ADE7880 datasheet(39/104 Pages) AD

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Data Sheet

ADE7880

Rev. A | Page 39 of 104

•

If the ADE7880 registers located in the data memory RAM

have not been modified, write 0x0001 into the Run register to

start the DSP.

•

If the ADE7880 registers located in the data memory RAM

have to be modified, first execute a software or a hardware

reset, initialize all ADE7880 registers at desired values,

enable the write protection and then write 0x0001 into the

Run register to start the DSP.

As mentioned in the Power Management section, when the

ADE7880 switch out of PSM0 power mode, it is recommended to

stop the DSP by writing 0x0000 into the Run register (see Table 10

and Table 11 for the recommended actions when changing

power modes).

ROOT MEAN SQUARE MEASUREMENT

Root mean square (rms) is a 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

power in the load. Mathematically, the rms value of a conti-

nuous signal f(t) is defined as

()dt

rms

∫

(10)

For time sampling signals, rms calculation involves squaring the

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

[]

∑

rms

(11)

Equation 10 implies that for signals containing harmonics, the

rms calculation contains the contribution of all harmonics, not

only the fundamental. The ADE7880 uses two different methods

to calculate rms values. The first one is very accurate and is active

only in PSM0 mode. The second one is less accurate, uses the

estimation of the mean absolute value (mav) measurement, is

active in PSM0 and PSM1 modes.

The ADE7880 also computes the rms values of various

fundamental and harmonic components of phase currents,

phase voltages and neutral current as part of the harmonic

calculations block. Refer to Harmonics Calculations section for

details.

The first method is to low-pass filter the square of the input

signal (LPF) and take the square root of the result (see Figure 65).

(

∑

∞

sin

)

(

)

(12)

then,

()

(

∑

∞

≠

∞

−

sin

)

cos(

)

(

(13)

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

value of f(t) is obtained by

∑

∞

(14)

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 second method computes the absolute value of the input

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

the absolute mean value of the input. If the input signal in

Equation 12 has a fundamental component only, 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 for

the fundamental components only. If harmonics are present in the

current channel, the mean absolute value is no longer

proportional to rms.

Current RMS Calculation

This section presents the first approach to compute the rms

values of all phase and neutral currents. The ADE7880 also

computes the rms of the sum of the instantaneous values of the

phase currents if Bit 2 (INSEL) in the CONFIG3 register is set

to 1. Note that the instantaneous value of the sum is stored into

ISUM register presented in the Neutral Current Mismatch

section. In 3-phase four wired systems that only require sensing

the phase currents, this value provides a measure of the neutral

current.

Figure 65 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, BIRMS, CIRMS,

NIRMS registers. The update rate of the current rms measurement

is 8 kHz. If Bit 2 (INSEL) of the CONFIG3 register is 0 (default),

the NIRMS register contains the rms value of the neutral

current. If the INSEL bit is 1, the NIRMS register contains the

rms value of the sum of the instantaneous values of the phase

currents.

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. If

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