ADE7758ARWRL datasheet(52/68 Pages) AD

52 / 68 page

ADE7758

Rev. A | Page 52 of 68

Calibration of IRMS and VRMS Offset

Step 11: Calculate the value written to the xVAROS registers

according to the following equations.

IRMSOS and VRMSOS are used to cancel noise and offset

contributions from the inputs. The calibration method is the

same whether calibrating using the pulse outputs or line

accumulation. Reading the registers is required for this

calibration since there is no rms pulse output. The rms offset

calibration should be performed before VAGAIN calibration.

The rms offset calibration also removes offset from the VA

calculation. For this reason, no VA offset register exists in the

ADE7758.

TEST

MIN

TEST

xVARHR

Offset

–

(54)

]

[

CLKIN

AccumTime

Offset

xVAROS

(55)

Example—Power Offset Calibration Using Line

Accumulation

The low-pass filter used to obtain the rms measurements is not

ideal, therefore it is recommended to synchronize the readings

with the zero crossings of the voltage waveform and to average a

few measurements when reading the rms registers.

This example only shows Phase A of the phase active power

offset calibration. Both active and reactive power offset for all

phases can be calibrated simultaneously using the method

explained in the Power Offset Calibration Using Line

Accumulation section.

The ADE7758 IRMS measurement is linear over a 500:1 range,

and the VRMS measurement is linear over a 20:1 range. To

measure the voltage VRMS offset (xVRMSOS), measure rms

values at two different nonzero current levels, for example,

VNOM and VNOM/20. To measure the current rms offset

(IRMSOS), measure rms values at two different nonzero current

levels, for example, ITEST and IMAX/500). This translates to three

test conditions: ITEST and VNOM, IMAX/500 and VNOM, ITEST and

VNOM/20. Figure 84 shows a flow chart for calibrating the rms

measurements.

For this example, ITEST = 10 A, IMIN = 100 mA, VNOM = 220 V,

Power Factor = unity, Frequency = 50 Hz, and LINECYC =

0xFFF.

At ITEST, the example ADE7758 meter shows 192489d in the

AWATTHR (0x01) register after gain calibration at 0xFFF line

cycles. At IMIN, the meter shows 1919d in the AWATTHR

offset, therefore, using Equation 53, the value written to

AWATTOS is 94d.

–

489

192

–

1919

Offset

MHz

29 =

AWATTOS

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