State of the art automatic condition diagnostic, monitoring, and balancing software

5. The Technical Means of Diagnostics and Balancing.

Question 5.2.

In many analyzing instruments low register analog-digit converters are used, for example with eight digits. And it is specified that the instrument can separate in spectral analysis two harmonic components the amplitude of which differ from one another in 1000 times. Is it really possible? The value of one digit in this case is 0.25%, but not 0.1%.
(The question was asked by the students of the system-engineering specialty.)

The question is answered by Alexej V. Barkov:

Really when the analog signal is converted into digital form all its range occurred to be 1/256 of the amplitude range, then it cannot be detected. But if the total signal amplitude is higher than the level of the first register of the analog-digital converter and further the signal is converted by the Fourier transform then the total dynamic range is increased in several times (in about half of the sqare root of n times, where n is the number of the digital signal readings used in the Fourier transform). So, when you have an 8 digit ADC then on the background of an intensive spectrum component it is possible to detect a component with an amplitude 1000 times less.

By the same reason the amplitude of the internal instrument noise, when the wave form of the signal is measured (time domain), becomes much higher than the component amplitudes of the internal noise in the spectral signal analysis.