Cirrus Logic CS5490 - Manual (Page 17)

CS5490
DS982F3 17
SampleCount register should not be changed from its
default value of 4000, and bit AFC of the Config2
register must be set. During continuous conversion, the
host processor should not change the SampleCount
register.
4.8.3 RMS Current & Voltage
The root mean square (RMS in Figure 7 ) calculations
are performed on N instantaneous voltage and current
samples using Equation 1:
4.8.4 Active Power
The instantaneous voltage and current samples are
multiplied to obtain the instantaneous power (P) (see
Figure 6 ). The product is then averaged over N samples
to compute active power (P
AVG
).
4.8.5 Reactive Power
Instantaneous reactive power (Q) is the sample rate
result obtained by multiplying instantaneous current (I)
by instantaneous quadrature voltage (Q). These values
are created by phase shifting instantaneous voltage (V)
90° using first-order integrators (see
Figure 6 ). The gain
of these integrators is inversely related to line
frequency, so their gain is corrected by the Epsilon
register, which is based on line frequency. Reactive
power (Q
AVG
) is generated by integrating the
instantaneous quadrature power over N samples.
4.8.6 Apparent Power
By default, the CS5490 calculates the apparent power
(S) as the product of RMS voltage and current. See
Equation 2:
The CS5490 also provides an alternate apparent power
calculation method. The alternate apparent power
method uses real power (P
AVG
) and reactive power
(Q
AVG
) to calculate apparent power. See Equation 3.
The APCM bit in the Config2 register controls which
method is used for apparent power calculation.
4.8.7 Peak Voltage & Current
Peak current (I
PEAK
) and peak voltage (V
PEAK
) are cal-
culated over N samples and recorded in the corre-
sponding channel peak register documented in the
register map. This peak value is updated every
N samples.
4.8.8 Power Factor
Power factor (PF) is active power divided by apparent
power, as shown below. The sign of the power factor is
determined by the active power. See Equation 4.
4.9 Average Active Power Offset
The average active power offset register, P
OFF
, can be
used to offset erroneous power sources resident in the
system not originating from the power line. Residual
power offsets are usually caused by crosstalk into the
current channel from the voltage channel, or from ripple
on the meter’s or chip’s power supply, or from
inductance from a nearby transformer.
These offsets can be either positive or negative,
indicating crosstalk coupling either in phase or out of
phase with the applied voltage input. The power offset
register can compensate for either condition.
To use this feature, measure the average power at no
load and take the measured result (from the P
AVG
register), invert (negate) the value, and write it to the
associated power offset register, P
OFF
.
4.10 Average Reactive Power Offset
The average reactive power offset register, Q
OFF
, can
be used to offset erroneous power sources resident in
the system not originating from the power line. Residual
reactive power offsets are usually caused by crosstalk
into the current channel from the voltage channel, or
from ripple on the meter’s or chip’s power supply, or
from inductance from a nearby transformer.
These offsets can be either positive or negative,
depending on the phase angle between the crosstalk
coupling and the applied voltage. The reactive power
offset register can compensate for either condition. To
use this feature, measure the average reactive power at
no load. Take the measured result from the Q
AVG
register, invert (negate) the value and write it to the
reactive power offset register, Q
OFF
.
IRMS
In
2
n 0 =
N 1 –

N
--------------------
= VRMS
Vn
2
n 0 =
N 1 –

N
----------------------
=
[Eq. 1]
S V
RMS
I
RMS
 = [Eq. 2]
S Q
AVG
2 P
AVG
2 + =
[Eq. 3]
PF
P
ACTIVE
S
---------------------- =
[Eq. 4]