ADE7754 Ver la hoja de datos (PDF) - Analog Devices
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ADE7754 Datasheet PDF : 44 Pages
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ADE7754
pattern. Current rms measurements of Phase A are corrupted by
the signal on the Phase C current input, current rms measure-
ments of Phase B are corrupted by the signal on the Phase A
current input, and current rms measurements of Phase C are
corrupted by the signal on the Phase B current input. This
crosstalk is present only on the current rms measurements and
does not affect the regular active power measurements. The
level of the crosstalk is dependent on the level of the noise
source and the phase angle between the noise source and the
corrupted signal. The level of the crosstalk can be reduced by
writing 01F7h to the address 3Dh. This 16-bit register is
reserved for factory operation and should not be written to any
other value. When the current inputs are 120° out of phase and
the register 3Dh is set to 01F7h, the level of the current rms
crosstalk is below 2%.
Current RMS Gain Adjust
The active power gain registers (AAPGAIN[11:0], BAPGAIN,
and CAPGAIN) affect the active power and current rms values.
Calibrating the current rms measurements with these registers is
not recommended. The conversion of the current rms registers
values to amperes has to be done in an external microcontroller
with a specific ampere/LSB constant for each phase. See the Cali-
bration of a 3-Phase Meter Based on the ADE7754 Application
Note AN-624. Due to gain mismatches between phases, the cali-
bration of the ampere/LSB constant has to be done separately for
each phase. One-point calibration is sufficient for this calibration.
The active power gain registers ease the calibration of the active
energy calculation in MODE 1 and 2 of the WATMODE register.
If the APGAIN registers are used for active power calibration
(WATMOD bits in WATMode register = 1 or 2), the current
rms values are changed by the active power gain register value
as described in the expression
Current rms register Phase A = rms ×
1+
AAPGAIN
212
done close to full scale and the other at approximately full scale/
100. The current offset compensation can then be derived using
these measurements. See the Calibration of a 3-Phase Meter Based
on the ADE7754 Application Note AN-624.
Voltage RMS Calculation
Figure 21 shows the details of the signal processing chain for the
rms calculation on one of the phases of the voltage channel. The
voltage channel rms value is processed from the samples used in
the voltage channel waveform sampling mode. The output of
the voltage channel ADC can be scaled by ± 50% by changing
VGAIN registers to perform an overall apparent power calibra-
tion. See the Apparent Power Calculation section. The VGAIN
adjustment affects the rms calculation because it is done before
the rms signal processing. The voltage rms values are stored in
unsigned 24-bit registers (AVRMS, BVRMS, and CVRMS).
256 LSB of the voltage rms register is approximately equivalent
to one LSB of a voltage waveform sample. The update rate of
the voltage rms measurement is CLKIN/12.
With the specified full-scale ac analog input signal of 0.5 V, the
LPF1 produces an output code that is approximately ± 10,217
decimal at 60 Hz. See the Voltage Channel ADC section. The
equivalent rms value of a full-scale ac signal is approximately
7,221d (1C35h), which gives a voltage rms value of 1,848,772d
(1C35C4h) in the VRMS register. With offset calibration, the
voltage rms measurement provided in the ADE7754 is accurate
within ± 0.5% for signal input between full scale and full scale/20.
VOLTAGE SIGNAL – V(t)
0.5/GAIN2
VRMSOS[11:0]
SGN 211 28
22 21 20
VA
LPF1
12
800h–7FFh
AVGAIN[11:0]
VOLTAGE
SIGNAL – v(t)
LPF3
VOLTAGE
CHANNEL (rms)
+
24
+
For example, when 7FFh is written to the active power gain
register, the ADC output is scaled up by 22.5%. Similarly, 800h
= –2047d (signed twos complement) and ADC output is scaled
by 29.3%. These two examples are illustrated in Figure 20.
Current RMS Offset Compensation
The ADE7754 incorporates a current rms offset compensation
for each phase (AIRMSOS, BIRMSOS, and CIRMSOS). These
are 12-bit twos complement signed registers that can be used to
remove offsets in the current rms calculations. An offset may
exist in the rms calculation due to input noises that are inte-
grated in the dc component of V2(t). The offset calibration will
allow the contents of the IRMS registers to be maintained at zero
when no current is being consumed.
n LSB of the current rms offset are equivalent to 32768 ϫ n LSB
of the square of the current rms register. Assuming that the
maximum value from the current rms calculation is 1,898,124
decimal with full-scale ac inputs, then 1 LSB of the current rms
offset represents 0.0058% of measurement error at –40 dB
below full scale.
Irms = Irms02 + IRMSOS × 32768
where Irmso is the rms measurement without offset correction.
The current rms offset compensation should be done by testing the
rms results at two non-zero input levels. One measurement can be
4000h
28F5h
00000h
D70Ah
C000h
+ FS
– FS
ADC OUTPUT
WORD RANGE
2A50A6h
1C35C4h
E1AE2h
0000h
F1E51Eh
E3CA3Ch
D5AF5Ah
000h 7FFh 800h
+ 150% FS
+ 100% FS
+ 50% FS
AVGAIN[11:0]
– 50% FS
– 100% FS
– 150% FS
Figure 21. Voltage RMS Signal Processing
Voltage RMS Gain Adjust
The voltage gain registers (AVGAIN[11:0], BVGAIN, and
CVGAIN) affect the apparent power and voltage rms values.
Calibrating the voltage rms measurements with these registers is
not recommended. The conversion of the voltage rms registers
values to volts has to be done in an external microcontroller
with a specific volt/LSB constant for each phase. See the Cali-
bration of a 3-Phase Meter Based on the ADE7754 Application
Note AN-624. Due to gain mismatches between phases, the cali-
bration of the volt/LSB constant has to be done separately for
each phase. One point calibration is sufficient for this calibration.
The voltage gain registers are aimed to ease the calibration of the
apparent energy calculation in MODE 1 and MODE 2 of the
VAMODE register.
–16–
REV. 0
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