ADE7953ACPZ datasheet(23/68 Pages) AD

23 / 68 page

Data Sheet

ADE7953

Rev. A | Page 23 of 68

ROOT MEAN SQUARE MEASUREMENT

Root mean square (rms) is a measurement of the magnitude

of an ac signal. Specifically, the rms of an ac signal is equal to

the amount of dc required to produce an equivalent amount

of power in the load. The rms is expressed mathematically in

Equation 1.

∫

RMS

)

(

(1)

For time-sampled signals, rms calculation involves squaring

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

∑

RMS

]

[

(2)

As implied by Equation 2, the rms measurement contains

information from the fundamental and all harmonics over

a 1.23 kHz measurement bandwidth.

The ADE7953 provide rms measurements for Current

Channel A, Current Channel B, and the voltage channel

simultaneously. These measurements have a settling time of

approximately 200 ms and are updated at a rate of 6.99 kHz.

CURRENT CHANNEL RMS CALCULATION

The ADE7953 provides rms measurements for both Current

Channel A and Current Channel B. Figure 42 shows the signal

path for this calculation. The signal processing is identical for

Current Channel A and Current Channel B.

CURRENT

SIGNAL

FROM HPF OR

INTEGRATOR

(IF ENABLED)

LPF

√

212

×IRMSOS[23:0]

IRMSx[23:0]

Figure 42. Current Channel RMS Signal Processing

As shown in Figure 42, the current channel ADC output samples

are used to continually compute the rms. The rms is achieved by

low-pass filtering the square of the output signal and then taking

a square root of the result. The 24-bit unsigned rms measurements

for Current Channel A and Current Channel B are available in

the IRMSA (Address 0x21A and Address 0x31A) and IRMSB

(Address 0x21B and Address 0x31B) registers, respectively. Both

of these registers are updated at a rate of 6.99 kHz. With full-

scale inputs on Current Channel A and Current Channel B,

the expected reading on the IRMSA and IRMSB register is

9032007d.

Because the LPF used in the rms signal path is not ideal, it is

recommended that the IRMSx registers be read synchronously

to the zero-crossing signal (see the Zero-Crossing Detection

section). This helps to stabilize reading-to-reading variation

by removing the effect of any 2ω ripple present on the rms

measurement.

VOLTAGE CHANNEL RMS CALCULATION

The ADE7953 provides an rms measurement on the voltage

channel. Figure 43 shows the signal path for this calculation.

VOLTAGE

SIGNAL

FROM HPF

LPF

√

212

VRMSOS[23:0]

VRMS[23:0]

Figure 43. Voltage Channel RMS Signal Processing

As shown in Figure 43, the voltage channel ADC output

samples are used to continually compute the rms. The rms is

achieved by low-pass filtering the square of the output signal

and then taking a square root of the result. The 24-bit unsigned

voltage channel rms measurement is available in the VRMS

updated at a rate of 6.99 kHz. With full-scale inputs on the

voltage channel, a VRMS reading of 9032007d can be expected.

Because the LPF used in the rms signal path is not ideal, it is

recommended that the VRMS register be read synchronously to

the zero-crossing signal (see the Zero-Crossing Detection section).

This helps to stabilize reading-to-reading variation by removing

the effect of any 2ω ripple present on the rms measurement.

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