ADE7754 datasheet(18/44 Pages) AD

18 / 44 page

REV. PrG 01/03

PRELIMINARY TECHNICAL DATA

ADE7754

– 18 –

Current RMS Gain Adjust

The Active power Gain registers (AAPGAIN[11:0],

BAPGAIN and CAPGAIN) have an effect on the Active

Power and current rms values. It is not recommended to

calibrate the current rms measurements with these registers.

The conversion of the current rms registers values to Am-

peres has to be done in an external Micro-controller with a

specific Ampere/LSB constant for each phase - see Calibration

of a 3-phase meter based on the ADE7754. Due to gain

mismatches between phases, the calibration of the Ampere/

LSB constant has to be done for each phase separately. One

point calibration is sufficient for this calibration. The Active

Power Gain registers are aimed to ease the calibration of the

Active energy calculation in MODE 1 and 2 of the VAMODE

If the APGAIN registers are used for Active Power calibra-

tion (WATMOD bits in WATTMode register = 1 or 2), the

current rms values are changed by Active Power Gain register

value as described in the expression below:

Current RMS

phase A

RMS

AAPGAIN

=×









For example, when 7FFh is written to the Active Power Gain

800h = -2047d (signed 2’s Complement) and ADC output is

scaled by 29.3%. These two examples are illustrated graphi-

cally in Figure 18.

Current RMS offset compensation

The ADE7754 incorporates a current RMS offset compen-

sation

for

each

phase

(AIRMSOS,

BIRMSOS

and

CIRMSOS). These are 12-bit 2-complement signed regis-

ters which 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 integrated in the DC component of

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 x

n LSB of the square of the Current RMS register. Assuming

that the maximum value from the Current RMS calculation

is 1,898,124d with full scale AC inputs, then 1 LSB of the

current RMS offset represents 0.0058% of measurement

error at -40dB down of full scale.

IRMSOS

rms

32768

where Irmso is the RMS measurement without offset correc-

tion.

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 compensa-

tion can then be derived from these measurements - see

Calibration of a 3-phase meter based on the ADE7754.

Voltage RMS calculation

Figure 19 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 calibration -see Apparent Power calculation.

The VGAIN adjustment affects the RMS calculation as 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 wave-

form sample. The update rate of the voltage RMS measurement

is CLKIN/12.

With the specified full scale AC analog input signal of 0.5V,

the LPF1 produces an output code which is approximately

±10,217d at 60 Hz- see Voltage channel ADC. The equivalent

RMS value of a full-scale AC signal is approximately 7,221d

(1C35h), which gives a voltage RMS value of 1,848,772

(1C35C4h) in the VRMS register.

With offset calibration, the voltage rms measurement pro-

vided in the ADE7754 is accurate within +/-0.5% for signal

input between Full scale and Full scale/20.

LPF3

Voltage Signal - V(t)

LPF1

SGN 211 28

VRMSOS[11:0]

800Hex - 7FFHex

0.5/GAIN2

Voltage Signal - v(t)

00000h

4000h

C000h

28F5h

D70Ah

+ FS

- FS

ADC Output

word Range

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

Voltage Channel (RMS)

AVGAIN[11:0]

Figure 19 - Voltage RMS signal processing

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