EVAL-ADE7858EBZ datasheet(36/76 Pages) AD

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ADE7858

Preliminary Technical Data

Rev. PrA| Page 36 of 76

24 bit unsigned number

0000 0000

WTHR[47:0]

24 bit signed number

0000 0000

WTHR0[31:0]

WTHR1[31:0]

Figure 43. WTHR[47:0] is communicated as two 32-bit registers

This discrete time accumulation or summation is equivalent to

integration in continuous time following the description in

expression (24).





























Lim

Energy

(24)

where:

is the discrete time sample number.

is the sample period.

In the ADE7858, the total phase active powers are accumulated

in AWATTHR[31:0], BWATTHR[31:0] and CWATTHR[31:0]

32-bit signed registers. The active energy register content can

roll over to full-scale negative (0x80000000) and continue

increasing in value when the active power is positive.

Conversely, if the active power is negative, the energy register

would underflow to full-scale positive (0x7FFFFFFF) and

continue decreasing in value.

Bit 0 (AEHF) in STATUS0[31:0] register is set when bit 30 of

one of xWATTHR, x=A, B, C registers changes, signifying one

of these registers is half full. If the active power is positive, the

watt-hr register becomes half full when it increments from

0x3FFF FFFF to 0x4000 0000. If the active power is negative,

the watt-hr register becomes half full when it decrements from

0xC000 0000 to 0xBFFF FFFF.

Setting bit 0 in MASK0[31:0] register enables the AEHF

interrupt. If enabled, the

IRQ pin is set low and the status bit

is set to 1 whenever one of the energy registers xWATTHR (for

AEHF interrupt), x=A,B,C becomes half full. The status bit is

cleared and

IRQ pin is set to logic high by writing STATUS0

Setting bit 6 (RSTREAD) of LCYMODE[7:0] register enables a

read-with-reset for all watt-hr accumulation registers, that is,

the registers are reset to 0 after a read operation.

Integration Time Under Steady Load

The discrete time sample period (T) for the accumulation register

is 125μs (1/8KHz). With full-scale sinusoidal signals on the

analog inputs and the watt gain registers set to 0x00000, the average

word value from each LPF2 is PMAX=33,516,139=0x1FF6A6B.

If the WTHR[47:0] threshold is set at PMAX level, this means

the DSP generates a pulse that is added at watt-hr registers

every 125 μs.

The maximum value that can be stored in the watt-hr

accumulation register before it overflows is 231 − 1 or

0x7FFFFFFF. The integration time is calculated as

min

125

FFFF

FFF

Time









(25)

Energy Accumulation Modes

The active power accumulated in each watt-hr accumulation

32-bit register (AWATTHR, BWATTHR, CWATTHR) depends

on the configuration of bits 5, 4 (CONSEL) in ACCMODE[7:0]

Table 12. Inputs to Watt-Hr Accumulation Registers

CONSEL

AWATTHR

BWATTHR

CWATTHR

VA × IA

VB × IB

VC × IC

VA × IA

VC × IC

VA × IA

VB × IB

VB = −VA − VC

VC × IC

VA × IA

VB × IB

VB = −VA

VC × IC

Depending on the poly-phase meter service, the appropriate

formula should be chosen to calculate the active energy. The

American ANSI C12.10 Standard defines the different

configurations of the meter. Table 13 describes which mode

should be chosen in these different configurations.

Table 13. Meter Form Configuration

ANSI Meter Form

CONSEL

5S/13S

3-Wire Delta

6S/14S

4-Wire Wye

8S/15S

4-Wire Delta

9S/16S

4-Wire Wye

Bits 1, 0 (WATTACC[1:0]) in ACCMODE[7:0] register

determine how CF frequency output may be generated function

of the total and fundamental active powers. While the watt-hr

accumulation registers accumulate the active power in a signed

format, the frequency output may be generated in signed mode

or in absolute mode, function of WATTACC[1:0]. See Energy

to Frequency Conversion chapter for details.

Line Cycle Active Energy Accumulation Mode

In line cycle energy accumulation mode, the energy accumula-

tion is synchronized to the voltage channel zero crossings so

that active energy is accumulated over an integral number of

half line cycles. The advantage of summing the active energy

over an integer number of line cycles is that the sinusoidal

component in the active energy is reduced to 0. This eliminates

any ripple in the energy calculation and allows the energy to be

accumulated accurately over a shorter time. By using the line

cycle energy accumulation mode, the energy calibration can be

greatly simplified, and the time required to calibrate the meter

can be significantly reduced. In line cycle energy accumulation

mode, the ADE7858 transfers the active energy accumulated in

the 32-bit internal accumulation registers into

xWATHHR[31:0], x=A,B,C registers after an integral number of

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