AS8228 datasheet(23/123 Pages) AMSCO

23 / 123 page

Data Sheet AS8218 / AS8228

Revision 3.0, 31-May-06

Page 23 of 123

High-Pass Filters

The high pass filters in the voltage and current channels, with corner frequencies of <10Hz, correct for DC

offsets introduced into the input channels.

Each of the voltage and current channels has a separate high pass filter in order to avoid any phase shift being

introduced between the voltage and the two current channels.

The high pass filters may also be by-passed (sel_hp), if so desired.

Corner frequency:

<10Hz

RMS Calculations

The DSP provides the voltage and current channel data in ‘sum-of-squares’ format. To calculate RMS values

from the voltage (sos_v) and current (sos_i1 and sos_i2), the following formula should be applied for the

voltage and current respectively:

∑

nsamp

rms

nsamp

where

∑

nsamp

is the sos_v value

∑

nsamp

rms

nsamp

where

∑

nsamp

is the sos_i value

nsamp should be selected in order to achieve coherent sampling as close as possible:

e.g. fs = 3.4956kHz (fosc = 3.579545MHz) ⇒ nsamp = 3496 should be selected if the MCU has to be

interrupted every 1 second. When fmains = 50Hz, 70 mains

periods are monitored.

Refer to Squareroot Block (SQRT) for a detailed description of the programming sequence of the squareroot

input operand.

AS8228 Datasheet(PDF) 23 Page - ams AG