Making pulsed DC measurement

[superbrew]

Hello, I am having trouble making a pulsed DC measurement using a 34401A DMM. The unit that we are trying to measure generates a 350uS pulse of approximately 950mV at a rate determined by the trigger input, which we are using a simple pulse generator for. We can measure the pulse amplitude on the scope, but we need something more accurate. We set the meter to trigger externally and connected the signal that we are using to trigger the unit under test. The problem is that the meter triggers on the negative edge of the trigger pulse and the UUT requires that the trigger pulse be longer than 350uS, so it is not measuring the pulse amplitude. We can't find anywhere in the manual where it gives a setup for this situation, other than a short section on external triggering. We also have a 3458A available. Thank you for any help.

 

[Rolf]

Hello, I am having trouble making a pulsed DC measurement using a 34401A DMM. The unit that we are trying to measure generates a 350uS pulse of approximately 950mV at a rate determined by the trigger input, which we are using a simple pulse generator for. We can measure the pulse amplitude on the scope, but we need something more accurate. We set the meter to trigger externally and connected the signal that we are using to trigger the unit under test. The problem is that the meter triggers on the negative edge of the trigger pulse and the UUT requires that the trigger pulse be longer than 350uS, so it is not measuring the pulse amplitude. We can't find anywhere in the manual where it gives a setup for this situation, other than a short section on external triggering. We also have a 3458A available. Thank you for any help.

Just set the oscope input to DC and use it as a comparetor, use a variable power supply set to the amplitude of the pulse you are measuering. You should be able to get close to the accuracy of your DVM.

 

[superbrew]

Thanks for the suggestion Rolf. We would like to be able to do this with a minimum amount of equipment, though. Also, some of our test points are < 100mV, so setting the power supply at those points can be tricky.

 

[crutschow]

The 34401 is probably not fast enough for you measurement but the 3458A voltmeter has a maximum read rate of 100k samples/sec so it should be able to take your reading with that in the external trigger mode. It does trigger on the negative edge of the trigger pulse so you will need to invert the pulse. If you have a pulse generator available you can use that to invert (and delay the pulse if necessary) to the point on the waveform you want to measure.

 

[Rolf]

Thanks for the suggestion Rolf. We would like to be able to do this with a minimum amount of equipment, though. Also, some of our test points are < 100mV, so setting the power supply at those points can be tricky.

I should have said a stable adjustable voltage source. You might even be able to use what you have with the addition of two ten turn pots, one for course and one for fine adjustment.
When you don't state how much accuracy you need it is difficult to help.

 

[Ubergeek63]

the scope should be fine for a repetitive pulse. you need a good digital scope for a single impulse.

 

[superbrew]

The spec for the unit is 1% of indicated value, so we need at least .25%. My main question was for technique required to measure this pulse with a DMM. I will see what I can do with the 3458A. I wil porbably just make a test jig to invert the trigger pulse from the generator. Thanks for the help all.

 

[Ubergeek63]

perhaps I am reading to much into it...a peak detector?