ade7880 Analog Devices, Inc., ade7880 Datasheet - Page 37

• Manufacturer Part Number: ade7880
• Description: Polyphase Multifunction Energy Metering Ic With Harmonic Monitoring
• Manufacturer: Analog Devices, Inc.

Preliminary Technical Data
estimation of the mean absolute value (mav) measurement, is
active in PSM0 and PSM1 modes.
The ADE7880 also computes the rms values of various
fundamental and harmonic components of phase currents,
phase voltages and neutral current as part of the harmonic
calculations block. Please refer to Harmonics calculations
section for details.
The first method is to low-pass filter the square of the input
signal (LPF) and take the square root of the result (see Figure 40).
Then
After the LPF and the execution of the square root, the rms
value of f(t) is obtained by
The rms calculation based on this method is simultaneously
processed on all seven analog input channels. Each result is
available in the 24-bit registers: AIRMS, BIRMS, CIRMS,
AVRMS, BVRMS, CVRMS, and NIRMS.
The second method computes the absolute value of the input
signal and then filters it to extract its dc component. It computes
the absolute mean value of the input. If the input signal in
Equation 12 has a fundamental component only, its average
value is
The calculation based on this method is simultaneously processed
only on the three phase currents. Each result is available in the
20-bit registers AIMAV, BMAV, and CMAV. Note that the
proportionality between mav and rms values is maintained for
the fundamental components only. If harmonics are present in the
current channel, the mean absolute value is no longer
proportional to rms.
+
F
F
F
f
f
DC
DC
) (
2
2
=
t
) (
k
k
t
∑
,
≠
=
m
∞
=
=
m
k
∑
=
=
∞
T
=
2
1
1
π
k
2
∑
1
∞
=
∑
k
×
F
1
∞





×
=
k
F
T
1
∫
F
2
2
0
F
k
k
k
2
2
×
2
2
−
×
×
F
sin
m
∑
k
F
F
∞
=
1
1
1
sin
×
F
(
k
k
sin(
2
t ω
(
k
cos(
t ω
+
t ω
γ
+
2
)
dt
k
k
γ
)
t ω
k
−
)
×
T
T
+
∫
2
sin
γ
2
k
)
(
×
m
+
F
t ω
1
×
+
sin(
γ
m
t ω
)
)
dt





Rev. PrE | Page 37 of 103
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(12)
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Current RMS Calculation
This section presents the first approach to compute the rms
values of all phase and neutral currents. The ADE7880 also
computes the rms of the sum of the instantaneous values of the
phase currents if the bit 2 (INSEL) in CONFIG3 register is set
to 1. Note that the instantaneous value of the sum is stored into
ISUM register presented in Neutral Current Mismatch section.
In 3 phase 4 wires systems that only require sensing the phase
currents, this value provides a measure of the neutral current.
Figure 40 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. The current rms values are signed
24-bit values and they are stored into the AIRMS, BIRMS, CIRMS,
NIRMS registers. The update rate of the current rms
measurement is 8 kHz. If bit 2 (INSEL) of CONFIG3 register is
0 (default), the NIRMS register contains the rms value of the
neutral current. If bit INSEL is 1, the NIRMS register contains
the rms value of the sum of the instantaneous values of the
phase currents.
With the specified full-scale analog input signal of 0.5 V, the
ADC produces an output code that is approximately ±5,326,737.
The equivalent rms value of a full-scale sinusoidal signal is
3,766,572 (0x39792C), independent of the line frequency. If
the integrator is enabled, that is, when Bit 0 (INTEN) in the
CONFIG register is set to 1, the equivalent rms value of a full-
scale sinusoidal signal at 50 Hz is 3,759,718 (0x395E66) and at
60 Hz is 3,133,207 (0x2FCF17).
The accuracy of the current rms is typically 0.1% error from
the full-scale input down to 1/1000 of the full-scale input when
PGA = 1. Additionally, this measurement has a bandwidth of 3.3
kHz. It is recommended to read the rms registers synchronous to
the voltage zero crossings to ensure stability. The IRQ1
rupt can be used to indicate when a zero crossing has occurred
(see the Interrupts section). Table 12 shows the settling time for
the Irms measurement, which is the time it takes for the rms
register to reflect the value at the input to the current channel
when starting from 0.
Table 12. Settling Time for I rms Measurement
Integrator Status
Integrator Off
Integrator On
As stated in the Current Waveform Gain Registers section, the
serial ports of the ADE7880 work on 32-, 16-, or 8-bit words.
Similar to the register presented in Figure 39, the AIRMS,
BIRMS, CIRMS, NIRMS 24-bit signed registers are accessed as
32-bit registers with the eight MSBs padded with 0s.
50 Hz Input signals
580 ms
700 ms
60 Hz Input signals
580 ms
700 ms
ADE7880
inter-