Datasheet

MAX71020
Single-Chip Electricity Meter AFE
31Maxim Integrated
The meter constant Kh is derived from WRATE and rep-
resents the amount of energy measured for each pulse. If
Kh = 1Wh/pulse and 120V and 30A is applied in-phase to
the meter, the meter will produce one pulse per second
(120V and 30A results in a load of 3600W, or put another
way, energy consumption of one watt-hour per second).
If the load is 240V at 150A, 10 pulses per second are
generated. To compute the WRATE value, see Table 16.
The maximum pulse rate is 7.56kHz.
See the VPULSE and WPULSE section for details on how
to adjust the timing of the output pulses. The maximum
time jitter is 1/6 of the multiplexer cycle period (nomi-
nally 67µs) and is independent of the number of pulses
measured. Thus, if the pulse generator is monitored for
one second, the peak jitter is 67ppm. After 10s, the peak
jitter is 6.7ppm. The average jitter is always zero. If it
is attempted to drive either pulse generator faster than
its maximum rate, it simply outputs at its maximum rate
without exhibiting any rollover characteristics. The actual
pulse rate, using WSUM as an example, is:
S
46
WRATE WSUM f X
RATE Hz
2
× ××
=
where f
S
= sampling frequency (2520.6Hz), X =
pulse speed factor derived from the CE variables
PULSE_SLOW (register 0x020[0]) and PULSE_FAST
(register 0x020[1]).
Other CE Parameters
Table 17 shows the CE parameters used for suppression
of noise due to scaling and truncation effects.
CE Calibration Parameters
Table 18 lists the parameters that are typically entered to
effect calibration of meter accuracy.
CE Flow Diagrams
Figure 10 to Figure 12 show the data flow through the
CE in simplified form. Functions not shown include delay
compensation, sag detection, scaling, and the process-
ing of meter equations.
Table 16. CE Pulse Generation Parameters
CE
ADDRESS
NAME DEFAULT DESCRIPTION
0x21 WRATE 547
where:
K = 42.7868
The default value yields 1.0 Wh/pulse for V
MAX
= 600V and I
MAX
= 208A. The
maximum value for WRATE is 32,768 (2
15
).
0x22 KVAR 6444 Scale factor for VAR measurement
0x45 WPULSE_CTR 0 WPULSE counter
0x46 WPULSE_FRAC 0
Unsigned numerator, containing a fraction of a pulse. The value in this register
always counts up towards the next pulse.
0x47 WSUM_ACCUM 0 Rollover accumulator for WPULSE
0x49 VPULSE_CTR 0 VPULSE counter
0x4A VPULSE_FRAC 0
Unsigned numerator, containing a fraction of a pulse. The value in this register
always counts up towards the next pulse.
0x4B VSUM_ACCUM 0 Rollover accumulator for VPULSE
MAX MAX
KV I
Kh Wh pulse
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××
=
××