Re: Accuracy of DVM is "+/-0.8% +1"...what does this mean.
Hello,
The accuracy of the Tenma 72-7765 DVM on the 400mV range is quoted on page 1 as "+/-0.8% +1".
..do you know what this means?....the "+1" bit?
Tenma 72-7765 DVM Datasheet:-
https://www.electro-tech-online.com/custompdfs/2013/04/1661853.pdf
Accuracy specifications: ±(% of reading + number of LSD)
LSD = Least Significant Digit
:wink:
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Read from here:
DMM accuracy
Some instruction manuals list basic meter accuracy as ± % of reading. For example, if the basic meter accuracy in the dc volts range is ±1 %, and the true voltage is 1.00 V, the meter is expected to display a reading of 1.00 V ±1 %, or 0.99 V to 1.01 V. However, basic accuracy does not take into account the inner workings of the ADC (that is at the heart of every DMM) and other circuitry on the analog side. These circuits and the ADC have tolerances, nonlinearities, and offsets that vary from function to function. In addition, signal noise might require limiting the resolution. To give meter users a more accurate value, DMM manufacturers present accuracy specifications in the following format:
Complete accuracy specifications: ±(% of reading + number of LSD)
Where:
Reading = the true value of the signal that the DMM measures
LSD = least significant digit
The LSD represents the magnitude of uncertainty due to internal offsets, noise, and rounding errors. For a given DMM the number of LSDs varies from function to function and even from range to range for the same function. Accuracy and range selection need to be considered independently, otherwise a misunderstanding can lead to gross errors. For example, consider the following:
A 3½-digit display DMM measures an output of a precision 1.2-V reference. Presume that the true voltage is 1.200 V. The DMM manual shows the dc volts accuracy specification as ±(0.5% + 3). How should you measure the voltage and interpret the reading?
First, set the meter to the 200-V range. The display will indicate the measured voltage as XX.X. The percentage of reading is (1.200)(0.5)/100 = 0.006 V, which cannot even be seen on the display because only one digit after the decimal point is shown. However, when accounting for the three allowed LSD counts, realize that the last digit on the display can vary by ±3 counts. So, the meter can display a value of 1.2 ±0.3 V, or a range of 0.9 V to 1.5 V. This is a ± 25% potential error with all factors combined and is not acceptable for a precision measurement.
Set the switch to the 20-V range and it will display the value as X.XX, which improves the accuracy. The complete accuracy can be a calculated as ± (1.200)(0.5)/100 +0.03) = ± 0.036 V. So, any reading between 1.16 V and 1.23 V is within the accuracy specifications. This complete accuracy is ± 3% of reading, which is better, but still not accurate enough.
Finally, set the DMM to the 2-V range. The display format changes to X.XXX. The percentage of reading does not change, but the third LSD becomes a smaller factor. The complete accuracy can be a determined as ± (1.200)(0.5)/100 +0.003) = ± 0.009 V. The meter display is only allowed to be within the narrow 1.191 V to 1.209 V range. Now the complete accuracy is only ± 0.75% of reading, which is sufficient for the measurement. So, selecting the lowest measurement range before the DMM over-ranges reduces the negative effect of the number of LSDs and gives the most accurate results.
http://www.designworldonline.com/how-to-determine-digital-multimeter-accuracy/#_