LSBrms Unit Meaning for the System Noise Specification

Issue Details

The product data sheet for my DAQ device gives a specification for system noise in units of LSBrms. How do I interpret the LSBrms value?

Solution

LSBrms stands for least significant bit root-mean-square and it measures the power of the system noise in terms of the smallest detectable change in voltage.

The "rms" of LSBrms refers to the amount of power in the system noise and the "LSB" in LSBrms refers to the smallest detectable change in voltage. Therefore, the LSBrms gives the power of the system noise in terms of the smallest detectable change in voltage. For example, if we have a 16-bit DAQ board and we are measuring from a range of -10 to 10 volts, then our smallest detectable change in voltage is equal to (range/resolution) = (20/(2^16)) = 0.305 mV. If our LSBrms value for system noise is 0.5, then our system noise is (0.5*0.305) = 0.153 mVrms.

Additional Information

LSB: The least significant bit is often used to refer to the smallest voltage change detectable by an A/D converter, or the smallest voltage change that can be generated by a D/A converter. Every analog voltage signal that you measure or generate must be represented in the computer as a binary value. The number of bits in this binary value depend on the specific DAQ device that you are using (12-bit, 16-bit, etc.). The number of bits determines the number of discrete values we can use to represent an analog signal for a given voltage range. For example, a 16-bit number can be used to represent 2^16 different values. If our voltage range is 0 to 10 volts, then we can translate our analog signal into 2^16 different values spaced equally between 0 volts and 10 volts. The difference between one value to the next is determined by the least significant bit.

RMS: The root-mean-square voltage of a signal is used for most electronic and physical measurements in order to represent the power carrying capability of a signal.