ADE7878 datasheet(27/92 Pages) AD

27 / 92 page

ADE7878

Rev. 0 | Page 27 of 92

Current Channel Sampling

The waveform samples of the current channel are taken at the

output of HPF and stored into the IAWV, IBWV, ICWV, and

INWV 24-bit signed registers at a rate of 8 kSPS. All power and

rms calculations remain uninterrupted during this process.

Bit 17 (DREADY) in the STATUS0[31:0] register is set when the

IAWV, IBWV, ICWV, and INWV registers are available to be

read using the I2C or SPI serial port. Setting Bit 17 (DREADY)

in the MASK0[31:0] register enables an interrupt to be set when

the DREADY flag is set. See the Digital Signal Processor section

for more details on Bit DREADY.

As previously stated, the serial ports of the ADE7878 work on

32-, 16-, or 8-bit words. When the IAWV, IBWV, ICWV, and

INWV 24-bit signed registers are read from the ADE7878, they

are transmitted sign extended to 32 bits. See Figure 34 for

details.

24 23 22

24-BIT SIGNED NUMBER

BITS[31:24] ARE

EQUAL TO BIT 23

BIT 23 IS A SIGN BIT

Figure 34. 24-Bit IxWV Register Transmitted as 32-Bit Signed Word

The ADE7878 contains a high speed data capture (HSDC) port

that is specially designed to provide fast access to the waveform

sample registers. See the HSDC Interface section for more details.

di/dt CURENT SENSOR AND DIGITAL INTEGRATOR

The di/dt sensor detects changes in the magnetic field caused by

the ac current. Figure 35 shows the principle of a di/dt current

sensor.

MAGNETIC FIELD CREATED BY CURRENT

(DIRECTLY PROPORTIONAL TO CURRENT)

+ EMF (ELECTROMOTIVE FORCE)

– INDUCED BY CHANGES IN

MAGNETIC FLUX DENSITY (di/dt)

Figure 35. 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 and

neutral current datapath, there is a built-in digital integrator to

recover the current signal from the di/dt sensor. The digital inte-

grator is disabled by default when the ADE7878 is powered up

and after reset. Setting Bit 0 (INTEN) of the CONFIG[15:0]

the magnitude and phase response of the digital integrator.

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 sig-

nificant high frequency noise. An antialiasing filter of at least

the second order is needed to avoid noise aliasing back in the

band of interest when the ADC is sampling (see the Antialiasing

Filter section).

–50

–100

–50

500

1000

1500

2000

2500

3000

3500

4000

FREQUENCY (Hz)

0.01

0.1

100

1000

FREQUENCY (Hz)

Figure 36. Combined Gain and Phase Response of the

Digital Integrator

The 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 remain at 0x000000 in that case.

–30

–15

–20

–25

–89.96

–89.97

–89.98

–89.99

FREQUENCY (Hz)

Figure 37. Combined Gain and Phase Response of the

Digital Integrator (40 Hz to 70 Hz)

As previously stated, the serial ports of the ADE7878 work on 32-,

16-, or 8-bit words. Similar to the registers shown in Figure 32, the

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