ADE7953 datasheet(35/68 Pages) AD

35 / 68 page

Data Sheet

ADE7953

Rev. A | Page 35 of 68

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.

xWATTOS

xWATT

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 cali-

bration 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 V2(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

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.

VRMSOS

VRMS

(33)

where VRMS0 is the initial VRMS reading prior to offset

calibration.

The current rms offset is calibrated in a similar way. The AIRMSOS

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.

AIRMSOS

IRMSA

(34)

BIRMSOS

IRMSB

(35)

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