ADE7754 datasheet(17/44 Pages) AD

17 / 44 page

REV. PrG 01/03

PRELIMINARY TECHNICAL DATA

ADE7754

– 17 –

ROOT MEAN SQUARE MEASUREMENT

Root Mean Square (RMS) is a fundamental measurement of

the magnitude of an AC signal. Its definition can be both

practical and mathematical. Defined practically, the RMS

value assigned to an AC signal is the amount of DC required

to produce an equivalent amount of heat in the same load.

Mathematically: the RMS value of a continuous signal f(t) is

defined as:

ft dt

rms

=⋅

()

∫

(1)

For time sampling signals, rms calculation involves squaring

the signal, taking the average and obtaining the square root:

rms

=⋅

∑

() (2)

The method used to calculate the RMS value in the ADE7754

is to low-pass filter the square of the input signal (LPF3) and

take the square root of the result.

With Vt

rms

( )

sin

=⋅

⋅

()

then

rms

()

cos

=−⋅

()

The RMS calculation is simultaneously processed on the six

analog input channels. Each result is available on separate

registers.

Current RMS calculation

Figure 18 shows the detail of the signal processing chain for

the RMS calculation on one of the phases of the current

channel. The current channel RMS value is processed from

the samples used in the current channel waveform sampling

mode. It should be noticed that the APGAIN adjustment

affects the result of the RMS calculation - see Current RMS

Gain adjust. The current RMS values are stored in an

unsigned 24-bit registers (AIRMS, BIRMS and CIRMS).

One LSB of the current RMS register is equivalent to one

LSB of a current waveform sample. The update rate of the

current RMS measurement is CLKIN/12.

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

ADC will produce an output code which is approximately

±2,684,354d - see Current channel ADC. The equivalent

RMS values of a full-scale AC signal and full scale DC signal

are respectively 1,898,124d (1CF68Ch) and 2,684,354d

(28F5C2h).

With offset calibration, the current rms measurement pro-

vided in the ADE7754 is accurate within +/-2% for signal

input between Full scale and Full scale/100.

Note: A crosstalk between phases can appear in the ADE7754

current rms measurements. This crosstalk follows a specific

pattern: Current rms measurements of Phase A are corrupted

by the signal on the Phase C current input, Current rms

measurements 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 only present 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 0x01F7 to the address 0x3D. 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

0x3D is set to 0x01F7, the level of the current rms crosstalk

is below 2%.

LPF3

Current Signal - i(t)

-100% to +100% FS

Irms(t)

1CF68Ch

00h

IRMS

SGN 211 210 29

IRMSOS[11:0]

AAPGAIN

HPF

00000h

400000h

C00000h

28F5C2h

D70A3Eh

+ FS

- FS

ADC Output

word Range

0000h

1CF68Ch

+ 100% FS

- 100% FS

E30974h

+ 122.5% FS

+ 70.7% FS

- 70.7% FS

- 122.5% FS

2378EDh

147AE0h

EB8520h

DC8713h

AAPGAIN[11:0]

000h

7FFh

800h

Current Channel (RMS)

Figure 18 - Current RMS signal processing

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