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| Why doesn't my RMS multimeter agree with my Rigol 1054Z scope on the AC VRMS? |
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| Mike Meschi:
Here’s the circuit and my problem. My function generator is simply parallel connected to a multimeter and my scope. No load is connected and I am not using a scope probe, just a BNC connector with alligator clips. I was just checking to see if the readings on my scope for AC VRMS were the same as the AC VRMS readings on my multimeter. My multimeter is a true RMS meter. The scope is calibrated and has a 1M ohm input impedance. The function generator has a 50 ohm output impedance. So I set the function generator to put out 2 volts AC RMS (as read by my multimeter) at 500 Hz. (as entered on the function generator). Then I read the AC VRMS voltage on the scope and it does not match the multimeter. Not even close. If the multimeter reads 2 volts AC RMS then the scope reads 500 millivolts AC RMS. The multimeter does read the correct frequency. If I crank up the frequency of the function generator to say 1K Hz., then 10K Hz., the difference in AC VRMS readings is lesser on the scope but the same on my multimeter. Same thing happens if I crank up the amplitude of the function generator. The readings are always different but never by the same amount. So I tried a different multimeter and get the same results. The waveform on the scope always shows the right frequency and is a perfect sine wave but never reads the same AC VRMS or peak to peak voltage as the multimeter. If I take the peak to peak voltage given to me by the scope and multiply it by .707 to get the RMS voltage, it results in the same reading as the AC VRMS on the scope. Shouldn’t they be the exact same readings? What is going on? What is the best way to check the accuracy of the AC VRMS readings you get on the scope? It would seem the scope is reading incorrectly since both multimeters read the same. |
| Fungus:
Some setting is wrong on the 'scope but it's impossible to say without screenshots. eg. Is the 'scope in AC mode? Is the displayed signal centered around zero? |
| AG6QR:
It does sound like the oscilloscope may have an issue. If your 'scope has more than one channel, have you tried each of them? Also, as a sanity check, try measuring a DC voltage with your 'scope. Something like an AA battery should measure somewhere around 1.2-1.5V. Be sure the 'scope is set to DC coupling. Something to be aware of is that a multimeter typically has a much lower bandwidth than an oscilloscope. So at high frequencies, the response of the meter will fall off much faster than the response of the oscilloscope. But this doesn't seem to be the cause of your particular problem, because it would cause the multimeter to read lower, not higher than the oscilloscope. Still, it's not a bad idea to do an initial test at 60Hz, since any multimeter that measures true RMS ought to be able to handle mains power frequencies perfectly well. And an oscilloscope ought to handle it perfectly, as well. Once that works, try higher frequencies. |
| wasedadoc:
--- Quote from: Mike Meschi on August 09, 2023, 05:21:38 pm ---. If I take the peak to peak voltage given to me by the scope and multiply it by .707 to get the RMS voltage, it results in the same reading as the AC VRMS on the scope. --- End quote --- For a sine wave the peak-peak voltage is 2.828 times the RMS. Or expressed the other way, multiply the peak-peak by 0.3535 to get RMS. |
| alm:
The values should be pretty close (within 1% or so) as long as the signal is well within the bandwidth of both instruments. I agree with Fungus that we need much more information. Screenshots, models of the function generator and DMM, and a picture of the setup showing cabling would help. My first guesses would indeed either be that the scope is set to AC coupling, which forms a high pass filter. Check the manual for the lower bandwidth limited when AC coupling is enabled. What is the probe attenuation setting on the scope? Is it 1x or some other number? |
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