ADE7758_15 datasheet(53/72 Pages) AD

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Data Sheet

ADE7758

Rev. E | Page 53 of 72

Power Offset Calibration Using Line Accumulation

Power offset calibration should be used for outstanding

performance over a wide dynamic range (1000:1). Calibration

of the power offset is done at or close to the minimum current.

The ADE7758 has power offset registers for watts and VAR,

xWATTOS (0x39 to 0x3B) and xVAROS (0x3C to 0x3E). Offsets in

the VA measurement are compensated by adjusting the rms offset

registers (see the Calibration of IRMS and VRMS Offset section).

More line cycles could be required at the minimum current to

minimize the effect of quantization error on the offset

calibration. For example, if a current of 40 mA results in an

active energy accumulation of 113 after 2000 half line cycles,

one LSB variation in this reading represents an 0.8% error. This

measurement does not provide enough resolution to calibrate

out a <1% offset error. However, if the active energy is

accumulated over 37,500 half line cycles, one LSB variation

results in 0.05% error, reducing the quantization error.

Figure 84 shows the steps to calibrate the power offsets using

the line accumulation mode.

Step 1: If the values change after gain calibration, Step 1, Step 3,

and Step 4 from the gain calibration should be repeated to

configure the LCYCMODE, LINECYC, and MASK registers.

Select Phase A, Phase B, or Phase C for a line period measure-

ment with the FREQSEL[1:0] bits in the MMODE register (0x14).

For example, clearing Bit 1 and Bit 0 selects Phase A for line

period measurement.

Step 2: Set the test system for IMIN, VNOM, and unity power factor.

Step 3: Reset the interrupt status register by reading RSTATUS

(0x1A).

Step 4: Read all xWATTHR energy registers (0x01 to 0x03) after

the LENERGY interrupt and store the values.

Step 4a: If it is not known, the line period is available in the

ADE7758 frequency register, FREQ (0x10). To configure line

period measurement, select the phase for period measurement

in the MMODE[1:0] and set LCYCMODE[7].

Step 5: Calculate the value to be written to the xWATTOS

registers according to the following equations:

TEST

MIN

ITEST

IMIN

TEST

LINECYC

xWATTHR

Offset

TEST

MIN

–

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

(73)

[]

CLKIN

AccumTime

Offset

xWATTOS

(74)

where:

AccumTime is defined in Equation 61.

is the value in the energy register at ITEST.

is the value in the energy register at IMIN.

LINECYCIMIN is the number of line cycles accumulated at IMIN.

LINECYCIMAX is the number of line cycles accumulated at IMAX.

TEST

xWATTHR

MIN

xWATTHR

Step 6: Write to all xWATTOS registers (0x39 to 0x3B).

Step 7: Set the test system for IMIN, VNOM, and zero power factor

inductive to calibrate VAR gain.

Step 8: Repeat Steps 3, 4, and 5.

Step 9: Calculate the value written to the xVAROS registers

according to the following equations:

TEST

MIN

ITEST

IMIN

TEST

LINECYC

xVARHR

Offset

TEST

MIN

–

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

(75)

202

]

[

[11

FREQ

CLKIN

AccumTime

Offset

xVAROS

(76)

where the FREQ[11:0] register is configured for line period

readings.

Example: Power Offset Calibration Using Line Accumulation

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.

For this example, IMIN = 50 mA, ITEST = 10 A, VNOM = 220 V,

VFULLSCALE = 500 V, IFULLSCALE = 130 A, MC = 3200 impulses/kWh,

Power Factor = 1, Frequency = 50 Hz, and CLKIN = 10 MHz.

Also, LINECYCITEST = 0x800 and LINECYCIMIN = 0x4000.

After accumulating over 0x800 line cycles for gain calibration at

ITEST, the example ADE7758 meter shows 14804d in the

AWATTHR (0x01) register. At IMIN, the meter shows 592d in the

AWATTHR register. By using Equation 73, this is equivalent to

0.161 LSBs of offset; therefore, using Equation 61 and Equation 74,

the value written to AWATTOS is 0d.

0.16

–

0.05

0x800

0x4000

14804

–

592

⎟

⎠

⎞

⎜

⎝

⎛

Offset

2085

4000

−

AccumTime

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