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I have a circuit where I have to measure tens of uA of currents.

When measuring it with a 50Eur multimeter, the multimeter shows the currents with acceptable accuracy and so far no significant influence the was observed doe to the resistance of the multimeter.

When trying to achieve the same with a 12bit oscilloscope, I have to use 500Ohm shunts to get a measurable voltage. This resistance has to much of an influence on the whole measurement.

How is it that a 6 times cheaper device (the multimeter) is so much better in measuring low values than the oscilloscope?

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    \$\begingroup\$ for the same reason that a much cheaper bicycle is better at weaving in and out of stationary traffic than your more expensive car. You wouldn't use your multimeter to examine a 1MHz signal, and you wouldn't use your bike to transport a wardrobe to the next city. Each is good at some things, and not others' \$\endgroup\$
    – Neil_UK
    Commented Sep 6, 2016 at 15:37
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    \$\begingroup\$ The multimeter can average lots of slow measurements with a high settling time. The scope is optimised for speed rather than sensitivity. \$\endgroup\$
    – pjc50
    Commented Sep 6, 2016 at 15:39
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    \$\begingroup\$ Because multimeter shunts are much less than 500Ohm, but instead in the mOhms range. There are preamplifiers available to measure small currents: eevblog.com/projects/ucurrent \$\endgroup\$
    – 0x6d64
    Commented Sep 6, 2016 at 15:40
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    \$\begingroup\$ The faster you have to do a measurement the more difficult it is the be accurate. A DVM has a sampling rate in the range of 1Sps (Sample per second). Scopes have sampling rates in the range of GSps. There is a factor of \$10^9\$. \$\endgroup\$
    – Curd
    Commented Sep 6, 2016 at 16:13
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    \$\begingroup\$ If you took all the data collected by the scope, it is possible that you could filter it down and analyze to provide a measurement not far off. But that would only be filtering in the digital domain - the meter is quieter at a given gain because it also has far lower analog bandwidth to its gain. It may also have a better ADC. 100 MSPS 16 bit ADCs filtered down to a narrow bandwidth don't do too badly against 20-24 bit audio class ones, but a 1 GSPS 12 bit one may not be comparable. \$\endgroup\$
    – Chris Stratton
    Commented Sep 6, 2016 at 16:38

2 Answers 2

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  • if both are 12bit then, the DMM has a much smaller full scale reading , eg 200mV,
  • whereas DSO might be 100mV/div .
  • 10uA is not a suitable parameter for DSO's.
  • If you wanted 100mV on DSO then Rs would be 100mV/10uA= 10k not 0.5k Ohm

Thus the problem may be your test method, not the instruments.

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As the sensitivity (boxes per volt) of an oscilloscope is increased, the error band described by the constant minimum thickness of the oscilloscope's best focused trace increases, with respect to the signal.

Digital multimeters generally have a noise floor below that of oscilloscopes, don't have an indecision band as wide as an oscilloscope's trace and, consequently, can exhibit greater precision for static measurements.

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