Appendix A Specifications - Agilent Technologies 3458A Calibration Manual

Appendix A
Introduction
The 3458A accuracy is specified as a part per
million (ppm) of the reading plus a ppm of
range for dcV, Ohms, and dcl. In acV and acl,
the specification is percent of reading plus
percent of range. Range means the name of the
scale, e.g. 1 V, 10 V, etc.; range does not mean
the full scale reading, e.g. 1.2 V, 12 V, etc.
These accuracies are valid for a specific time
from the last calibration.
Absolute versus Relative
Accuracy
All 3458A accuracy specifications are relative
to the calibration standards. Absolute accuracy
of the 3458A is determined by adding these
relative accuracies to the traceability of your
calibration standard. For dcV, 2 ppm is the
traceability error from the Agilent factory.
That means that the absolute error relative to
the U.S. National Institute of Standards and
Technology (NIST) is 2 ppm in addition to the
dcV accuracy specifications. When you
recalibrate the 3458A, your actual traceability
error will depend upon the errors from your
calibration standards. These errors will likely
be different from the Agilent error of 2 ppm.
Example 1: Relative Accuracy; 24 Hour
Operating temperature is Tcal ± 1°C
Assume that the ambient temperature for the
measurement is within ± 1°C of the
temperature of calibration (Tcal). The 24 hour
accuracy specification for a 10 V dc
measurement on the 10 V range is 0.5 ppm ±
0.05 ppm. That accuracy specification means:
0.5 ppm of Reading + 0.05 ppm of Range
For relative accuracy, the error associated with
the measurement is:
(0.5/1,000,000 x 10 V) + (0.05/1,000,000 x 10 V) =
± 5.5 µV or 0.55 ppm of 10 V
Errors from temperature
changes
The optimum technical specifications of the
3458A are based on auto-calibration (ACAL)
of the instrument within the previous 24 hours
and following ambient temperature changes of
less than ±1°C. The 3458A's ACAL capability
corrects for measurement errors resulting from
the drift of critical components from time and
temperature.
The following examples illustrate the error
correction of auto-calibration by computing
the relative measurement error of the 3458A
for various temperature conditions. Constant
conditions for each example are:
90 day accuracy specifications
Example 2: Operating temperature is
28°C;
With ACAL
This example shows basic accuracy of the
3458A using auto-calibration with an
operating temperature of 28°C. Results are
rounded to 2 digits.
(4.1 ppm x 10 V) + (0.05 ppm x 10 V) = 42 µV
Total relative error = 42 µV
Example 3: Operating temperature is
38°C;
Without ACAL
The operating temperature of the 3458A is
38°C, 14°C beyond the range of Tcal ±1°C.
Additional measurement errors result because
of the added temperature coefficient without
using ACAL.
(4.1 ppm x 10 V) + (0.05 ppm x 10 V) = 42 µV
Temperature Coefficient (specification is per
°C):
(0.5ppm x 10V + 0.01 ppm x 10V) x 14°C = 71 µV
Total error = 113 µV
Example 4: Operating temperature is
38°C;
With ACAL
Assuming the same conditions as Example 3,
but using ACAL significantly reduces the
error due to temperature difference from
calibration temperature. Operating
temperature is 10°C beyond the standard range
of Tcal ±5°C.
(4.1 ppm x 10 V) + (0.05 ppm x 10 V) = 42 µV
Temperature Coefficient (specification is per
°C):
(0.15ppm x 10V + 0.01ppm x 10V) x 10°C = 16µV
Total error = 58 µV
Specifications
Example 5: Absolute Accuracy; 90 Day
Assuming the same conditions as Example 4,
but now add the traceability error to establish
absolute accuracy.
(4.1 ppm x 10 V) + (0.05 ppm x 10 V) = 42 µV
10 V DC input
Temperature Coefficient (specification is per
10 V DC range
°C):
Tcal = 23°C
(0.15ppm x 10V + 0.01ppm x 10V) x 10°C = 16µV
Agilent factory traceability error of 2 ppm:
Additional errors
When the 3458A is operated at power line
cycles below 100, additional errors due to
noise and gain become significant. Example 6
illustrates the error correction at 0.1 PLC.
Example 6: operating temperature is
28×C; 0.1 PLC
Assuming the same conditions as Example 2,
but now add additional error.
(4.1 ppm x 10 V) t (0.05 ppm x 10 V) = 42 µV
Referring to the Additional Errors chart and
RMS Noise Multiplier table, additional error
at 0.1 PLC is:
(2 ppm x 10 V) + (0.4 ppm x 1 x 3 x 10 V) = 32 µV

Appendix A Specifications

(2 ppm x 10 V) = 20 µV
Total absolute error = 78 µV
Total relative error = 74 µV
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