ADE7878 datasheet(40/92 Pages) AD

40 / 92 page

ADE7878

Rev. 0 | Page 40 of 92

Voltage Channel RMS Calculation

Figure 57 shows the detail 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. The voltage rms values are signed 24-bit

values, and they are stored into the AVRMS[23:0], BVRMS[23:0],

and CVRMS[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.

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

full-scale input down to 1/1000 of the full-scale input. 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 interrupt can be used to

indicate when a zero crossing has occurred (see the

section).

Interrupts

The settling time for the V rms measurement is 440 ms for both

50 Hz and 60 Hz input signals. The V rms measurement is the

time it takes for the rms register to reflect the value at the input

to the voltage channel when starting from 0.

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 AVRMS, BVRMS, and CVRMS 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

VOLTAGE SIGNAL

FROM HPF

LPF

√

xVRMSOS[23:0]

xVRMS[23:0]

Figure 57. Voltage RMS Signal Processing

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