ADE7878 datasheet(37/92 Pages) AD

37 / 92 page

ADE7878

Rev. 0 | Page 37 of 92

REFERENCE CIRCUIT

The nominal reference voltage at the REFIN/OUT pin is 1.2 ±

0.075% V. This is the reference voltage used for the ADCs in the

ADE7878. The REFIN/OUT pin can be overdriven by an external

source, for example, an external 1.2 V reference. The voltage of

the ADE7878 reference drifts slightly with temperature; see the

Specifications section for the temperature coefficient specification

(in ppm/°C). The value of the temperature drift varies from part

to part. Because the reference is used for all ADCs, any x% drift

in the reference results in a 2x% deviation of the meter accuracy.

The reference drift resulting from temperature changes is

usually very small and typically much smaller than the drift

of other components on a meter. Alternatively, the meter can

be calibrated at multiple temperatures.

If Bit 0 (EXTREFEN) in the CONFIG2[7:0] register is cleared to 0

(the default value), the ADE7878 uses the internal voltage refer-

ence. If the bit is set to 1, then the external voltage reference is used.

Set the CONFIG2 register during the PSM0 mode. Its value is

maintained during the PSM1, PSM2, and PSM3 power modes.

DIGITAL SIGNAL PROCESSOR

The ADE7878 contains a fixed function digital signal processor

(DSP) that computes all powers and rms values. It contains

various memories: program memory ROM, program memory

RAM, and data memory RAM.

The program used for the power and rms computations is

stored in the program memory ROM, and the processor executes

it every 8 kHz. The end of the computations is signaled by

setting Bit 17 (DREADY) to 1 in the STATUS0[31:0] register.

An interrupt attached to this flag can be enabled by setting

Bit 17 (DREADY) in the MASK0[31:0] register. If enabled, the

IRQ0 pin is set low and Status Bit DREADY is set to 1 at the end

of the computations. The status bit is cleared and the IRQ0 pin

is set to high by writing to the STATUS0[31:0] register with

Bit 17 (DREADY) set to 1.

The registers used by the DSP are located in the data memory

RAM at addresses between 0x4000 and 0x43FF. The width of

this memory is 28 bits.

As seen in the Power-Up Procedure section, at power-up or

after a hardware or software reset, the DSP is in idle mode. No

instruction is executed. All the registers located in the data

memory RAM are initialized at 0, their default values. The

cleared to 0x0000. The Run[15:0] register must be written with

0x0001 for the DSP to start code execution. It is recommended

to first initialize all ADE7878 registers located in the data

memory RAM with their desired values and then write the

Run[15:0] register with 0x0001. In this way, the DSP starts the

computations from a desired configuration.

There is no obvious reason to stop the DSP if the ADE7878 is

maintained in PSM0 normal mode. All ADE7878 registers,

including ones located in the data memory RAM, can be

modified without stopping the DSP. However, to stop the DSP,

0x0000 must be written into Register Run[15:0]. To start the

DSP again, one of the following procedures must be followed:

•

If the ADE7878 registers located in the data memory RAM

have not been modified, write 0x0001 into Register Run[15:0]

to start the DSP.

•

If the ADE7878 registers located in the data memory RAM

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

reset, initialize all ADE7878 registers at desired values, and

then write 0x0001 into Register Run[15:0] to start the DSP.

As mentioned in the Power Management section, when the

ADE7878 switches out of PSM0 power mode, it is recommended

to stop the DSP by writing 0x0000 into the Run[15:0] 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 continuous

signal f(t) is defined as

()dt

Frms

∫

(9)

For time sampling signals, rms calculation involves squaring the

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

[]

∑

Frms

(10)

Equation 10 implies that for signals containing harmonics, the

rms calculation contains the contribution of all harmonics, not

only the fundamental. The ADE7878 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, and is active in PSM0 and PSM1 modes. 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 53).

(

∑

∞

sin

)

(

)

(11)

Then

()

∑

∞

≠

∞

−

sin

)

cos(

)

(

(12)

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