AS8218 datasheet(22/123 Pages) AMSCO

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Data Sheet AS8218 / AS8228

Revision 3.0, 31-May-06

Page 22 of 123

Calculating Phase Correction Factors

The measured phase_error in percentage is defined by the following formula

[]

100

)

cos(

)

cos(

)

shift

phase

cos(

error

phase

−

while the phase_shift in degrees, is calculated as follows:

[ ]

−

⎥

⎦

⎤

⎢

⎣

⎡

⎟

⎠

⎞

⎜

⎝

⎛ +

)

cos(

100

error

phase

arccos

shift

phase

shift

phase

correction

phase

−

The required phase correction factor can be determined from error measurements with a power factor (PF) less

than 1.

Assuming that at PF = 1 the meter has been calibrated and the error is approximately 0 for Ical (calibration

current), the PF is reduced and the effect of phase differences results in an increased error (‘phase_error’).

Example: The phase_error at PF = 0.5 (

ϕ = 60°) is measured to be 9.2 %.

The related phase shift can be calculated using the following formula:

[ ]

−

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

⎟

⎠

⎞

⎜

⎝

⎛ +

cos

100

error

phase

cos

arc

shift

phase

where the phase_error is the measured error in percentage and cosΦ is the phase angle.

For phase_error = 9.2[%] the phase_shift is -3.0° and the phase correction is 3.0°.

If fosc = 3.579545MHz and fmains = 50Hz, one phase correction unit represents 2.41’, which is 0.04023°.

Thus the phase correction factor must be set to

units

04023

= 75 units.

The pcorr register has to be set to 4bh.

Equalization Filters

The equalization filters in the voltage and current channels correct for the attenuation effects introduced by the

decimation filters around the frequency band limit. The resulting transfer curve after the equalization filter has

approximately 0dB attenuation over the entire frequency band.

The equalization filters may be by-passed (sel_equ), if required.

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