ADE7754AR Analog Devices Inc, ADE7754AR Datasheet - Page 21

• Manufacturer Part Number: ADE7754AR
• Description: IC ENERY METER 3PHASE 24-SOIC
• Manufacturer: Analog Devices Inc

Specifications of ADE7754AR

• Input Impedance: 370 KOhm
• Measurement Error: 0.1%
• Voltage - I/o High: 2.4V
• Voltage - I/o Low: 0.8V
• Current - Supply: 7mA
• Voltage - Supply: 4.75 V ~ 5.25 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 24-SOIC (0.300", 7.50mm Width)
• Meter Type: 3 Phase
• For Use With: EVAL-ADE7754EBZ - BOARD EVALAUTION FOR ADE7754
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Other names: AD71049AR; AD71049AR

A digital to frequency converter (DFC) is used to generate the
CF pulsed output. The DFC generates a pulse each time one
LSB in the active energy register is accumulated. An output
pulse is generated when CFDEN/CFNUM pulses are generated
at the DFC output. Under steady load conditions, the output
frequency is proportional to the active power. The maximum
output frequency (CFNUM = 00h and CFDEN = 00h) with
full scale ac signals on the three phases (i.e., current channel
and voltage channel is approximately 96 kHz).
The ADE7754 incorporates two registers to set the frequency of
CF (CFNUM[11:0] and CFDEN[11:0]). These are unsigned
12-bit registers that can be used to adjust the frequency of CF
to a wide range of values. These frequency scaling registers are
12-bit registers that can scale the output frequency by 1/2
with a step of 1/2
If the value 0 is written to any of these registers, the value 1
would be applied to the register. The ratio CFNUM/CFDEN
should be smaller than 1 to ensure proper operation. If the ratio
of the registers CFNUM/CFDEN is greater than 1, the CF
frequency can no longer be guaranteed to be a consistent value.
For example, if the output frequency is 18.744 kHz and the
contents of CFDEN are zero (000h), then the output frequency
can be set to 6.103 Hz by writing BFFh to the CFDEN register.
The output frequency will have a slight ripple at a frequency
equal to twice the line frequency because of imperfect filtering
of the instantaneous power signal used to generate the active
power signal. See the Active Power Calculation section. Equa-
tion 5 gives an expression for the instantaneous power signal.
This is filtered by LPF2, which has a magnitude response given
by Equation 11.
REV. 0
ACTIVE POWER
PHASE B
ACTIVE POWER
PHASE A
ACTIVE POWER
PHASE C
Figure 29. ADE7754 Energy to Frequency Conversion
| ( )|
H f
=
+
1
12
+
1
.
+
8
f
2
2
53
TOTAL ACTIVE
POWER
0
DFC
11
11
CFDEN[11:0]
CFNUM[11:0]
0
0
12
to 1
(11)
CF
–21–
The active power signal (output of the LPF2) can be rewritten as
where f
From Equation 8
Equation 13 shows that there is a small ripple in the energy
calculation due to a sin(2 t) component. This is graphically
displayed in Figure 30. The ripple becomes larger as a percentage
of the frequency at larger loads and higher output frequencies.
Choosing a lower output frequency at CF for calibration can
significantly reduce the ripple. Also, averaging the output fre-
quency by using a longer gate time for the counter achieves the
same results.
No Load Threshold
The ADE7754 includes a selectable “no load threshold” or
“startup current” feature that eliminates any creep effects in the
active energy measurement of the meter. When enabled, this
function is independently applied on each phase’s active power
calculation. This mode is selected by default and can be disabled
p t
E t
E(t)
( )
( )
l
is the line frequency (e.g., 60 Hz).
=
=
VI
VIt
Figure 30. Output Frequency Ripple
−
−


 







 





4
1
π
+
VI
f



l
2
8
1
f
VI
l
+
t



2





 





2
VIt
8
×
f
l
cos
–



2
4


 





(
4
f
×
I
π
1
sin
VI
+
f t
l
( (
2
(
8
4
)
f
ADE7754
I
π
2
f t
l
sin
)
(
4
f t
I
)
(12)
(13)