ADE7754_15 datasheet(16/44 Pages) AD

16 / 44 page

REV. 0

–16–

ADE7754

pattern. Current rms measurements of Phase A are corrupted by

the signal on the Phase C current input, current rms measure-

ments of Phase B are corrupted by the signal on the Phase A

current input, and current rms measurements of Phase C are

corrupted by the signal on the Phase B current input. This

crosstalk is present only on the current rms measurements and

does not affect the regular active power measurements. The

level of the crosstalk is dependent on the level of the noise

source and the phase angle between the noise source and the

corrupted signal. The level of the crosstalk can be reduced by

writing 01F7h to the address 3Dh. This 16-bit register is

reserved for factory operation and should not be written to any

other value. When the current inputs are 120

° out of phase and

the register 3Dh is set to 01F7h, the level of the current rms

crosstalk is below 2%.

Current RMS Gain Adjust

The active power gain registers (AAPGAIN[11:0], BAPGAIN,

and CAPGAIN) affect the active power and current rms values.

Calibrating the current rms measurements with these registers is

not recommended. The conversion of the current rms registers

values to amperes has to be done in an external microcontroller

with a specific ampere/LSB constant for each phase. See the Cali-

bration of a 3-Phase Meter Based on the ADE7754 Application

Note AN-624. Due to gain mismatches between phases, the cali-

bration of the ampere/LSB constant has to be done separately for

each phase. One-point calibration is sufficient for this calibration.

The active power gain registers ease the calibration of the active

energy calculation in MODE 1 and 2 of the WATMODE register.

If the APGAIN registers are used for active power calibration

(WATMOD bits in WATMode register = 1 or 2), the current

rms values are changed by the active power gain register value

as described in the expression

Current rms register Phase A

rms

AAPGAIN

=×













For example, when 7FFh is written to the active power gain

= –2047d (signed twos complement) and ADC output is scaled

by 29.3%. These two examples are illustrated in Figure 20.

Current RMS Offset Compensation

The ADE7754 incorporates a current rms offset compensation

for each phase (AIRMSOS, BIRMSOS, and CIRMSOS). These

are 12-bit twos complement signed registers that can be used to

remove offsets in the current rms calculations. An offset may

exist in the rms calculation due to input noises that are inte-

grated in the dc component of V

2(t). The offset calibration will

allow the contents of the IRMS registers to be maintained at zero

when no current is being consumed.

n LSB of the current rms offset are equivalent to 32768

n LSB

of the square of the current rms register. Assuming that the

maximum value from the current rms calculation is 1,898,124

decimal with full-scale ac inputs, then 1 LSB of the current rms

offset represents 0.0058% of measurement error at –40 dB

below full scale.

IRMSOS

rms

32768

where Irmso is the rms measurement without offset correction.

The current rms offset compensation should be done by testing the

rms results at two non-zero input levels. One measurement can be

done close to full scale and the other at approximately full scale/

100. The current offset compensation can then be derived using

these measurements. See the Calibration of a 3-Phase Meter Based

on the ADE7754 Application Note AN-624.

Voltage RMS Calculation

Figure 21 shows the details of the signal processing chain for the

rms calculation on one of the phases of the voltage channel. The

voltage channel rms value is processed from the samples used in

the voltage channel waveform sampling mode. The output of

the voltage channel ADC can be scaled by

±50% by changing

VGAIN registers to perform an overall apparent power calibra-

tion. See the Apparent Power Calculation section. The VGAIN

adjustment affects the rms calculation because it is done before

the rms signal processing. The voltage rms values are stored in

unsigned 24-bit registers (AVRMS, BVRMS, and CVRMS).

256 LSB of the voltage rms register is approximately equivalent

to one LSB of a voltage waveform sample. 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

±10,217

decimal at 60 Hz. See the Voltage Channel ADC section. The

equivalent rms value of a full-scale ac signal is approximately

7,221d (1C35h), which gives a voltage rms value of 1,848,772d

(1C35C4h) in the VRMS register. With offset calibration, the

voltage rms measurement provided in the ADE7754 is accurate

within

±0.5% for signal input between full scale and full scale/20.

VOLTAGE

SIGNAL – v(t)

VOLTAGE

CHANNEL (rms)

0000h

1C35C4h

+ 100% FS

– 100% FS

E3CA3Ch

+ 150% FS

+ 50% FS

– 50% FS

– 150% FS

2A50A6h

E1AE2h

F1E51Eh

D5AF5Ah

AVGAIN[11:0]

000h

7FFh

800h

00000h

4000h

C000h

28F5h

D70Ah

+ FS

– FS

ADC OUTPUT

WORD RANGE

800h–7FFh

LPF1

211

SGN

VRMSOS[11:0]

LPF3

AVGAIN[11:0]

VOLTAGE SIGNAL – V(t)

0.5/GAIN2

Figure 21. Voltage RMS Signal Processing

Voltage RMS Gain Adjust

The voltage gain registers (AVGAIN[11:0], BVGAIN, and

CVGAIN) affect the apparent power and voltage rms values.

Calibrating the voltage rms measurements with these registers is

not recommended. The conversion of the voltage rms registers

values to volts has to be done in an external microcontroller

with a specific volt/LSB constant for each phase. See the Cali-

bration of a 3-Phase Meter Based on the ADE7754 Application

Note AN-624. Due to gain mismatches between phases, the cali-

bration of the volt/LSB constant has to be done separately for

each phase. One point calibration is sufficient for this calibration.

The voltage gain registers are aimed to ease the calibration of the

apparent energy calculation in MODE 1 and MODE 2 of the

VAMODE register.

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