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ADE7566/ADE7569/ADE7166/ADE7169

Rev. A | Page 55 of 144

PHASE COMPENSATION

RMS CALCULATION

The ADE7566/ADE7569/ADE7166/ADE7169 must work with

transducers that can have inherent phase errors. For example, a

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

transformer (CT). These phase errors can vary from part to part,

and they must be corrected to perform accurate power

calculations. The errors associated with phase mismatch are

particularly noticeable at low power factors. The ADE7566/

ADE7569/ADE7166/ADE7169 provide a means of digitally

calibrating these small phase errors. The part 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 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 root mean square (rms) value of a continuous signal V(t) is

defined as

∫

rms

)

(

(3)

For time sampling signals, rms calculation involves squaring the

signal, taking the average, and obtaining the square root. The

ADE7566/ADE7569/ADE7166/ADE7169 implement this

method by serially squaring the input, averaging them, and then

taking the square root of the average. The averaging part of this

signal processing is done by implementing a low-pass filter

(LPF3 in Figure 59, Figure 61, and Figure 62). This LPF has a

−3 dB cut-off frequency of 2 Hz when MCLK = 4.096 MHz.

()

)

sin(

(4)

The phase calibration register (PHCAL[7:0]) is a twos complement,

signed, single-byte register that has values ranging from 0x82

(−126d) to 0x68 (+104d).

where V is the rms voltage.

( t

−

cos

)

(

)

(5)

The PHCAL register is centered at 0x40, meaning that writing

0x40 to the register gives 0 delay. By changing this register, the

time delay in the voltage channel signal path can change from

−231.93 μs to +48.83 μs (MCLK = 4.096 MHz). One LSB is

equivalent to 1.22 μs (4.096 MHz/5) time delay or advance. A

line frequency of 60 Hz gives a phase resolution of 0.026° at the

fundamental (that is, 360° × 1.22 μs × 60 Hz).

When this signal goes through LPF3, the cos(2ωt) term is attenu-

ated and only the dc term Vrms2 goes through (see Figure 59).

LPF3

INPUT

V 2

(t) = V2 – V2 cos(2ωt)

V 2

(t) = V2

(t) = √2 × V sin(ωt)

Figure 58 illustrates how the phase compensation is used to

remove a 0.1° phase lead in the current channel due to the

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

channel, a phase lead must also be introduced into the voltage

channel. The resolution of the phase adjustment allows the

introduction of a phase lead in increments of 0.026°. The phase

lead is achieved by introducing a time advance into the voltage

channel. A time advance of 4.88 μs is made by writing −4 (0x3C)

to the time delay block, thus reducing the amount of time delay

by 4.88 μs, or equivalently, a phase lead of approximately 0.1° at a

line frequency of 60 Hz (0x3C represents −4 because the register is

centered with 0 at 0x40).

Figure 59. RMS Signal Processing

The Irms signal can be read from the waveform register by setting

the WAVMODE register (0x0D) and setting the WFSM bit in

the Interrupt Enable 3 SFR (MIRQENH, 0xDB). Like the

current and voltage channels waveform sampling modes, the

waveform data is available at sample rates of 25.6 kSPS,

12.8 kSPS, 6.4 kSPS, or 3.2 kSPS.

It is important to note that when the current input is larger than

40% of full scale, the Irms waveform sample register does not

represent the true processed rms value. The rms value processed

with this level of input is larger than the 24-bit read by the wave-

form register, making the value read truncated on the high end.

PGA1

IPA

ADC 1

HPF

PGA2

ADC 2

LPF2

60Hz

0.1°

CHANNEL 2 DELAY

REDUCED BY 4.48µs

(0.1°LEAD AT 60Hz)

0x0B IN PHCAL[7:0]

PHCAL[7:0]

–231.93µs TO +48.83µs

60Hz

DELAY BLOCK

1.22µs/LSB

Figure 58. Phase Calibration

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