Data Sheet AS8118

Revision 1.8, 15-Feb-05

Page 7 of 30

Digital Filters

The current and voltage channels have been identically implemented with digital high pass filters in both channels, thus

eliminating offsets.

The filters ensure that there are no phase errors introduced between the voltage and current channels, enabling single point

calibration.

Energy to Pulse Output Conversion

The energy value is accumulated in the energy accumulator and compared with the default or programmed threshold level,

following each sample. The threshold represents the pulse equivalent energy value. If the energy value goes above the

threshold, a pulse is generated and presented to the output. Each time a pulse is generated the threshold value is subtracted

from the contents of the energy accumulator.

The remaining energy, namely the energy value above the threshold value is retained in the accumulator. Further measured

energy is added to the retained value in the accumulator and a pulse is again generated and presented to the output, when

the value again exceeds the threshold value. Thus no energy is lost during the energy to pulse output conversion process.

follows:

3.57945 MHz / 8*256 =

1.7478 kHz

The number of measured harmonics is defined by the sample rates of the voltage and current input signals. The maximum

bandwidth, which is half the sample frequency, is calculated as follows:

1.7478/2 kHz =

873.9 Hz

Thus, depending on the mains frequency, the measured energy is up to the following harmonics:

50Hz mains = 17th harmonic

60Hz mains = 14th harmonic

Energy Pulse Outputs

The AS8118 has three different pulse outputs. All the outputs are derived directly from the measured energy; thus, the

outputs can be used for energy accumulation and for calibration purposes. The output options include the following:

-

CAL: A higher pulse rate output for fast calibration

-

MOP/MON: Low pulse rate outputs for directly driving a stepper motor

-

LED: A low pulse rate output which may be used to directly drive a LED for displaying power consumption

Calibration Pulse Output (CAL):

The CAL output is a high frequency output, the frequency of which is proportional to the real

power measured.

The output pulse rate is programmable via the on-chip memory and allows for 4 pulse rate options:

Parameter: F_cal_sel

Setting

CAL Output Pulse Rate

00

MON/MOP x 8

01

MON/MOP x 16

10

MON/MOP x 32

11

MON/MOP x 64