UNAOHM
NG500 (Rev. 0)
17
On a practical level, the noise spectrum is always limited by the bandwidth of the device in
which it is generated or through which it transits.
With respect to the result, this is equivalent to filtering the noise voltage from the start, picking
up only the one that crosses the device with the B bandwidth.
It may therefore be represented by the circuit in Figure 8 in which :
Figura 8 Equivalent circuit with pass band filter
K = 1.38 10
-23
Joule degrees Kelvin (Boltzmann constant).
T = absolute temperature in degrees Kelvin ( zero degrees centigrade corrisponds to 273 K).
B = bandwidth in Hz.
R = resistance value in
Ω
.
For example:
If “R” has a value of 1000
Ω
at an ambient temperautre of 20° C = (273 + 20 = 293 K) and the
filter has a bandwidth of 1 MHz, the voltage that will develop at the output will be :
______________________________
Vn =
√
4 x 1.38 10
-23
x 293 x 10
6
x 1000 = 4 µV
