Electronics > Beginners

MM Vs. Scope

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eev_carl:
Hi,

I have two resistors in series connected to a sine wave voltage source.  I'm getting two different readings with my multimeter and scope and was wondering if someone could explain the scope reading.  My multimeter gives me an RMS reading that matches the function generator (1.765V), but the scope gives me a lesser value (1.69V).  Is this just a difference in accuracy?

Both the multimeter and scope leads are connected to the voltage source and to the ground.

Thanks,
Carl

eb4fbz:
An oscilloscope is designed to look at waveforms, it's voltage calibration and resolution is secondary. However, a multimeter is carefully calibrated and use a higher resolution (although slower) ADC.

Gyro:
You haven't indicated what frequency you are running the function generator at. The frequency response of the meter and scope are different. At some lowish frequency point, the meter reading will start to drop off.

Kleinstein:
The amplitude calibration of scopes is often not that good - depending on the model some 2-5% are possible. So the difference could be just from the tolerances. In addition a probe (e.g. 10:1) might add some error. So does possible AC coupling if the frequency is low.

In addition the waveform from the generator may not be perfect and the DMMs might not include all the harmonics. Also AC accuracy of DMMs is often not that high, especially if using only a small part of the range. At higher frequencies the scope might be more accurate than the DMM, while around 50 Hz the DMM is usually more accurate.

eev_carl:
My input Vs is 5Vpp at 1kHz.

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