Datasheet

Data Sheet ADE7953
Rev. B | Page 35 of 72
OFFSET CALIBRATION
Power Offsets
The ADE7953 includes offset calibration registers for the active,
reactive, and apparent powers on Current Channel A and Current
Channel B. Offsets can exist in the power calculations due to
crosstalk between channels on the PCB and in the ADE7953.
The offset calibration allows these offsets to be removed to
increase the accuracy of the measurement at low input levels.
The active power offset can be corrected on Current Channel A
and Current Channel B by adjusting the AWATTOS (Address
0x289 and Address 0x389) and BWATTOS (Address 0x295 and
Address 0x395) registers, respectively. The xWATTOS registers
are 24-bit, signed twos complement registers with default
values of 0. One LSB in the xWATTOS register is equivalent
to 0.001953 LSBs in the active power measurement. The
xWATTOS value is, therefore, applied to the xWATT register,
shifted by nine bits, as shown in Figure 58.
23 9 0
xWATTOS
23 0
xWATT
09320-027
Figure 58. xWATTOS and xWATT Registers
With full-scale inputs on the voltage and current channels, the
expected power reading is approximately 4862401 LSBs (deci-
mal). At −60 dB (1000:1) on Current Channel A and Current
Channel B, the expected readings in the AWATT and BWATT
registers, respectively, are approximately 4862 (decimal). One
LSB of the xWATT register, therefore, corresponds to
0.000039% at −60 dB.
The reactive power offset can be corrected on Current Channel A
and Current Channel B by adjusting the AVAROS (Address
0x28A and Address 0x38A) and BVAROS (Address 0x296 and
Address 0x396) registers, respectively. The xVAROS registers
affect the reactive power in the same way that the xWATTOS
registers affect the active power.
The apparent power offset can be corrected on Current
Channel A and Current Channel B by adjusting the AVAOS
(Address 0x28B and Address 0x38B) and BVAOS (Address 0x297
and Address 0x397) registers, respectively. The xVAOS registers
affect the apparent power in the same way that the xWATTOS
registers affect the active power.
RMS Offsets
The ADE7953 includes offset calibration registers to allow offset
in the rms measurements to be corrected. Offset calibration
registers are available for the IRMS measurements on Current
Channel A and Current Channel B, as well as for the VRMS
measurement. Offset can exist in the rms calculation due to
input noise that is integrated in the dc component of V
2
(t). The
offset calibration allows these offsets to be removed to increase
the accuracy of the measurement at low input levels.
The voltage rms offset can be corrected by adjusting the VRMSOS
register (Address 0x288 and Address 0x388). This 24-bit, signed
twos complement register has a default value of 0, indicating
that no offset is added. The VRMSOS value is applied prior to
the square root function. Equation 33 shows the effect of the
VRMSOS register on the VRMS measurement.
12
2
2 VRMSOSVRMS VRMS
0
 (33)
where
VRMS
0
is the initial VRMS reading prior to offset
calibration.
The current rms offset is calibrated in a similar way. The AIRMSOS
register (Address 0x286 and Address 0x386) compensates for
offsets in the IRMSA measurement, and the BIRMSOS register
(Address 0x292 and Address 0x392) compensates for offsets in
the IRMSB measurement. Both registers are 24-bit, signed twos
complement registers. The xIRMSOS registers affect the IRMS
measurements in the same way that the VRMSOS register
affects the VRMS measurement. Equation 33 can therefore be
modified to represent the offset calibration on the IRMS, as
shown in Equation 34 and Equation 35.
12
2
2 AIRMSOSIRMSA IRMSA
0
 (34)
12
2
2 BIRMSOSIRMSB IRMSB
0
 (35)
Refer to the AN-1118 Application Note,
Calibrating a Single-
Phase Energy Meter Based on the ADE7953
, for a more detailed
explanation on how to calibrate an energy meter based on the
ADE7953.