cDAQ Effective Number of Bits (ENOB)

Issue Details

What is the effective number of bits (ENOB) for various CompactDAQ input modules ?

Solution

Effective number of bits (ENOB) is a measure of the actual performance of an ADC after noise and non-linearity are included. The ENOB is computed as the signal-to-noise and distortion ratio of the ADC (in dB) minus 1.76, divided by 6.02. 

ENOB = (SINAD - 1.76) / 6.02

Where SINAD = 20*log10([half of full range / sqrt(2) rms] / input noise rms)

SINAD and ENOB are typically a function of signal frequency and amplitude. However, you can estimate low-frequency SINAD and ENOB from a device's noise specifications. 


Example:

The NI 9239 has an input noise of 70 uVrms.The biggest sine wave that fits in the +/-10 V ADC range is 10 V/sqrt(2) rms; thus a good estimate of the SINAD is 10 V/sqrt(2) / 70 uV and expressed in decibels (20*log10) equals 100 dB. 
Using the formula above leads to an ENOB of around 16.3.


Note:  If a device's specification is listed in "Input noise in ppm of RangeRMS", the calculation can be performed as the example below:

(Input noise in ppm/1000000) * full voltage range = Input noise RMS
Input noise in ppm = 7.6
Full range = ±60V = 120V
(7.6/1000000) * 120 = 912uV

Additional Information

This is the calculation for C series devices such as the NI 9239 analog input and the NI 9219 universal analog input.