Datasheet

Data Sheet ADE7758
Rev. E | Page 29 of 72
Current RMS Offset Compensation
The ADE7758 incorporates a current rms offset compensation
register for each phase (AIRMSOS, BIRMSOS, and CIRMSOS).
These are 12-bit signed registers that can be 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
2
(t). Assuming that the maximum value from
the current rms calculation is 1,914,753d with full-scale ac
inputs (60 Hz), one LSB of the current rms offset represents
0.94% of the measurement error at 60 dB down from full scale.
The IRMS measurement is undefined at zero input. Calibration
of the offset should be done at low current and values at zero
input should be ignored. For details on how to calibrate the
current rms measurement, see the Calibration section.
IRMSOSIRMSIRMS
2
0
 16384
(9)
where
IRMS
0
is the rms measurement without offset correction.
Table 7. Approximate IRMS Register Values
Frequency (Hz) Integrator Off (d) Integrator On (d)
50 1,921,472 2,489,581
60 1,914,752 2,067,210
Voltage Channel RMS Calculation
Figure 64 shows the details of the signal path for the rms
estimation on Phase A of the voltage channel. This voltage rms
estimation is done in the ADE7758 using the mean absolute
value calculation, as shown in Figure 64.The voltage channel
rms value is processed from the waveform samples after the
low-pass filter LPF1. The output of the voltage channel ADC
can be scaled by ±50% by changing VRMSGAIN[11:0] registers
to perform an overall rms voltage calibration. The VRMSGAIN
registers scale the rms calculations as well as the apparent
energy calculation because apparent power is the product of the
voltage and current rms values. The voltage rms values are
stored in 24-bit registers (AVRMS, BVRMS, and CVRMS). One
LSB of a voltage waveform sample is approximately equivalent to
256 LSBs of the voltage rms register. 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 63% of its
full-scale value, that is, ±9,372d, at 60 Hz (see the Voltage
Channel ADC
section). The equivalent rms value of a full-scale
ac signal is approximately 1,639,101 (0x1902BD) in the VRMS
register.
The accuracy of the VRMS measurement is typically 0.5% error
from the full-scale input down to 1/20 of the full-scale input.
Additionally, this measurement has a bandwidth of 260 Hz. It is
recommended to read the rms registers synchronous to the
voltage zero crossings to ensure stability. The IRQ can be used
to indicate when a zero crossing has occurred (see the
Interrupts section).
VAN
AVRMSGAIN[11:0]
0x2748
LPF OUTPUT
WORD RANGE
0x0
60Hz
0xD8B8
0x2797
LPF OUTPUT
WORD RANGE
0x0
50Hz
0xD869
LPF1
VOLTAGE SIGNAL–V(t)
0.5
GAIN
0x193504
50Hz
0x0
0x1902BD
60Hz
0x0
| X |
A
VRMS[23:0]
LPF3
SGN 2
16
2
15
2
14
2
8
2
7
2
6
V
RMSOS[11:0]
+
+
04443-063
Figure 64. Voltage RMS Signal Processing
Table 8 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 8. Settling Time for VRMS Measurement
63% 100%
100 ms 960 ms
Voltage RMS Offset Compensation
The ADE7758 incorporates a voltage rms offset compensation
for each phase (AVRMSOS, BVRMSOS, and CVRMSOS).
These are 12-bit signed registers that can be used to remove
offsets in the voltage rms calculations. An offset can exist in the
rms calculation due to input noises and offsets in the input
samples. It should be noted that the offset calibration does not
allow the contents of the VRMS registers to be maintained at 0
when no voltage is applied. This is caused by noise in the
voltage rms calculation, which limits the usable range between
full scale and 1/50th of full scale. One LSB of the voltage rms
offset is equivalent to 64 LSBs of the voltage rms register.
Assuming that the maximum value from the voltage rms
calculation is 1,639,101d with full-scale ac inputs, then 1 LSB of
the voltage rms offset represents 0.042% of the measurement
error at 1/10 of full scale.
VRMS = VRMS
0
+ VRMSOS × 64 (10)
where
VRMS
0
is the rms measurement without the offset
correction.
Table 9. Approximate VRMS Register Values
Frequency (Hz) Value (d)
50 1,678,210
60 1,665,118