AS8218_1 Datasheet, PDF (16/123 Pages) ams AG – Highly Integrated Single Phase 2 Current Energy Metering Integrated Circuits

Data Sheet AS8218 / AS8228
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 AS8218 /
AS8228. The DSP offers programming flexibility and provides for fast and efficient meter production calibration
procedures.
A power supply monitor (PSM) is also described in this section. The PSM ensures that a reset is generated
independently of the rise and fall times of the supply voltage (VDD).
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 AS8218 /
AS8228 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 30 ppm/K typical.
Voltage Reference Specifications
Parameter
Output voltage
Temperature coefficient
Symbol
V_ref
TK
Min
1.217
Typ
1.219
30
Max
1.221
Unit
V
ppm/K
Notes
Current Inputs for Energy Calculation
The AS8218 / AS8228 ICs have 2 identical mains 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 AS8218 / AS8228
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 AS8218 / AS8228 ICs may also be used in a conventional single
current configuration with either a current transformer or shunt resistor being used for current sensing.
Revision 3.0, 31-May-06
Page 16 of 123