EVAL-ADE7880EBZ datasheet(42/104 Pages) AD

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ADE7880

Data Sheet

Rev. A | Page 42 of 104

Voltage Channel RMS Calculation

Figure 69 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 Registers AVRMS, BVRMS,

and CVRMS. 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 ±5,326,737.

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

3,766,572 (0x39792C), 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 3.3 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 V rms measurement settling time is 580 ms for both 50 Hz

and 60 Hz input signals. The V rms measurement settling time

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 stated in the Current Waveform Gain Registers section, the

serial ports of the ADE7880 work on 32-, 16-, or 8-bit words.

Similar to the register presented in Figure 57, the AVRMS,

BVRMS, and CVRMS 24-bit signed registers are accessed as

32-bit registers with the eight MSBs padded with 0s.

0xAEB86F =

–5,326,737

0x14791 =

+5,326,737

VOLTAGE SIGNAL

FROM HPF

LPF

xVRMSOS[23:0]

xVRMS[23:0]

Figure 69. Voltage RMS Signal Processing

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