CS5460A
DS284PP4
17
operation, the instantaneous voltage/current data is
low-pass filtered by a Sinc
2
filter. Then the data is
decimated to every Nth sample. Because of the
Sinc
2
filter operation, the first output for each chan-
nel will be invalid (i.e. all RMS calculations are in-
valid in the ‘single computation cycle’ data
acquisition mode and the first RMS calculation re-
sults will be invalid in the ‘continuous computation
cycles’ data acquisition mode). However, all ener-
gy calculations will be valid since energy calcula-
tions do not require this Sinc
2
operation.
After the user issues the ’Start Conversions’ com-
mand to the CS5460A (see Section 3.1, Commands
(Write Only)), and if the ‘C’ bit in this command is
set to a value of ‘1’, the device will remain in its ac-
tive state. Once commanded into continuous com-
putation
cycles
data
acquisition
mode,
the
CS5460A will continue to perform A/D conver-
sions on the voltage/current channels, as well as all
subsequent
calculations,
until
a)
the
‘Pow-
er-Up/Halt’ command is received through the serial
interface, or b) the device looses power, or c) the
RS bit in the Configuration Register is asserted by
the user (‘software reset’), or d) the /RESET pin is
asserted and then de-asserted (‘hardware reset’).
2.3 Basic Application Circuit
Configurations
Figure 6 shows the CS5460A connected to a ser-
vice to measure power in a single-phase 2-wire sys-
tem operating from a single power supply. Note
that in this diagram the shunt resistor used to mon-
itor the line current is connected on the “Line” (hot)
side of the power mains. In most residential power
metering applications, the power meter’s cur-
rent-sense shunt resistor is intentionally placed on
the ‘hot’ side of the power mains in order to help
detect any attempt by the subscriber to steal power.
In this type of shunt-resistor configuration, note
that the common-mode level of the CS5460A must
be referenced to the hot side of the power line. This
means that the common-mode potential of the
CS5460A will typically oscillate to very high posi-
tive voltage levels, as well as very high negative
voltage levels, with respect to earth ground poten-
tial. The designer must therefore be careful when
attempting to interface the CS5460A’s digital out-
put lines to an external digital interface (such as a
LAN connection or other communication net-
work). Such digital communication networks may
require that the CMOS-level digital interface to the
meter is referenced to an earth-ground. In such cas-
es, the CS5460A’s digital serial interface pins must
be isolated from the external digital interface, so
that there is no conflict between the ground refer-
ences of the meter and the external interface. The
CS5460A and associate circuitry should be en-
closed in a protective insulated case when used in
this configuration, to avoid risk of harmful electric
shock to humans/animals/etc.
Figure 7 shows how the same single-phase
two-wire system can be metered while achieving
complete isolation from the power lines. This iso-
lation is achieved using three transformers. One
transformer is a general-purpose voltage trans-
former, used to supply the on-board DC power to
the CS5460A. A second transformer is a high-pre-
cision, low-impedance voltage transformer (often
called a ‘potential transformer’) with very little
roll-off/phase delay, even at the higher harmonics.
A current transformer is then used to sense the line
current. A burden resistor placed across the sec-
ondary of the current transformer creates the cur-
rent-sense voltage signal, for the CS5460A’s
current channel inputs. Because the CS5460A is
not directly connected to the power mains, isolation
is not required for the CS5460A’s digital interface.
Figure 8 shows the CS5460A configured to mea-
sure power in a single-phase 3-wire system. In
many 3-wire residential power systems within the
United States, only the two Line terminals are
available (neutral is not available). Figure 9 shows
how the CS5460A can be configured to meter a
3-wire system when no neutral is available.
Содержание CS5460A
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