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AT73C500

Chip Set Solution for Watt-Hour Meters

Atmel Corporation 

AT73C501 and AT73C502

The AT73C501 consists of six, 16-bit analog-to-digital con-

verters. The converters are equipped with single-ended

inputs. For differential ended applications, the AT73C502

chip is used.

The converters contain a reference voltage generator, volt-

age monitoring block and serial output interface. Both con-

verters are based on high-performance, oversampling

Sigma-Delta modulators and digital decimation filters.

Figure 6. Block diagram of the single-ended ADC chip, AT73C501

VOLTAGE

MONITORING

SIGMA-DELTA

MODULATOR

SIGMA-DELTA

MODULATOR

SIGMA-DELTA

MODULATOR

SIGMA-DELTA

MODULATOR

SIGMA-DELTA

MODULATOR

SIGMA-DELTA

MODULATOR

DECIMATION

FILTER

DECIMATION

FILTER

DECIMATION

FILTER

DECIMATION

FILTER

DECIMATION

FILTER

DECIMATION

FILTER

SERIAL OUTPUT

LOGIC

VOLTAGE

REFERENCE

In a 50 Hz meter, the nominal decimated sampling rate of

3200 Hz is used. This corresponds to 64 samples per each

line frequency cycle. 60 Hz meters operate with 3840 Hz

sample rate. The master clock frequency of the ADC is

1024 times higher than the above frequencies, i.e. 3.2768

MHz in 50 Hz meters and 3.93216 MHz in 60 Hz systems.

The default meter constant of AT73C500 energy counters

is based on the above sample rates.

Other sample frequencies can be used, but the energy

results have to be scaled accordingly. If higher sampling

rate is selected, the meter constant will also be increased

by the same ratio.

The three current inputs of AT73C501 are fed from second-

ary outputs of current transformers, from Hall sensors or

other similar sensors. In differential-ended applications,

such as with current shunt resistors, the AT73C502 ADC

can be used. On any of these converters, the voltage

inputs must be equipped with simple external voltage divid-

ers.

The input voltage range of each converter is 2VPP. The

characteristics of a Watt-hour meter operating, according to

IEC1036 specification, are based on a certain basic cur-

rent, IB. As a default, the basic current of AT73C500

chipset is to 6.25% of the current input full scale value. This

means that if a meter is designed for IB = 5ARMS, the full

scale range of the current channels will be:

TIMING AND

CONTROL

IFS = 5 ARMS × 6-1---.-02---0-5-- = 80 ARMS

The following current transformer and voltage divider con-

figuration is recommended for a 230V, 3-phase system,

with 5A basic current:

Converter full-scale input

Corresponding full-scale

line voltage / current

Voltage Inputs

2.0VPP

270VRMS

Current Inputs

2.0VPP

80ARMS

With the above settings, the nominal pulse rate of the

meter constant outputs is 1250 impulses/kWh (1250

impulses/kVArh) and the rate of four display outputs 100

impulses/kWh (100 imp/kVArh).

When used in a 5A transformer operated meter, the maxi-

mum current range can be scaled down to 8A for example.

In this case, the meter constant will be ten times higher

than in an 80A meter, i.e. 12500 impulses/kWh. Similarly,

the starting current level will be transferred 2mA from

20mA.

6

AT73C500

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