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3 rms current & voltage, 4 active power, 5 reactive power – Cirrus Logic CS5490 User Manual

Page 17: 6 apparent power, 7 peak voltage & current, 8 power factor, 9 average active power offset, 10 average reactive power offset, Cs5490

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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]