MAXQ3181 Maxim, MAXQ3181 Datasheet - Page 50

• Manufacturer Part Number: MAXQ3181
• Description: The MAXQ3181 is a dedicated electricity measurement front-end that collects and calculates polyphase voltage, current, active power and energy, and many other metering parameters of a polyphase load
• Manufacturer: Maxim

Low-Power, Active Energy, Polyphase AFE
• Energy accumulation (including energy pulse output
• Overvoltage detection
• Overcurrent detection
• Undervoltage detection
On every ADC frame, the input samples are processed
as follows:
• The voltage and current samples are read. The cur-
• Both the current and voltage signals are passed
• The current and voltage signals are now split into
The result is a set of accumulated values that represent
squared voltage, squared current, and real (active)
energy for both the entire usable spectrum and as fil-
tered by the peak filter. The real energy at this point
does not yet represent real power; to obtain usable
power values further processing is required. Each of
these values is further processed at the end of each
DSP cycle.
Figure 9. Per Sample Operations
50
function)
rent sample is shifted to account for the gain applied
in the PGA. The phase- and gain-corrected samples
are passed to the next stage.
through highpass filters (HPF) specified by the
HPF_C variable.
several components. The first of these components is
squared and accumulated to begin the RMS current
and voltage process. The second is processed and
accumulated to begin the real power calculation.
______________________________________________________________________________________
CURRENT INPUT
Per Sample Operations
VOLTAGE INPUT
GAIN SEL
ADC
ADC
V_GAIN
I_GAIN
At the end of each DSP cycle, accumulated information
is available that is used to calculate all other opera-
tional results in the meter. DSP cycles track the line fre-
quency and have a duration of the number of cycles
specified in the CYCNT register. On each phase, the
time required for CYCNT cycles to complete is calculat-
ed and this value is used to update the duration of one
DSP cycle, specified in the NS register.
NS contains the number of ADC frame periods in a sin-
gle DSP cycle. Because line frequency varies slightly
from cycle to cycle, and because the ADC frame clock
is not synchronized to the line, the value of NS is not an
integer, and varies slightly from DSP cycle to DSP cycle.
Because the value of NS is so critical to accurate calcu-
lation of energy, ensuring that it is correct on every
cycle is essential. There are two ways to manage the
slight variation of NS from cycle to cycle: first, one
could simply replace the old value of NS with the newly
calculated value on each DSP cycle. This means that
NS (and every other value in the meter, since they
depend on NS) would have a significant amount of
uncertainty. A better method is to use each newly cal-
culated value of NS as an input to a filter. The output of
the filter is then the value of NS that is actually used in
calculations. In the MAXQ3181, this filter is controlled
by the AVG_NS register.
A second problem with updating NS on every line
cycles is the fact that noise impulses that occur at near-
ly the same time as the zero crossing can shift the zero
HPF
HPF
X
X
2
2
Per DSP Cycle Operations
I
V
2
2
E
P