Why is there a difference in the voltage drop across the same resistor when using an oscilloscope and a multimeter?

Question

I am measuring the voltage drop across a 1 ohm resistor within a circuit with an oscilloscope and a multimeter.

The impedance of the oscilloscope is 1M ohm and the impedance of the multimeter is 10M ohm.

I am reading approx 5mV when using the multimeter and approx 2mV with the oscilloscope.

Which of these two values would be most accurate, if any and if not, how would I go about ensuring the voltage drop measured across the resistor is accurate?

Edit -

Accuracy of equipment:

• Oscilloscope

  • [3%×(|reading|+|offset|)+1%×|offset|+0.2div+2mV], ≤100mV/div

• Multimeter

  • 1.0% + 3 counts

Settings used:

• Oscilloscope
  • 10mV / div
• Multimeter
  • mV setting with resolution of 0.1mV

Answer 1

I would normally trust a DVM over a oscilloscope for accuracy but given the published specifications we have:

DVM: reading 5mV, mV range resolution 0.1mV. 3 counts is therefore 0.3mV.

Specification states reading is between:

\$ 0.99 \times \text{actual} - 0.3\text{ mV} \$ and \$ 1.01 \times \text{actual} + 0.3\text{ mV} \$

Rearranging this we find actual is between:

\$ \dfrac{\text{reading} - 0.3\text{ mV}}{1.01} \$ and \$ \dfrac{\text{reading} + 0.3\text{ mV}}{0.99} \$.

Making it between \$ 4.6534 \text{ mV} \$ and \$ 5.4536 \text{ mV} \$

Now for the oscilloscope we have an offset which is where 0V is on the screen. Let's assume it is in the middle making \$ \text{offset} = 0 \text{ mV} \$

We have a reading of 2mV, 10mV per division making 0.2 divisions 2mV.

This gives us an actual output voltage between.

\$ 0.97 \times{reading - (2+2)\text{ mV}} \$ and \$ 1.03 \times{reading + (2+2)\text{ mV}} \$.

That is between \$ -2.06 \text{ mV} \$ and \$ 6.06 \text{ mV} \$

Answer 2

There is no way to know which of these readings is more accurate unless you can tell us the uncertainty specifications for the two instruments. There are lousy oscilloscopes and great multimeters, great oscilloscopes and lousy multimeters.

If you want to determine the accuracy of the measurements you must consult the error/uncertainty specifications for the two instruments. Make sure that they have been recently calibrated using the manufacturer's recommended procedure. Make sure you are using the instruments under the conditions (temperature, humidity, warm-up time, etc.) specified by the manufacturer. Then, and only then, you can apply the uncertainty specifications from the manufacturer and determine the uncertainty (lack of accuracy) in the measurements.