Trying to figure out the correct way to calculate accuracy with LSD. I looked at a few sources, but I'm still confused on the topic. The question:

A digital voltmeter can display counts up to 1999999 (6 1 /2 digits). 
The range is set to 1 volt and has an input of 0.724 DCV. 
With an accuracy specified as ±(4 ppm rdg + 10 LSD), the accuracy is ± ______ DCV.

This is what I came up with:

10 of 199999 = .0005%
0.724 * 0.000004 + 0.0000005 = 0.000007896

However, it ended up being wrong. Just need a little help understanding it.


1 Answer 1


This accuracy is defined by gain error (4 ppm ) and offset error (10 LSD).
You then add these two for worst case.

0.724000 \$~~~~~\$ input
0.000002896 gain error
0.xxxx10 \$~~~~~~\$ offset error

0.000013 tolerance error

Remember gain and offset in future when working with ADC errors as well as incremental missing codes usually caused by logic ground noise on Vref.


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