Electronics > Beginners

MM Vs. Scope

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eev_carl:

--- Quote from: Doctorandus_P on July 12, 2018, 10:55:03 pm ---What are the values of your series resistors?

What kind of DMM / Oscilloscope are you using?
Toy's, or decent stuff?
If your scope is "way off" @ 100kHz, I suspect you have a toy like the DSO150 or DSO nano or similar, or your function geneator is not capable of generating proper signals for such a frequency.

--- End quote ---

Hi,

Resistors are 1k and 10k.

I'm using a Klein MM700 (True RMS).  It's listed at 50-400Hz, so I switched my Vs to 5Vpp at 400Hz.  My scope is a Rigol MSO1104Z.  The function generator is a DG1022Z.

I watched a W2AEW video on probes and adjusted my compensating capacitor.  That seems to bring the values closer.

bitseeker:
Also, for amplitude measurements on a digital scope, be sure to use as much of the vertical scale as possible. Setting the V/div too high doesn't fully utilize the ADC and will not give as accurate a result. You want the waveform to span as many divisions as possible without going out of bounds in order to use as many of those bits as possible.

Kleinstein:
The limited resolution of the ADC is not such a big problem for RMS calculation for the scope. It can use quite some samples and thus adds resolution from oversampling. With the usual 1:2:5 steps at the scope, it should be possible to use at least 30% of the ADC range.

The scope might be off at low frequency due to AC coupling roll off. Otherwise I would not expect too much deviation towards low frequency here. If the DMM is specified till 400 Hz, it might already be off quite a bit at 2 kHz - enough to compensate for some difference in calibration of the scope and DMM. Heaving the same reading on the DMM and scope does not mean the reading is accurate.

The cheap DMM might also have quite some error for the RMS value, especially if the signal is less than 20% of full scale. So with the typical 1:10 steps, some AC voltages might be hard to measure accurately with such a meter.

The input impedance of the scope and AC input of DMMs is often not that different: scopes are usually 1 M + some 20 pF and DMMs  are often 10 M and sometimes (especially the better ones) 1 M in AC mode, with usually a little more capacitance.

Circlotron:

--- Quote from: bitseeker on July 13, 2018, 04:51:51 am ---Also, for amplitude measurements on a digital scope, be sure to use as much of the vertical scale as possible. Setting the V/div too high doesn't fully utilize the ADC and will not give as accurate a result. You want the waveform to span as many divisions as possible without going out of bounds in order to use as many of those bits as possible.

--- End quote ---
Not only that, make sure you have several cycles visible on the screen. On my scope when you wind the frequency down and the ends of the cycle stretch off each side of the screen the digital volts measurement reads very low.

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