currently testing automotive brushed dc motors. Occasionally, I'm getting interruptions in my data acquisition system that I believe is being caused by transients coming from some of the motors under test. Below is a transient that I was able to capture (I'm getting positive spikes just as often too). Could I get rid of this with a bypass capacitor? If so, based on the graph below and the timing of this transient, how do I size the capacitor? Is there another device, such as TVS, that would work better?
I placed an 18V TVS diode directly across the motor under test, which is roughly 6 meter away from the test system. The scope measurement below was taken at the test system. How is this possible? Shouldn't it be clamped at 18V in the positive direction, and around -0.6V in the negative?