Differences between oscilloscope, multimeter, and power supply voltage readings

Question

So I recently got myself an oscilloscope and am trying to learn how to use it by starting with the basics, a circuit with a voltage source, and a resistor. When I set my power supply to 5V, the multimeter reads 4.995V and the oscilloscope reads 5.02V rms. I have found a thread already explaining how to tell which one is more accurate, but I am looking for more of an explanation of the other things at play here causing these to be different.

I figure the power supply is just giving an estimated reading, it was a fairly cheap one so I understand if it dent exactly output the voltage it reads.

After this picture I realized the different locations of where I am measuring might be a factor as breadboards tend to have undesirable effects to circuits. It didn't change anything.

What are some other possible sources to the discrepancy and what can I do to try and remediate it (already calibrated the oscilloscope), if anything?

Answer 1

When I set my power supply to 5V, the multimeter reads 4.995V and the oscilloscope reads 5.02V rms.

5.02 V / 4.995 V = 1.005. That's a 0.5% difference. 5.00 V / 4.995 V = 1.001. That's a 0.1% difference. The specified accuracy of your oscilloscope is +-3%, the multimeter is +-0.5% +3 counts, and the power supply meter's accuracy is unknown.

There is no discrepancy. All the measurements are within their error bars for an actual value of between 4.97 V and 5.02 V, so they are all effectively the same.

Answer 2

This is fine. None of those devices are precision instruments (and the power supply is probably not the least accurate among them!), and you're seeing a difference of < 0.5% between the highest and the lowest. There's nothing you should be doing, unless you're involved in some high-precision physics work, in which case you should get your lab director to buy you some new gear and a better place to put it than the kitchen counter. For hobby work, you're already doing more than good enough.

Answer 3

Of the three instruments you have on your table, the multimeter will be the most accurate and precise for measuring a DC voltage.

Scopes have larger input offset errors as well as lower ADC resolution than a multimeter, but have a much higher sampling rate. Scopes can also have overall gain error and certain non-linear sampling errors. So there’s a definite trade off when using a scope to measure DC or DC-biased signals. Truth be told, most scopes kind of suck at it really, but if you're measuring DC-biased dynamic signals like the voltage across a resistive current sensor, using a scope is essential.

Your scope can be calibrated better, but it’s still within its margin of error. If this matters for your measurements and you want a quick fix, you can characterize the scope DC offset vs. the meter and subtract the offset in subsequent measurements to improve accuracy. This is good enough for most work. (n.b., that scope input offset can differ from range to range, so you need to characterize the ranges separately.)

If you (or your employer) really, really care about this (for example, for compliance with a QA standard like ISO9001) you will need to spend more money on the scope with more ADC bits, fancier input amplifiers, and NIST/NBS-traceable calibration for it from time to time. You might also need to invest in some precision calibration references for your lab use.

Relevant: Accuracy vs. Precision, https://blog.minitab.com/en/real-world-quality-improvement/accuracy-vs-precision-whats-the-difference

Answer 4

Your Scope will tell you Vpeak and Vrms(root mean square). Your multimeter gives, if it's a true rms, well that is should give the same rms value as your scope, if the signal is a pure sinus. Your multimeter can also give wrong values depending on the frequency it can handle. Good multimeters are true rms up to 1MHz. Be sure to put the Power supply behind a safety transformer when you need to measure the primary part of it, or use a differential probe so you won't create a short and blow up either your scope or the Power Supply.