Section 5: Basic Digital Multimeter (DMM) Operation
Series 3700 System Switch/Multimeter User's Manual
5-76
3700S-900-01 Rev. C / July 2008
As an example of how to calculate the actual readings limits for dB, with a user-
defined VREF of 10V, you must calculate the voltage accuracy and apply it to
the above equation.
To calculate a -60dB measurement, assume 10mV RMS for a VREF of 10V.
Using the 100mV range, one-year, 10Hz - 20kHz frequency band, and SLOW
rate, the voltage limits are as follows:
Accuracy =
±[(0.06% of reading) + (0.03% of range)]
±[(0.0006 × 10mV) + (0.0003 × 100mV)]
±(6µV + 30µV)
±36µV
Thus, the actual reading accuracy is 10mV ±36mV 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 for the following
conditions:
1. -0.4mV with open inputs and the 10M-ohm divider enabled
2. +/-(8ppm or r 5uV) with autozero off for +/-1 degree C and
<=10minutes
3. For 2-wire ohms, add 100m-ohm to "ppm of range" with REL
4. Add 0.1% to 10M-ohm range when measuring through a Model 3700 card
>50%RH humidity
Before calculating accuracy, study the associated specifications very carefully to
see if any derating factors apply.