ade7854 Analog Devices, Inc., ade7854 Datasheet - Page 33

• Manufacturer Part Number: ade7854
• Description: Poly Phase Multifunction Energy Metering Ic With Neutral Current Measurement
• Manufacturer: Analog Devices, Inc.

Preliminary Technical Data
to read the rms registers synchronous to the voltage zero
crossings to ensure stability. The
indicate when a zero crossing has occurred (see the Interrupts
section).
Table 10 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.
Table 10. Settling Time for IRMS Measurement
Integrator Off
Integrator On
As previously stated, the serial ports of the ADE7854 work on
32, 16 or 8-bit words. Similar to the register presented in Figure
17, AIRMS, BIRMS and CIRMS 24-bit signed registers are
accessed as 32-bit registers with 8 most significant bits padded
with 0s.
Current RMS Offset Compensation
The ADE7854 incorporates a current rms offset compensation
register for each phase: AIRMSOS[23:0], BIRMSOS[23:0] and
CIRMSOS[23:0]. These are 24-bit signed registers and are used
to remove offsets in the current rms calculations. An offset can
exist in the rms calculation due to input noises that are
integrated in the dc component of I
rms offset compensation register is equivalent to 128 LSBs of
the square of the current rms register. Assuming that the
maximum value from the current rms calculation is 4,191,400
with full-scale ac inputs (50 Hz), one LSB of the current rms
offset represents 0.00036% of the rms measurement at 60 dB
down from full scale. Calibration of the offset should be done at
low current and values at zero input should be ignored.
where IRMS
As previously stated, the serial ports of the ADE7854 work on
32, 16 or 8-bit words and the DSP works on 28 bits. Similar to
registers presented in Figure 16, AIRMSOS, BIRMSOS and
CIRMSOS 24-bit signed registers are accessed as 32-bit registers
with 4 most significant bits padded with 0s and sign extended
to 28 bits.
IRMS
VOLTAGE SIGNAL
FROM HPF
=
IRMS
0
is the rms measurement without offset correction.
Figure 41. Voltage RMS Signal Processing
50Hz Input signals
530msec
550msec
0
2
+
x
0xA9A9A8=
0x565658=
2
-5658200
5658200
IRMSOS
0V
LPF
⋅
2
128
xVRMSOS[23:0]
7
IRQ interrupt can be used to
2
Σ
(t). One LSB of the current
1
60Hz Input signals
530msec
500msec
xVRMS[23:0]
(12)
Rev. PrC| Page 33 of 71
Voltage Channel RMS Calculation
Figure 41 shows the detail 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. The voltage rms values are signed 24-bit
values and they are stored into the registers AVRMS[23:0],
BVRMS[23:0] and CVRMS[23:0]. The update rate of the
current rms measurement is 8KHz.
With the specified full-scale analog input signal of 0.5 V, the
ADC produces an output code that is approximately
±5,928,256. The equivalent rms value of a full-scale sinusoidal
signal at 4,191,910 (0x3FF6A6), independent of the line
frequency.
The accuracy of the voltage rms is typically 0.1% error from the
full-scale input down to 1/1000 of the full-scale input. Additionally,
this measurement has a bandwidth of 4 kHz. It is recommended
to read the rms registers synchronous to the voltage zero
crossings to ensure stability. The
indicate when a zero crossing has occurred (see the Interrupts
section).
Table 11 shows the settling time for the VRMS measurement,
which is the time it takes for the rms register to reflect the value
at the input to the voltage channel.
Table 11. Settling Time for VRMS Measurement
50Hz Input signals
530 ms
As previously stated, the serial ports of the ADE7854 work on
32, 16 or 8-bit words. Similar to the register presented in Figure
17, AVRMS, BVRMS and CVRMS 24-bit signed registers are
accessed as 32-bit registers with 8 most significant bits padded
with 0s.
Voltage RMS Offset Compensation
The ADE7854 incorporates a voltage rms offset compensation
for each phase AVRMSOS[23:0], BVRMSOS[23:0], and
CVRMSOS[23:0]. These are 24-bit signed registers used to
remove offsets in the voltage rms calculations. An offset can
exist in the rms calculation due to input noises that are
integrated in the dc component of V
rms offset compensation register is equivalent to 128 LSBs of
the square of the voltage rms register. Assuming that the
maximum value from the voltage rms calculation is 4,191,400
with full-scale ac inputs (50 Hz), one LSB of the current rms
offset represents 0.00036% of the rms measurement at 60 dB
down from full scale. Calibration of the offset should be done at
low current and values at zero input should be ignored.
where VRMS
correction.
VRMS
=
VRMS
0
is the rms measurement without offset
0
2
+
VRMSOS
⋅
128
60Hz Input signals
530 ms
IRQ interrupt can be used to
2
(t). One LSB of the voltage
1
ADE7854
(13)