Specifications A-11
Calculating dB characteristics accuracy
The relationship between voltage and dB is as follows:
As an example of how to calculate the actual readings limits for dB, with a user-defined V
REF
of 10V, you must calculate the voltage accuracy and apply it to above equation.
To calculate a -60dB measurement, assume 10mVRMS for a V
REF
of 10V. Using the 100mV
range, one-year, 10Hz - 20kHz frequency band, and SLOW rate, the voltage limits are as fol-
lows:
Accuracy = ±[(0.06% of reading) + (0.03% of range)]
±[(0.006
×
10mV) + (0.0003
×
100mV)]
±[6µV + 30µV]
±36µV
Thus, the actual reading accuracy is 10mV
±
36
µ
V or 10.036mV to 9.964mV. Applying the
voltage reading accuracy into the dB equation yields:
Thus, the actual reading accuracy is -60dB + 0.031213dB to -60dB - 0.031326dB.
dBm and dB for other voltage inputs can be calculated in exactly the same manner using per-
tinent specifications, ranges, and other reference voltages.
Additional derating factors
In some cases, additional derating factors must be applied to calculate certain accuracy val-
ues. For example, an additional derating factor must be added for DC voltages over 500V. Before
calculating accuracy, study the associated specification notes carefully to see if any derating fac-
tors apply.
dB = 20 log
V
IN
V
REF
---------------
dBm = 20 log
10.036mV
10V
------------------------- -59.96879dB=
dBm = 20 log
9.964mV
10V
---------------------- -60.03133dB=
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