Calculating Db Characteristics Accuracy; Additional Derating Factors - Keithley 2016 User Manual

A-8 Specs and Accessories

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
of 10V, you must calculate the voltage accuracy and apply it to above equation.
To calculate a -60dB measurement, assume 10mVRMS for a V
range, one-year, 10Hz - 20kHz frequency band, and SLOW rate, the voltage limits are as
follows:
Accuracy =
Thus, the actual reading accuracy is 10mV ± 36µV 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
pertinent specifications, ranges, and other reference voltages.

Additional derating factors

In some cases, additional derating factors must be applied to calculate certain accuracy
values. 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 factors apply.
V
dB = 20 log -------------- -
V
REF
±[(0.06% of reading) + (0.03% of range)]
±[(0.006 × 10mV) + (0.0003 × 100mV)]
±[6µV + 30µV]
±36µV
10.036mV
dBm = 20 log ------------------------ - =
10V
9.964mV
dBm = 20 log --------------------- - =
10V
IN
REF
-59.96879dB
-60.03133dB
REF
of 10V. Using the 100mV