ade7854 Analog Devices, Inc., ade7854 Datasheet - Page 22

• Manufacturer Part Number: ade7854
• Description: Poly Phase Multifunction Energy Metering Ic With Neutral Current Measurement
• Manufacturer: Analog Devices, Inc.

ADE7854
di/dt CURENT SENSOR AND DIGITAL INTEGRATOR
The di/dt sensor detects changes in the magnetic field caused by
the ac current. Figure 20 shows the principle of a di/dt current
sensor.
The flux density of a magnetic field induced by a current is
directly proportional to the magnitude of the current. The
changes in the magnetic flux density passing through a conductor
loop generate an electromotive force (EMF) between the two
ends of the loop. The EMF is a voltage signal that is propor-
tional to the di/dt of the current. The voltage output from the
di/dt current sensor is determined by the mutual inductance
between the current carrying conductor and the di/dt sensor.
Due to the di/dt sensor, the current signal needs to be filtered
before it can be used for power measurement. On each phase
current data path, there is a built-in digital integrator to recover
the current signal from the di/dt sensor. The digital integrator is
disabled by default when the ADE7854 is powered up and after
reset. Setting bit 0 (INTEN) of the CONFIG[15:0] register turns
on the integrator. Figure 21 to Figure 24 show the magnitude and
phase response of the digital integrator.
+0.5V/GAIN
-0.5V/GAIN
V
IN
0V
IAP
IAN
V
IN
ANALOG INPUT RANGE
Figure 20. Principle of a di/dt Current Sensor
PGA1
x1, x2, x4,x8,
PGA1 bits
GAIN[2:0]
x16
MAGNETIC FIELD CREATED BY CURRENT
(DIRECTLY PROPORTIONAL TO CURRENT)
REFERENCE
+ EMF (ELECTROMOTIVE FORCE)
– INDUCED BY CHANGES IN
ADC
MAGNETIC FLUX DENSITY (di/dt)
0xA58AC0=
0x5A7540=
-5,928,256
5,928,256
0V
AIGAIN[23:0]
DSP
ADC OUTPUT RANGE
CURRENT CHANNEL
DATA RANGE
Figure 19. Current Channel Signal Path
HPFDIS[23:0]
HPF
Rev. PrC| Page 22 of 71
INTEGRATOR
CONFIG[0]
INTEN bit
DIGITAL
Note that the integrator has a −20 dB/dec attenuation and
approximately −90° phase shift. When combined with a di/dt
sensor, the resulting magnitude and phase response should be a
flat gain over the frequency band of interest. However, the di/dt
sensor has a 20 dB/dec gain associated with it and generates
significant high frequency noise. An antialiasing filter of at least
the second order is needed to avoid that the noise alias back in
the band of interest when the ADC is sampling (see the
Antialiasing Filter section).
DICOEFF[23:0] 24-bit signed register is used in the digital
integrator algorithm. At power up or after a reset, its value is
0x000000. Before turning on the integrator, this register must be
initialized with 0xFF8000. DICOEFF[23:0] is not used when
the integrator is turned off and can be left at 0x000000 in this
case.
As previously stated, the serial ports of the ADE7854 work on
32, 16 or 8-bit words. Similar to the registers presented in
Figure 16, DICOEFF[23:0] 24-bit signed register is accessed as a
0xA58AC0=
0x5A7540=
-5,928,256
5,928,256
0V
ACTIVE AND REACTIVE
LPF1
TOTAL/FUNDAMENTAL
POWER CALCULATION
CURRENT RMS (IRMS)
CURRENT PEAK,
OVERCURRENT
SAMPLE REGISTER
IAWV WAVEFORM
CALCULATION
CURRENT CHANNEL
DATA RANGE AFTER
DETECT
Figure 22. Combined Phase Response of the
Figure 24. Combined Phase Response of the
Figure 21. Combined Gain Response of the
Figure 23. Combined Gain Response of the
INTEGRATOR
Digital Integrator (40 Hz to 70 Hz)
Digital Integrator (40 Hz to 70 Hz)
Preliminary Technical Data
ZX DETECTION
Digital Integrator
Digital Integrator
TBD
TBD
TBD
TBD
0xA58AC0=
0x5A7540=
-5,928,256
5,928,256
0V
ZX SIGNAL
DATA RANGE