ADE9178 datasheet(40/122 Pages) AD

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Data Sheet

ADE9178

THEORY OF OPERATION

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Rev. A | 40 of 122

CF Pulse Generation

Many electricity meters are required to provide a pulse output that

is proportional to the energy being accumulated, with a given pulse

per kWh meter constant. The ADE9178 includes two pulse outputs

that are proportional to the energy accumulation.

A block diagram of the CFx pulse generation and apparent da-

tapath is shown in Figure 56 and Figure 57, respectively. The

ACCMODE[8:7] bits in the CFx_CONFIG register decides the CF

accumulation mode (signed, positive, or negative accumulation for

active power and signed or positive accumulation only for apparent

power). The PHASESEL[6:4] bits in the CFx_CONFIG register

determines which phases to include for CF accumulation. The

PWRSEL[3:1] bits in the CFx_CONFIG register decides whether

active power or apparent power is used for CF accumulation. If the

application wants to clear CF accumulator, it can be done using the

ACC_CLEAR[10] bit in the CFx_CONFIG register. The CF pulses

are generated when accumulated value from previous CF pulse

exceeds CFx_THR × 28.

The low-pulse width of CF pulse can be controlled by CF_LTMR

(in μs), for example, CF_LTMR = 50000 decimal sets the low-pulse

width to 50 ms register as per Table 20. If WIDTHSEL is set to 1,

the maximum active low-pulse width is 80 ms (CF_LTMR = 80000

decimal = 0x13880). Note that the CF_LTMR must be set ≥ 0x1 and

≤ 0x13880.

Note that for the best CF performance, CF frequency must be read

from the falling edge of the signal.

Table 20. WIDTHSEL Configuration

WIDTHSEL

Active Low-Pulse

Width for Low

Frequencies

(<6.25 Hz)

Active Low-Pulse

Width for High

Frequencies

(>6.25 Hz)

Behavior when

Entering No

Load

80 ms

50% of period

If CFx is low,

finish current

pulse, return high

CF_LTMR (in μs)

50% of period

CFx_THR register can be used to control the frequency of the puls-

es. The ADE9178 issues a pulse whenever accumulated energy

from previous CF pulse crosses (CFx_THR × 256). To generate

M impulses per kWHr, use the following formula for setting the

threshold:

CFx_THR=1000×60×60×POWFS_CODES×4000

M×IFS×VFS_T×256

(24)

where, M is the required impulses per kWHr at full-scale inputs, IFS

is full-scale current in amperes RMS, and VFS_T is full-scale voltage

in volts RMS. Note that the fully-differential VFS_T is used for this

calculation. See the Worked Examples section on how to calculate

VFS_T for the system. POWFS_CODES is the full-scale power code

and can be obtained from full-scale code documentation.

The expected frequency of CF pulse at full-scale input can be

calculated as:

CFHz =POWFS_CODES×4000

CFx_THR×256

(25)

Note that the maximum recommended CF frequency of ADE9178 is

500 Hz. So minimum value of CFX_THR should be more than:

CFx_THR=POWFS_CODES×4000

500×256

=85829040×4000

500×256

=2682157

decimal (26)

for single-phase accumulation and more than assuming FS inputs.

CFx_THR=POWFS_CODES×4000

500×256

×3

=85829040×4000

500×256

×3=

8046472 decimal (27)

for 3-phase accumulation assuming FS inputs.

CFx_THR is an unsigned value. For signed, positive, and negative

accumulation, the absolute value of the internal accumulation is

compared to the CFx_THR.

The SUMxSIGN bits in the PHSIGN register indicate whether the

sum of the energy that goes into the last CFx pulse is positive or

negative. Bits [5:4] (REVPSUM2 and REVPSUM1) in the STATUS0

the last CFx pulse is positive active energy and the next CFx pulse

is negative active energy, then REVPSUMx bit is set. This bit is

updated when the CFx pin goes from high to low.

Example:

System Parameters:

►

VFS_T = 707 VRMS

►

IFS = 44.188 ARMS

►

M = 1000 impls/kWHr

CFx_THR=1000×60×60×85829040×4000

1000×44.188×707×256

154537194 decimal

(28)

CFHz =85829040×4000

154537194×256

×3=

26.034 Hz (29)

with all 3-phases active.

Therefore, if VNOMINAL = 230 VRMS and INOMINAL= 10 ARMS (3-

phase), expected CF pulse output is 1.9167 Hz.

For more details, refer to the CF Pulse Setup Tab in the Calibration

and Conversion excel for a CFx_THR calculator, available on the

product webpage.

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