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The Differences Between Accuracy and Bits of Resolution

Hardware: Multifunction DAQ (MIO)>>M Series

Problem:
I have a data acquisition card that has an ADC with a specified bits of resolution. I calculated the code width of my card using the formula:

Code Width = Range/Gain*(2^Resolution in Bits)

to determine the smallest change my ADC should be able to measure, and I expect to obtain an analog input measurement that falls within my code width tolerance. However, when I take an actual measurement with the card, my reading falls outside of my calculated code width. Why is this?

Solution:
The code width is not the only variable that affects the absolute accuracy of a DAQ card. Code width is a measure of how precise the Analog to Digital Convertor (ADC) can translate an analog signal into its digital form before it can be passed on to the computer.

However, before the signal can be received by the ADC, several things must happen:

• The proper input channel must be selected
• The appropriate amount of gain must be applied
• Some signal conditioning may need to occur.

To see how these components come together, a hardware model taken from the M series DAQ user manual is shown below as an example:

All of these components including the ADC will be affected by real world phenomena, which will introduce gain error, offset error, system noise, and temperature drift on the signal. Therefore, the resulting absolute accuracy of the device will depend on the environment that the card is used in.

The equation to calculate the expected noise is as follows:

Absolute Accuracy = ±((Input Voltage * % of reading) + Offset + System Noise + Temperature Drift

This formula requires a number of inputs that can all be referenced in tables that are typically found in the data and specifications sheets for the acquisition products. Some data sheets already have the absolute accuracy in various operating environments calculated for you.

For more information on accuracy, please refer to the links below.

Related Links:
KnowledgeBase 2X4HGEBG: How Do I Calculate Absolute Accuracy or System Accuracy?
NI Developer Zone: Data Acquisition Fundamentals
NI Developer Zone: Understanding Instrument Specifications -- How to Make Sense Out of the Jargon

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Report Date: 09/14/2005
Last Updated: 04/11/2008
Document ID: 3PD0S4DI

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