CS5490 datasheet(17/57 Pages) CIRRUS

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CS5490

DS982F3

SampleCount register should not be changed from its

default value of 4000, and bit AFC of the Config2

host processor should not change the SampleCount

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 (PAVG).

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

power (QAVG) 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 (PAVG) and reactive power

(QAVG) 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 (IPEAK) and peak voltage (VPEAK) are cal-

culated over N samples and recorded in the corre-

sponding channel peak register documented in the

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, POFF, 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

To use this feature, measure the average power at no

load and take the measured result (from the PAVG

associated power offset register, POFF.

4.10 Average Reactive Power Offset

The average reactive power offset register, QOFF, 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 QAVG

reactive power offset register, QOFF.

IRMS

–



--------------------

VRMS

–



----------------------

[Eq.1]

SVRMS IRMS



[Eq.2]

SQAVG2 PAVG2

[Eq.3]

PACTIVE

----------------------

[Eq.4]

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