Datasheet

71M6511/71M6511H
Single-Phase Energy Meter IC
DATA SHEET
NOVEMBER 2010
Page: 14 of 98 © 2005–2010 Teridian Semiconductor Corporation V2.7
A Maxim Integrated Products Brand
There is no correlation between the line signal frequency and the choice of PRE_SAMPS or SUM_CYCLES (even though when
SUM_CYCLES = 42 one set of SUM_CYCLES happens to sample a period of 16.6ms). Furthermore, sampling does not have to
start when the line voltage crosses the zero line.
Delay Compensation
When measuring the energy of a phase (i.e., Wh and VARh) in a service, the voltage and current for that phase must be
sampled at the same instant. Otherwise, the phase difference, Ф, introduces errors.
o
delay
o
delay
ft
T
t
360360 ⋅⋅=⋅=
φ
Where f is the frequency of the input signal and t
delay
is the sampling delay between voltage and current.
In traditional meter ICs, sampling is accomplished by using two A/D converters per phase (one for voltage and the other one
for current) controlled to sample simultaneously. Teridian’s Single-Converter Technology
®
, however, exploits the 32-bit signal
processing capability of its CE to implement “constant delay” all-pass filters. These all-pass filters correct for the conversion
time difference between the voltage and the corresponding current samples that are obtained with a single multiplexed A/D
converter.
The “constant delay” all-pass filters provide a broad-band delay β that is precisely matched to the difference in sample time
between the voltage and the current of a given phase. This digital filter does not affect the amplitude of the signal, but
provides a precisely controlled phase response. The delay compensation implemented in the CE aligns the voltage samples
with their corresponding current samples by routing the voltage samples through the all-pass filter, thus delaying the voltage
samples by β, resulting in the residual phase error β – Ф. The residual phase error is negligible, and is typically less than ±1.5
milli-degrees at 100Hz, thus it does not contribute to errors in the energy measurements.