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Multimeter vs. Oscilloscope

I am sure that some of you will wonder why we need all this complex stuff: amplifier, oscilloscope, additional software? Why all this when we could simply take a regular digital ammeter or multimeter and measure all necessary current with it?

Well, unfortunately, you can obtain more or less adequate results with a multimeter only in idle mode, when the HDD heads do not move. To illustrate this statement we took an oscillogram for Maxtor Atlas 15K II HDD consumption current when working in Intel IOMeter Random Read pattern. Red color stands for the current along +5V power rail, blue color – for +12V power rail, 0 is marked with a black horizontal line and horizontal sweep equals 5msec/div:

The main part of the power the HDD consumes along +12V rail is spent on moving the heads; impulses come in pairs: the first one corresponds to the beginning of the head movement (speeding up) and the second one to the end down of the movement (slowing down). The distance between them varies from almost 0 to the time it takes to move the head from one side of the drive to the other, depending on how “lucky” the HDD is with the sequential requests. Before the heads start moving, we can also see increase in power consumption along the +5V rail indicating that the HDD electronics dealing with the next request activates.

However, we are primarily interested not in the HDD mechanics, but in the impulse characteristics. As you see, first, their amplitude is pretty high (about 4-5 times higher than the constant), second, the front is almost vertical while the entire impulse may be less than a millisecond. How big is the chance that your multimeter will catch this peak?

Unfortunately, there is hardly any chance at all. Multimeters are devices that are intended to work with constant voltage (and constant current, respectively). They simply do not use fast ADC, because it doesn’t make any sense. A typical multimeter measures at a pace of a few tenths of a second, which is 20x higher than what a current impulse from the moving heads is.

To make this explanation more illustrative we translated the oscillogram above into a spectrum:

As you see, there is a big peak at 0 (direct current level), relatively high and more or less steady level in the interval up to a few tens of kHz, high jump to 42.8kHz and another one at 85.6kHz. So, we need a device that can work with at least up to 100kHz frequencies in order to measure adequately the parameters of a signal like that, and multimeter is definitely not the one for the task.

To check out this theory we used two almost randomly selected multimeters – and inexpensive Mastech M890G and a more serious Uni-Trend UT70D. The latter also allows to detect the average, minimum and maximum values within a given period of time.

 
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