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.