Rainbow Electronics AT73C502 - Manual (Page 6)

AT73C500
6
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
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 both of these converters, the voltage
inputs must be equipped with simple external voltage divid-
ers.
The input voltage range of each converter is 2V
PP
. The
characteristics of a Watt-hour meter operating, according to
IEC1036 specification, are based on a certain basic cur-
rent, I
B
. 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 I
B
= 5A
RMS
, the full
scale range of the current channels will be:
The following current transformer and voltage divider con-
figuration is recommended for a 230V, 3-phase system,
with 5A basic current:
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 to 2mA, from
20mA.
VOLTAGE
REFERENCE
SERIAL OUTPUT
LOGIC
VOLTAGE
MONITORING
TIMING AND
CONTROL
DECIMATION
FILTER
DECIMATION
FILTER
DECIMATION
FILTER
DECIMATION
FILTER
DECIMATION
FILTER
SIGMA-DELTA
MODULATOR
SIGMA-DELTA
MODULATOR
SIGMA-DELTA
MODULATOR
SIGMA-DELTA
MODULATOR
SIGMA-DELTA
MODULATOR
SIGMA-DELTA
MODULATOR
DECIMATION
FILTER
Voltage Inputs Current Inputs
Converter full-scale input 2.0V
PP
2.0V
PP
Corresponding full-scale
line voltage / current
270V
RMS
80A
RMS
I
FS
= 5 A
RMS
100
6.25
-----------
×
80 A
RMS
=