Question

Consider a 3 digit display for a DVM with an accuracy of ± 0.5 % for a reading of ± 1 digit. Compute the error for 5 V reading on a 10 V range.

(a) ± 10 V

(b) ± 0.035 V

(c) ± 0.05 V

(d) ± 1 V

This question was addressed to me in an international level competition.

My question is taken from Resolution and Sensitivity topic in section Electronic Instruments of Electrical Measurements

Answer 1

Right answer is (b) ± 0.035 V

Best explanation: The resolution of a DVM is,

R = ^1⁄10^n

Here, n is 3. Substituting n=3 in the equation for resolution we get,

R = ^1⁄10^3 = 10^-3 = 0.001

For 10 V range, the resolution is,

R10V = 10 × 0.001 = 0.01 V

Consider the reading of 5 V.

Error = ± 0.5 % of 5 = ^0.5⁄100 × 5 = ± 0.025 V

1 digit error = ± 0.01 V

 Total error = ±(0.025 V+ 0.01 V)=± 0.035 V.