Spec Interpretation Guide; Accuracy Specifications - Agilent Technologies 34970A Manual

Spec Interpretation Guide

The following pages list the technical specifications
for the Agilent 34970A Data Acquisition/Switch
Unit and its modules. The explanations and exam-
ples below are helpful in understanding how to
interpret these specifications:
• Measurement accuracy is specified as percent
of reading plus percent of range, where reading
is the actual measured value and range is the
name of the scale (1V, 10V, etc.) —not the full
scale value (1.2V, 12V, etc.).
• DMM measurement accuracies include all switch-
ing errors. Switching errors are also listed
separately in the module specifications section.
Temperature measurement accuracies include
ITS-90 conversion errors. The thermocouple
accuracies include the reference junction error
as well.
• Accuracies are listed as either 24-hour, 90-day,
or 1-year specifications. This refers to the
length of time since the instrument's last cali-
bration. Use the specification that matches
your calibration cycle. The 24-hour specifica-
tions are useful for determining short-term
relative performance.
EXAMPLE 1: Basic dcV accuracy
Calculate the accuracy of the following
measurement:
9 V dc input
10 V dc range
1-year accuracy specifications
Normal operating temperature (18°C–28°C)
From the following page, the 1-year accuracy is:
0.0035% of reading + 0.0005% of range
Which translates into:
(0.0035/100 x 9 V)+
(0.0005/100 x 10 V) = 365µV
For a total accuracy of:
365 µV / 9 V = 0.0041%
EXAMPLE 2: Extreme operating temperature
When the 34970A is used outside of its 18°C–28°C
temperature range, there are additional tempera-
ture drift errors to consider. Assume the same
conditions in example 1, but at a 35°C operating
temperature.
The basic accuracy is again:
0.0035% of reading + 0.0005% of range=365 µV.
Now, multiply the 10 V temperature coefficient
from the following page by the number of degrees
outside of operating range for additional error:
(0.0005% reading + 0.0001% range)
/°C x (35°C - 28°C) =
(0.0005% reading + 0.0001% range)
/°C x 7°C =
0.0035% reading + 0.0007% range = 385 µV
Total error is then:
365 µV + 385 µV = 750 µV or 0.008%
EXAMPLE 3: Thermocouple measurement accuracy
Calculating the total thermocouple reading error
is easy with the 34970A—just add the listed meas-
urement accuracy to the accuracy of your trans-
ducer. Switching, conversion, and reference junc-
tion errors are already included in the measure-
ment specification.
For this example, assume a J-type thermocouple
input reading 150°C.
From the following page, total error is:
Thermocouple probe accuracy + 1.0°C
The probe vendor specifies accuracy of 1.1°C
or 0.4%, whichever is greater.
Total error is then:
1.0ºC + 1.1 ºC = 2.1ºC total, or 1.4%
EXAMPLE 4: acV Accuracy
The acV function measures the true RMS value
of the input waveform, regardless of waveshape.
Listed accuracies assume a sinewave input. To
adjust accuracies for non-sinusoids, use the listed
crest factor adder.
For this example, assume a ±1 V square wave
input with 50% duty cycle and a 1 kHz frequency.
Accuracy for 1 V, 1 kHz sinusoid is:
0.06% reading + 0.04% range
A 50% duty cycle squarewave has a crest factor of
Peak Value / RMS value = 1 V / 1 V = 1
From Crest Factor table, add:
0.05% of reading
The total accuracy is:
0.11% of reading + 0.04% of range = 1.5 mV or 0.15%
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