ADE7978 datasheet(49/125 Pages) AD

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Data Sheet

ADE7978/ADE7933/ADE7932/ADE7923

Rev. D | Page 49 of 125

Time Interval Between Phases

The ADE7978 can measure the time delay between phase

voltages, between phase currents, or between the voltages and

currents of the same phase. The negative to positive transitions

identified by the zero-crossing detection circuit are used as the

start and stop measuring points. Because the zero-crossing events

are identified based on the fundamental components of the phase

currents and voltages, the time interval measurements relate to

the fundamental components. Only one set of time delay meas-

urements is available at one time; these measurements are based

on Bits[10:9] (ANGLESEL[1:0]) in the COMPMODE register

(Address 0xE60E).

When the ANGLESEL[1:0] bits are set to 00 (the default value),

the delays between voltages and currents on the same phase are

measured (see Figure 65). The delay between the Phase A voltage

and Phase A current is stored in the 16-bit unsigned ANGLE0

currents of Phase B and Phase C are stored in the ANGLE1 and

ANGLE2 registers, respectively.

PHASE A

CURRENT

ANGLE0

PHASE A

VOLTAGE

Figure 65. Delay Between Phase A Voltage and Phase A Current Is

Stored in the ANGLE0 Register

When the ANGLESEL[1:0] bits are set to 01, the delays between

phase voltages are measured. The delay between the Phase A volt-

age and the Phase C voltage is stored in the ANGLE0 register. The

delay between the Phase B voltage and the Phase C voltage is

stored in the ANGLE1 register, and the delay between the Phase A

voltage and the Phase B voltage is stored in the ANGLE2 register

(see Figure 66).

PHASE B

PHASE C

PHASE A

ANGLE2

ANGLE0

ANGLE1

Figure 66. Delays Between Phase Voltages or Phase Currents

When the ANGLESEL[1:0] bits are set to 10, the delays between

phase currents are measured. The delay between the Phase A

current and the Phase C current is stored in the ANGLE0 register,

the delay between the Phase B current and the Phase C current is

stored in the ANGLE1 register, and the delay between the Phase A

current and the Phase B current is stored in the ANGLE2 register

(see Figure 66).

The ANGLE0, ANGLE1, and ANGLE2 registers are 16-bit

unsigned registers with 1 LSB corresponding to 3.90625 µs

(256 kHz clock), which means a resolution of 0.0703° (360° ×

50 Hz/256 kHz) for 50 Hz systems and 0.0843° (360° × 60 Hz/

256 kHz) for 60 Hz systems. The delays between phase voltages

or between phase currents are used to characterize how balanced

the load is. The delays between phase voltages and currents are

used to compute the fundamental power factor on each phase,

as shown in Equation 15.

cosφx = cos













kHz

256

360

LINE

ANGLEx



(15)

where fLINE is the line frequency.

PERIOD MEASUREMENT

The ADE7978 provides the period measurement of the line in the

voltage channel. The period of each phase voltage is measured and

stored in three registers: APERIOD, BPERIOD, and CPERIOD

(Address 0xE905 to Address 0xE907). The 16-bit unsigned period

registers are updated every line period. Because of the LPF1 filter

(see Figure 61), the period measurement becomes stable after a

settling time of 30 ms to 40 ms.

The period measurement has a resolution of 3.90625 µs/LSB

(256 kHz clock), which represents 0.0195% (50 Hz/256 kHz)

when the line frequency is 50 Hz and 0.0234% (60 Hz/256 kHz)

when the line frequency is 60 Hz. The value of the period registers

for 50 Hz networks is approximately 5120 (256 kHz/50 Hz); the

value of the period registers for 60 Hz networks is approximately

4267 (256 kHz/60 Hz). The length of the registers enables the

measurement of line frequencies as low as 3.9 Hz (256 kHz/216).

The period registers are stable at ±1 LSB when the line is estab-

lished and the measurement does not change.

Use the following equations to compute the line period and

frequency using the period registers:

TL = xPERIOD[15:0]/256E3 (sec)

(16)

fL = 256E3/xPERIOD[15:0] (Hz)

(17)

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