ADE5166 datasheet(56/156 Pages) AD

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ADE5166/ADE5169/ADE5566/ADE5569

Rev. B | Page 56 of 156

Figure 59 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 intro-

duction 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 4.88µs

(0.1°LEAD AT 60Hz)

0x3C IN PHCAL[7:0]

PHCAL[7:0]

–231.93µs TO +48.83µs

60Hz

DELAY BLOCK

1.22µs/LSB

Figure 59. 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 the results, 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 60, Figure 61, Figure 62, and Figure 63).

This LPF has a −3 dB cutoff frequency of 2 Hz when MCLK =

4.096 MHz.

()

)

sin(

(2)

where V is the rms voltage.

( t

−

cos

)

(

)

(3)

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

nuated and only the dc term, Vrms2 (shown as V2 in Figure 60),

goes through.

LPF3

INPUT

V2 (t) = V2 –V2 cos(2ωt)

V2 (t) = V2

V(t) = √2 × V sin(ωt)

Figure 60. RMS Signal Processing

The Irms signal can be read from the waveform register by set-

ting the WAVMODE register (Address 0x0D) and setting the

WFSM bit (Bit 5) in the Interrupt Enable 3 SFR (MIRQENH,

Address 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, and 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.

Current Channel RMS Calculation

Each ADE5166/ADE5169/ADE5566/ADE5569 simultaneously

calculates the rms values for the current and voltage channels in

different registers. Figure 61 and Figure 62 show the detail of the

signal processsing chain for the rms calculation on the current

channel. The current channel rms value is processed from the sam-

ples used in the current channel waveform sampling mode and is

stored in the unsigned, 24-bit IRMS SFRs (IRMSL, Address 0xD4;

IRMSM, Address 0xD5; and IRMSH, Address 0xD6). One LSB of

the current channel rms register (IRMSL, IRMSM, and IRMSH)

is equivalent to 1 LSB of a current channel waveform sample.

The update rate of the current channel rms measurement is

4.096 MHz/5. To minimize noise in the reading of the register, the

Irms register can also be configured to update only with the zero

crossing of the voltage input. This configuration is done by setting

the ZXRMS bit (Bit 2) in the MODE2 register (Address 0x0C).

With the different specified full-scale analog input values, the ADC

produces an output code that is approximately ±0d2,684,354

(see the Current Channel ADC section). Similarly, the equiva-

lent rms value of a full-scale ac signal is 0d1,898,124 (0x1CF68C).

The current rms measurement provided in the ADE5166/

ADE5169/ADE5566/ADE5569 is accurate to within ±0.5%

for signal inputs between full scale and full scale/500. The

conversion from the register value to amps must be done

externally in the microprocessor using an amps/LSB constant.

Current Channel RMS Offset Compensation

The ADE5166/ADE5169/ADE5566/ADE5569 incorporate a cur-

rent channel rms offset compensation register (IRMSOS). This is

a 12-bit, signed register that can be used to remove offset in the

current channel rms calculation. An offset can exist in the rms

calculation due to input noises that are integrated into the dc

component of V2(t).

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