112
Functional Description
11.4
Voltage Inputs
The voltage inputs uses low-TC (temperature coefficient) voltage-divider resistor networks as signal
attenuators. Voltage inputs may be configured as a three-phase, three-element input, with four
connections (A, B, C, and N), or configured as a two-element input, with independent connections
to each element (A+, A–, C+, and C–, for example).
11.5
Self Calibration
Designed to provide exceptional stability over time and temperature, the input sections use several
high-performance and higher-priced components. Drift and temperature sensitivity are reduced,
in the programmable gain amplifiers (PGA’s) and analog-to-digital converter (ADC), by use of
an internal, continuous self-calibration process. The inputs to the PGA’s are multiplexed, each
to four different signals: the three current (or voltage) inputs, and a calibration signal generated
by a special source built in for the purpose. This enables the PGA’s and ADC’s to make use of
lower-cost components, since their drifts and initial errors are removed by self-calibration.
11.6
Error Analysis
All identified sources of error in the 1133A have been quantified using worst-case manufacturers’
performance data. These have then been combined using a root-sum-of-squares (RSS) method to
yield a performance estimate. Effects due to initial calibration, measurement noise, temperature,
and aging are all included. The reason for using RSS analysis is beyond the scope of this paper.
However, we have found in our many years’ experience building calibration instruments that this
method yields the most realistic estimate of actual worst-case performance, provided that numerous
errors contribute significantly to the overall performance (i.e., no one error dominates), and provided
that worst-case actual data is used for the analysis.
The error analysis, used for the 1133A power or energy measurements, is shown in Table 6-1.
Similar analyses have been performed for the other functions of the instrument, and include most
of the same factors shown here.
11.7
Signal Sampling
The signals, representing the scaled voltage and current inputs and the calibration signal, are time-
multiplexed into a two-channel ADC. Each signal is sampled at a rate of 10,240 samples per second.
The current and voltage signals for each phase are sampled simultaneously to eliminate errors in
power calculation, which would occur with non-simultaneous sampling. The sample clock in the
1133A is synchronized to UTC-USNO (GNSS) within one microsecond, allowing measurements of
phase angle across a power grid to be compared directly, and ensuring that revenue is billed at the
correct rates.
11.8
Power and Energy
Power and energy are determined by making twenty separate measurements per second of the cross
product of voltage and current for each phase. Each measurement uses 1024 samples (i.e., it takes
