AS8267 datasheet(17/136 Pages) AMSCO

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Data Sheet AS8267 / AS8268

Revision 1.0, 19-Jun-07

Page 17 of 136

8.1

Energy Measurement Front End (Including DSP)

The Energy Measurement Front End is made up of the analog front end and the digital signal processing block

(DSP), which performs the active energy measurement calculations for the microcontroller.

The analog front end comprises of the three Sigma-Delta modulators for the sampling of the mains voltage,

Line current and a second current channel, for the optional measurement of the Neutral current. Also included

in the analog front end is the voltage reference, which provides the temperature stability to the Sigma-Delta

modulators. Setting up for the optimum input conditions for the voltage and current channels is also described

in this section.

The digital signal processing block (DSP) provides the filtering and processing of the output data from the

sigma-delta modulators and ensures that the specified measurement accuracy is provided by the AS8267 /

AS8268. The DSP offers programming of measurement parameters and provides for fast and efficient meter

production calibration procedures.

A power supply monitor (PSM) ensures that a reset is generated independently of the rise and fall times of the

supply voltage (VDD). The PSM is also described in this section.

Analog Front End

The analog front end comprises of three identical Sigma-Delta modulators, which convert the differentially

connected analog voltage and current inputs into digital signals. The two current inputs are gain adjustable to

accommodate both directly connected or galvanically isolated current sensors.

The on-chip voltage reference (VREF) is the most important contributor to the accuracy of the AS8267 /

AS8268 ICs due to it providing temperature stability to the circuit. Considering that the voltage and current

signals are multiplied to derive the energy value, errors introduced prior to multiplication function results in

errors being multiplied. Thus the introduction of errors into the voltage and the current channel inputs will result

in a doubling of the percentage error after multiplication at the energy output.

The temperature coefficient of the VREF is specified at 15 ppm/K typical (30 ppm/K max.).

Current Inputs for Energy Calculation

The AS8267 / AS8268 ICs have 2 identical current inputs, I1P/I1N and I2P/I2N, for measurement of both the

Live and Neutral currents. Either of the two current inputs may be selected for calculating the energy value.

These two differential current inputs are second order Sigma-Delta modulators, with each of the inputs being

provided with selectable gains of 4, 16 and 20. The selectable gains are provided so that the AS8267 / AS8268

ICs may be easily adapted for use with either 2 current transformers or alternatively a shunt resistor and a

current transformer for current sensing. The AS8267 / AS8268 ICs may also be used in a conventional single

current configuration with either a current transformer or shunt resistor being used for current sensing.

The current input signal levels may be programmed by means of on-chip programmable gain settings. The

required gain setting is selected as follows:

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