EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: DonpK on December 22, 2021, 11:17:44 pm
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I'm using a Siglent DMM3055 to measure current drawn by a microcontroller. The controller is put in a low-power mode using a watch-dog timer library. The controller "sleeps" for 60 seconds then wakes up and runs a few lines of code, then goes back into the low-power mode.
The current measurement seems accurate - 25mA when awake and running the code and ca. 15µS when in the low-power mode. I am using the Trend Chart function on the DMM to track the current measurements. The measurements look okay - the current jumps to 25mA when the code is running, than back to 15µS. What doesn't look right is the length of time the current reading stays at 25mA. The current reading remains at 25mA for about a second even though the time it takes to run the code has to be only a few milliseconds.
My question is what is the minimum current pulse length the DMM can measure? There could be a problem with my code, of course, but I want to make sure I'm using the DMM appropriately.
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The meter doesn't care what the waveform is. It just reads the average. Of course, speed matters, so if the signal is very slow it will read first one value and then the other. The faster the jump between readings, the less time the meter has to go to the right value so it tends to read average.
However, the devil is in the details. If the meter samples the input, it can get confused if the input changes while it's generatiing a display. Thus you will get nonesense and the way out of it would be to add a filter.
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You could get lucky with the frequency counter function if your meter has it.
A little bit of maths
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Sounds like a great time to use an oscilloscope to check the pulse duration.
SDM3055 is advertised as “up to 150 readings/sec”, which means a measurement every 6.7ms or longer. Maybe it’s even slower in trend mode? Are you using it in triggered mode? Have you tried lowering the precision (e.g. to 3 1/2 digits), to get more readings/s?
Edit: Just checked the manual and learned that at 5 1/2 digit precision you can only get 5 readings/s in DCI, i.e. 200ms between readings, at best…
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You can power the circuit with a capacitor and measure the voltage drop of the capacitor when the circuit works. So you can easily measure the electrical charge consumed.
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Thanks for the several good suggestions. I'll follow them up.