ADE7754_15 datasheet(15/44 Pages) AD

15 / 44 page

REV. 0

ADE7754

–15–

are particularly noticeable at low power factors. The ADE7754

provides a means of digitally calibrating these small phase

errors. The ADE7754 allows a small time delay or time advance

to be introduced into the signal processing chain to compensate

for small phase errors. Because the compensation is in time, this

technique should be used only 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 registers (APHCAL, BPHCAL, and

CPHCAL) are twos complement, 5-bit signed registers that

can vary the time delay in the voltage channel signal path from

–19.2

µs to +19.2 µs (CLKIN = 10 MHz). One LSB is equiva-

lent to 1.2

µs. With a line frequency of 50 Hz, this gives a

phase resolution of 0.022

° at the fundamental (i.e., 360°

1.2

µs

50 Hz).

Figure 19 illustrates how the phase compensation is used to

remove a 0.091

° phase lead in IA of the current channel caused

by an external transducer. In order to cancel the lead (0.091

°)

in IA of the current channel, a phase lead must also be intro-

duced into VA of the voltage channel. The resolution of the

phase adjustment allows the introduction of a phase lead of

0.086

°. The phase lead is achieved by introducing a time advance

into VA. A time advance of 4.8

µs is made by writing –4 (1Ch)

to the time delay block (APHCAL[4:0]), thus reducing the

amount of time delay by 4.8

µs. See the Calibration of a 3-Phase

Meter Based on the ADE7754 Application Note AN-624.

0.1

50Hz

IAP

IAN

ADC

PGA1

HPF

VAP

ADC

PGA2

0.69 AT 50Hz, 0.022

0.83 AT 60Hz, 0.024

LPF2

VA DELAYED BY 4.8 s

(–0.0868 AT 50Hz) 1CH

50Hz

APHCAL[4:0]

–19.2 s TO +19.2 s

Figure 19. Phase Calibration

ROOT MEAN SQUARE MEASUREMENT

Root Mean Square (rms) is a fundamental measurement of the

magnitude of an ac signal. Its definition can be practical or

mathematical. Defined practically, the rms value assigned to an

ac signal is the amount of dc required to produce an equivalent

amount of heat in the same load. Mathematically the rms value

of a continuous signal f(t) is defined as

ft dt

rms

∫

2()

(1)

For time sampling signals, rms calculation involves squaring the

signal, taking the average, and obtaining the square root:

rms

∑

()

(2)

The method used to calculate the rms value in the ADE7754 is

to low-pass filter the square of the input signal (LPF3) and take

the square root of the result.

With

rms

()

sin(

)

=×

then

rms

()

cos(

)

×=

−

The rms calculation is simultaneously processed on the six analog

input channels. Each result is available on separate registers.

Current RMS Calculation

Figure 20 shows the detail of the signal processing chain for the

rms calculation on one of the phases of the current channel.

The current channel rms value is processed from the samples

used in the current channel waveform sampling mode. Note

that the APGAIN adjustment affects the result of the rms calcu-

lation. See the Current RMS Gain Adjust section. The current

rms values are stored in unsigned 24-bit registers (AIRMS,

BIRMS, and CIRMS). One LSB of the current rms register is

equivalent to 1 LSB of a current waveform sample. The update

rate of the current rms measurement is CLKIN/12. With the

specified full-scale analog input signal of 0.5 V, the ADC produces

an output code which is approximately

±2,684,354d. See the

Current Channel ADC section. The equivalent rms values of a

full-scale ac signal is 1,898,124d. With offset calibration, the

current rms measurement provided in the ADE7754 is accurate

within

±2% for signal input between full scale and full scale/100.

CURRENT

SIGNAL – i(t)

CURRENT

CHANNEL (rms)

0000h

1CF68Ch

+ 100% FS

– 100% FS

E30974h

+ 122.5% FS

+ 70.7% FS

– 70.7% FS

– 122.5% FS

2378EDh

147AE0h

EB852Fh

DC8713h

AAPGAIN[11:0]

000h

7FFh

800h

00000h

400000h

C00000h

28F5C2h

D70A3Eh

+ FS

– FS

ADC OUTPUT

WORD RANGE

AAPGAIN

HPF

LPF3

211

SGN

210

Irms(t)

–100% to +100% FS

1CF68Ch

00h

IRMS

IRMSOS[11:0]

Figure 20. Current RMS Signal Processing

Note that a crosstalk between phases can appear in the ADE7754

current rms measurements. This crosstalk follows a specific

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