ADE7763ARSZ datasheet(25/56 Pages) AD

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Data Sheet

ADE7763

Rev. C | Page 25 of 56

Figure 51. Figure 52 shows the magnitude response of the filter.

As seen from the plots, the phase response is almost 0 from

45 Hz to 1 kHz, which is all that is required in typical energy

measurement applications. However, despite being internally

phase-compensated, the ADE7763 must work with transducers,

which could have inherent phase errors. For example, a phase

error of 0.1° to 0.3° is not uncommon for a current transformer

(CT). Phase errors can vary from part to part and must be

corrected in order to perform accurate power calculations. The

errors associated with phase mismatch are particularly

noticeable at low power factors. The ADE7763 provides a

means of digitally calibrating these small phase errors by

allowing a short time delay or time advance to be introduced

into the signal processing chain to compensate for these errors.

Because the compensation is in time, this technique should only

be used for small phase errors in the range of 0.1° to 0.5°.

Correcting large phase errors using a time shift technique can

introduce significant phase errors at higher harmonics.

The phase calibration register (PHCAL[5:0]) is a twos comple-

ment, signed, single-byte register that has values ranging from

0x21 (–31d) to 0x1F (+31d).

The register is centered at 0Dh, so that writing 0Dh to the register

produces 0 delay. By changing the PHCAL register, the time

delay in the Channel 2 signal path can change from –102.12 µs

to +39.96 µs (CLKIN = 3.579545 MHz). One LSB is equivalent

to 2.22 µs (CLKIN/8) time delay or advance. A line frequency of

60 Hz gives a phase resolution of 0.048° at the fundamental (i.e.,

360° × 2.22 µs × 60 Hz). Figure 49 illustrates how the phase

compensation is used to remove a 0.1° phase lead in Channel 1

due to the external transducer. To cancel the lead (0.1°) in

Channel 1, a phase lead must also be introduced into Channel 2.

The resolution of the phase adjustment allows the introduction

of a phase lead in increments of 0.048°. The phase lead is

achieved by introducing a time advance in Channel 2. A time

advance of 4.44 µs is made by writing −2 (0x0B) to the time delay

block, thus reducing the amount of time delay by 4.44 µs, or

equivalently, a phase lead of approximately 0.1° at line frequency

of 60 Hz. 0x0B represents –2 because the register is centered

with 0 at 0Dh.

PGA1

V1P

V1N

ADC 1

HPF

PGA2

V2P

V2N

ADC 2

DELAY BLOCK

2.22

µs/LSB

LPF2

60Hz

0.1°

CHANNEL 2 DELAY

REDUCED BY 4.44

µs

(0.1°LEAD AT 60Hz)

0x0B IN PHCAL [5.0]

PHCAL[5:0]

–102.12

µs TO +39.96µs

60Hz

Figure 49. Phase Calibration

FREQUENCY (Hz)

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

–0.1

102

103

104

Figure 50. Combined Phase Response of HPF and

Phase Compensation (10 Hz to 1 kHz)

FREQUENCY (Hz)

0.20

0.18

0.16

0.14

0.12

0.10

0.08

0.02

0.04

0.06

Figure 51. Combined Phase Response of HPF and

Phase Compensation (40 Hz to 70 Hz)

FREQUENCY (Hz)

0.4

0.3

0.2

0.1

0.0

–0.1

–0.2

–0.3

–0.4

Figure 52. Combined Gain Response of HPF and Phase Compensation

ACTIVE POWER CALCULATION

Power is defined as the rate of energy flow from the source to

the load. It is defined as the product of the voltage and current

waveforms. The resulting waveform is called the instantaneous

power signal and is equal to the rate of energy flow at any given

time. The unit of power is the watt or joules/s. Equation 9 gives

an expression for the instantaneous power signal in an ac system.

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