ADE7758 datasheet(33/68 Pages) AD

33 / 68 page

ADE7758

Rev. A | Page 33 of 68

Integration Time Under Steady Load

The discrete time sample period (T) for the accumulation

on the analog inputs and the watt gain registers set to 0x000, the

average word value from each LPF2 is 0xCCCCD (see Figure 64

and Figure 66). The maximum value which can be stored in the

watt-hr accumulation register before it overflows is 215 − 1 or

0x7FFF. Because the average word value is added to the internal

overflows, the integration time under these conditions with

WDIV = 0 is calculated as

second

0.524

μs

0.4

0xCCCCD

FFFF

FFFF,

0xFF,

Time

When WDIV is set to a value different from 0, the time before

overflow is scaled accordingly as shown in Equation 10.

(

)

[ 0

WDIV

Time

]

(10)

Energy Accumulation Mode

The active power accumulated in each watt-hr accumulation

the configuration of the CONSEL bits in the COMPMODE

described in Table 6.

Table 6. Inputs to Watt-Hr Accumulation Registers

CONSEL[1, 0]

AWATTHR

BWATTHR

CWATTHR

VA × IA

VB × IB

VC × IC

VA × (IA – IB)

VC × (IC – IB)

VA × (IA – IB)

VC × IC

Reserved

Note that the contents of the watt-hr accumulation registers are

affected by both the current gain register (IGAIN) and the watt

gain register of the corresponding phase. IGAIN should not be

used when using Mode 0 of CONSEL, COMPMODE[0:1].

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 7 describes which mode should be chosen in these

different configurations.

Table 7. Meter Form Configuration

ANSI Meter Form

CONSEL (d)

TERMSEL (d)

5S/13S

3-Wire Delta

3, 5, or 6

6S/14S

4-Wire Wye

8S/15S

4-Wire Delta

9S/16S

4-Wire Wye

Different gain calibration parameters are offered in the

ADE7758 to cover the calibration of the meter in different

configurations. It should be noted that in CONSEL Mode 0d

the IGAIN and WGAIN registers have the same effect on the

end result. However, changing IGAIN also changes all other

calculations that use the current waveform. In other words,

changing IGAIN changes the active, reactive, and apparent

energy, as well as the RMS current calculation results.

Active Power Frequency Output

Pin 1 (APCF) of the ADE7758 provides frequency output

for the total active power. After initial calibration during

manufacturing, the manufacturer or end customer will often

verifies the energy meter calibration. One convenient way to

verify the meter calibration is for the manufacturer to provide

an output frequency that is proportional to the energy or active

power under steady load conditions. This output frequency can

provide a simple, single-wire, optically isolated interface to

external calibration equipment. Figure 68 illustrates the energy-

to-frequency conversion in the ADE7758.

INPUT TO BWATTHR

INPUT TO AWATTHR

INPUT TO CWATTHR

DFC

APCF

APCFNUM[11:0]

APCFDEN[11:0]

÷4

Figure 68. ADE7758 Active Power Frequency Output

A digital-to-frequency converter (DFC) is used to generate the

APCF pulse output from the total active power. TERMSEL bits

(Bit 2 to Bit 4) of the COMPMODE register can be used to select

which phases to include in the total power calculation. Setting

Bit 2, Bit 3, and Bit 4 includes the input to the AWATTHR,

BWATTHR, and CWATTHR registers in the total active power

calculation. The total active power is signed addition. However,

setting the ABS bit (Bit 5) in the COMPMODE register enables

the absolute only mode, that is, only the absolute value of the

active power is considered.

The output from the DFC is divided down by a pair of frequency

division registers before sending to the APCF pulse output.

Namely, APCFDEN/APCFNUM pulses are needed at the DFC

output before the APCF pin outputs a pulse. Under steady load

conditions, the output frequency is directly proportional to the

total active power. The pulse width of APCF is 64 × CLKIN if

APCFNUM and APCFDEN are both equal. If APCFDEN is

greater than APCFNUM, the pulse width depends on

APCFDEN. The pulse width in this case is T × (APCFDEN/2),

where T is the period of the APCF pulse and APCFDEN/2 is

rounded to the nearest whole number. An exception to this is

when the period is greater than 180 ms. In this case, the pulse

width is fixed at 90 ms.

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