AS8267 datasheet(19/136 Pages) AMSCO

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Data Sheet AS8267 / AS8268

Revision 1.0, 19-Jun-07

Page 19 of 136

Voltage Input for Energy Calculation

The voltage channel input consisting of inputs VP and VN which are differential, with VP connected to the tap

of a resistor divider circuit of the line voltage and VN connected to VSSA. For optimum operating conditions,

the input signal at VP should be set at 100mVp for the rated voltage condition.

The resistor values for an ideal voltage divider may be calculated as follows:

Assuming a Vmains of 230V (rms) → 325V (peak) and R2 = 470Ω (according to the voltage divider shown

below), the value of R1A+R1B may be calculated as follows:

mains

R1A+R1B

−

100

325

470

)

(

)

(

)

(

mains

thus R1A = 820k

Ω and R1B = 750kΩ resistors may be selected.

The mains voltage is also sampled at 3.4956kHz, assuming that the recommended crystal oscillator frequency

of 3.5795MHz is used.

Digital Signal Processing Block (DSP)

The digital signal processing (DSP) block provides the signal processing required to ensure that the specified

measured accuracy is performed and that the microcontroller (MCU) is provided with the appropriate data and

protocol to perform all the required meter functions. For the description below, please refer to the following

block diagram (Figure 3).

The DSP makes allowance for phase correction of the two current channels (i1 and i2) within the Sinc

decimation filters in the phase correction block. The applicable phase correction setting (pcorr_i1 or pcorr_i2)

is selected (sel_i), depending upon which current (i1 or i2) is being used for the power calculation.

The equalization filters on the voltage and current channels which may be by-passed (sel_equ), correct for the

attenuation introduced by the decimation filters at the edge of the input frequency band, while the high pass

filters, which may also be by-passed (sel_hp), eliminate any DC offsets introduced into the input channels.

Independent calibration of the voltage (cal_v) and current signals (cal_i1 and cal_i2) is done after the voltage

and current signals are provided for power calculation. This ensures that calibration of the voltage (sos_v),

current channel 1 (sos_i1), current channel 2 (sos_i2) has no influence on the power (np) calibration.

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