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?

enter image description here


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?

enter image description here

  • \$\begingroup\$ Why would a spike like the one shown affect your DAQ system? We only have info from what you present and you haven't said what wire this spike is on or where in the circuit this wire is. \$\endgroup\$ – Andy aka Jun 3 at 14:51
  • \$\begingroup\$ It is just a theory at the moment. It's a custom DAQ system and I believe this spike is coupling onto my 3.3V supply, then resetting some ADC's. The scope trace is the supply voltage to the motors under test. \$\endgroup\$ – gtetil Jun 3 at 14:55
  • \$\begingroup\$ No. If you want to measure AC noise on battery, use either a twin probe balanced differential probe method or better a 50 Ohm terminated AC coupled coax line. Long inductive leads to a DC couple DAQ with unknown ground reference may give ambiguous false readings. Unless you are: e.g. conducting 100A from a 40 mohm ESR battery to drop 4V \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jun 3 at 14:57
  • \$\begingroup\$ So start with block diagram of what is significant, measurements and show methods of EMI reduction used already such as STP cables , orientation of cables etc. BW of signal desired etc \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jun 3 at 15:01
  • \$\begingroup\$ I'm not trying to measure/log this event. I want to get rid of it because I believe it is screwing with the functionality of my test system. \$\endgroup\$ – gtetil Jun 3 at 15:02

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.