Datasheet

Data Sheet ADE5166/ADE5169/ADE5566/ADE5569
Rev. D | Page 57 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).
110100
1
7 0
PGA1
I
P
/I
PA
I
N
I
ADC 1
HPF
24
PGA2
V
P
V
N
V
ADC 2
24
LPF2
V
I
60Hz
0.1°
I
V
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
11
DELAY BLOCK
1.22µs/LSB
07411-035
Figure 59. Phase Calibration
RMS CALCULATION
Current Channel RMS Calculation
The root mean square (rms) value of a continuous signal, I(t), is
defined as
( )
∫
×=
T
rms
dttI
T
I
0
2
1
(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
on the current channel 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
and Figure 62).
This LPF has a −3 dB cutoff frequency of 2 Hz when MCLK =
4.096 MHz.
( )
)sin(2 tItI ω ×=
(2)
where V is the rms voltage.
( )
tIItI ω 2cos)(
222
−=
(3)
When this signal goes through LPF3, the cos(2ωt) term is atte-
nuated and only the dc term, I
rms
2
(shown as I
2
in Figure 60),
goes through.
IRMS
LPF3
INPUT
I
2
(t) = I
2
– I
2
cos(2ωt)
I
2
(t) = I
2
I(t) = √2 × I sin(ωt)
07411-036
Figure 60. IRMS Signal Processing
The I
rms
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 I
rms
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.
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 samples 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
I
rms
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 V
2
(t).