ADE5569 datasheet(51/148 Pages) AD

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Preliminary Technical Data

ADE5166/ADE5169/ADE5566/ADE5569

Rev. PrB | Page 51 of 148

PHASE COMPENSATION

The ADE5166/ADE5169/ADE5566/ADE5569 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 calcula-

tions. The errors associated with phase mismatch are particularly

noticeable at low power factors. The ADE5166/ADE5169/

ADE5566/ADE5569 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 phase calibration register (PHCAL[7:0]) is a twos complement,

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

(−126d) to 0x68 (+104d).

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 a 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).

Figure 37 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).

PGA1

IP/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 37. Phase Calibration

RMS CALCULATION

The root mean square (rms) value of a continuous signal V(t) is

defined as

∫

rms

)

(

(1)

For time sampling signals, rms calculation involves squaring the

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

ADE5166/ADE5169/ADE5566/ADE5569 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 38, Figure 40, and Figure 41). This LPF has a −3 dB cutoff

frequency of 2 Hz when MCLK = 4.096 MHz.

()

)

sin(

(2)

where V is the rms voltage.

()

−

cos

)

(

(3)

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

ated and only the dc term Vrms2 (shown as V2 in Figure 38) goes

through.

LPF3

INPUT

V2 (t) = V2 –V2 cos(2ωt)

V2 (t) = V2

V(t) = √2 × V sin(ωt)

Figure 38. 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.

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