ADE7878ACPZ datasheet(38/92 Pages) AD

38 / 92 page

ADE7878

Rev. 0 | Page 38 of 92

After the LPF and the execution of the square root, the rms

value of f(t) is obtained by

∑

∞

(13)

The rms calculation based on this method is simultaneously

processed on all seven analog input channels. Each result is

available in the 24-bit registers: AIRMS, BIRMS, CIRMS,

AVRMS, BVRMS, CVRMS, and NIRMS.

The other method computes the absolute value of the input

signal and then filters it to extract its dc component. It basically

computes the absolute mean value of the input. If the input

signal in Equation 12 has only fundamental component, its

average value is

⎥

⎦

⎤

⎢

⎣

⎡

−

∫

)

sin(

)

sin(

The calculation based on this method is simultaneously

processed only on the three phase currents. Each result is

available in the 20-bit registers: AIMAV, BMAV, and CMAV.

Note that the proportionality between mav and rms values is

maintained only for the fundamental components. If harmonics

are present in the current channel, then the mean absolute value

is not anymore proportional to rms.

Current RMS Calculation

This section presents the first approach to compute the rms

values of all phase and neutral currents.

Figure 53 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. The current rms values are signed

24-bit values, and they are stored into the AIRMS[23:0],

BIRMS[23:0], CIRMS[23:0], and NIRMS[23:0] registers. The

update rate of the current rms measurement is 8 kHz.

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

ADC produces an output code that is approximately ±5,928,256.

The equivalent rms value of a full-scale sinusoidal signal is

4,191,910 (0x3FF6A6), independent of the line frequency. If

the integrator is enabled, that is, when Bit 0 (INTEN) in the

CONFIG[15:0] register is set to 1, the equivalent rms value of

a full-scale sinusoidal signal at 50 Hz is 4,191,910 (0x3FF6A6)

and at 60 Hz is 3,493,258 (0x354D8A).

The accuracy of the current rms is typically 0.1% error from

the full-scale input down to 1/1000 of the full-scale input when

PGA = 1. Additionally, this measurement has a bandwidth of

2 kHz. It is recommended to read the rms registers synchronous

to the voltage zero crossings to ensure stability. The IRQ1 inter-

rupt can be used to indicate when a zero crossing has occurred

(see the

section).

shows the settling time for

the I rms measurement, which is the time it takes for the rms

when starting from 0.

Interrupts

Table 12

Table 12. Settling Time for I rms Measurement

Integrator Status

50 Hz Input Signals

60 Hz InputSignals

Integrator Off

440 ms

Integrator On

550 ms

500 ms

As previously stated, the serial ports of the ADE7878 work on

32-, 16-, or 8-bit words. Similar to the register presented in

Figure 33, the AIRMS, BIRMS, CIRMS, and NIRMS 24-bit

signed registers are accessed as 32-bit registers with the eight

MSBs padded with 0s.

0xA58AC0 =

–5,928,256

0x5A7540 =

5,928,256

CURRENT SIGNAL FROM

HPF OR INTEGRATOR

(IF ENABLED)

LPF

√

xIRMSOS[23:0]

xIRMS[23:0]

Figure 53. Current RMS Signal Processing

ADE7878ACPZ Datasheet(PDF) 38 Page - Analog Devices

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