ADE7756AN Analog Devices Inc, ADE7756AN Datasheet - Page 23

• Manufacturer Part Number: ADE7756AN
• Description: IC ENERGY METERING 20-DIP
• Manufacturer: Analog Devices Inc

Specifications of ADE7756AN

• Rohs Status: RoHS non-compliant
• Input Impedance: 390 KOhm
• Measurement Error: 0.1%
• Voltage - I/o High: 2.4V
• Voltage - I/o Low: 0.8V
• Current - Supply: 3mA
• Voltage - Supply: 4.75 V ~ 5.25 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Through Hole
• Package / Case: 20-DIP (0.300", 7.62mm)
• Meter Type: Single Phase
• Lead Free Status / RoHS Status: Not Compliant

POWER OFFSET CALIBRATION
The ADE7756 also incorporates an Active Power Offset regis-
ter (APOS[11:0]). This is a signed, two’s complement, 12-bit
register that can be used to remove offsets in the active power
calculation—see Figure 30. An offset may exist in the power
calculation due to crosstalk between channels on the PCB or
in the IC itself. The offset calibration will allow the contents
of the Active Power register to be maintained at zero when no
power is being consumed.
Sixteen LSBs (APOS = 010h) written to the Active Power Off-
set register are equivalent to 1 LSB in the Waveform Sample
register. Assuming the average value outputs from LPF2 to store
in the Waveform Register is CCCDh (52,429 in Decimal) when
inputs on Channels 1 and 2 are both at full scale. At –60 dB
down on Channel 1 (1/1000 of the full-scale input), the average
word value outputs from LPF2 is 52.429 (52,429/1,000). 1 LSB
in the Waveform register has a measurement error of 1/52.429 ×
100% = 1.9% of the average value. The Active Power Offset
register has a resolution equal to 1/16 LSB of the Waveform
register, hence the power offset correction resolution is 0.12%
(1.9%/16) at –60 dB.
ENERGY-TO-FREQUENCY CONVERSION
ADE7756 also provides energy-to-frequency conversion for
calibration purposes. After initial calibration at manufacture, the
manufacturer or end customer will often verify 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 equip-
ment. Figure 32 illustrates the Energy-to-Frequency conversion
in the ADE7756.
The energy-to-frequency conversion is accomplished by accumu-
lating the Active power signal in a 24-bit register. An output
pulse is generated when there is a zero to one transition on the
MSB (most significant bit) of the register. Under steady load con-
ditions the output frequency is proportional to the Active Power.
The output frequency at CF, with full-scale ac signals on Chan-
nel 1 and Channel 2 and CFDIV = 000h and APGAIN = 000h,
is approximately 5.593 kHz. This can be calculated as follows:
with the Active Power Gain register set to 000h, the average
value of the instantaneous power signal (output of LPF2) is
CCCDh or 52,429 decimal. An output frequency is generated
11
20
ACTIVE POWER
SIGNAL – P
LPF2
CFDIV[11:0]
11
23
23
ENERGY-TO-FREQUENCY
MSB TRANSITION
WAVEFORM [23:0]
APOS [11:0]
0
0
0
0
ACTIVE POWER
CALIBRATION
OFFSET
CF
on CF when the MSB in the Digital-to-Frequency register (24
bits) toggles, i.e., when the register accumulates 2
the register is updated 2
the update rate is 4/CLKIN or 1.1175 µs, the time between
MSB toggles (CF pulses) is given as:
Equation 8 gives an expression for the output frequency at CF
with the CFDIV register = 0.
This output frequency is easily scaled by the Calibration Fre-
quency Division register (CFDIV[11:0]). This frequency scaling
register is a 12-bit register that scales the output frequency by 1
to 2
For example, if the output frequency is 5.59286 kHz while the
content of CFDIV is zero (000h), the output frequency can be
set to 5.4618 Hz by writing 3FFh Hex (1023 Decimal) to the
CFDIV register. The power-up default value in CFDIV is 3Fh.
The output frequency will have a slight ripple at a frequency equal
to twice the line frequency. This is due to imperfect filtering of
the instantaneous power signal to generate the Active Power
signal—see Active Power Calculation section. Equation 3 gives
an expression for the instantaneous power signal. This is
filtered by LPF2, which has a magnitude response given by
Equation 10.
The Active Power signal (output of LPF2) can be rewritten as
where fl is the line frequency (e.g., 60 Hz)
From Equation 6
From Equation 12 it can be seen that there is a small ripple in
the energy calculation due to a sin(2 ωt) component. This is
shown graphically in Figure 33. The Active Energy calculation
is shown by the dashed straight line and is equal to V × I × t.
The sinusoidal ripple in the Active Energy calculation is also
shown. Since the average value of a sinusoid is zero, this ripple
will contribute nothing to the energy calculation over time. How-
ever, the ripple can be observed in the frequency output, especially
at higher output frequencies. The ripple will get larger as a
percentage of the frequency at larger loads and higher output
frequencies. The reason is simply that at higher output frequen-
cies the integration or averaging time in the energy-to-frequency
conversion process is shorter. As a consequence, some of the
sinusoidal ripple is observable in the frequency output. Choosing
a lower output frequency at CF for calibration can significantly
E t
| ( )|
CF Hz
Frequency
( )
H f
p t
12
( )
159.999 × 1.1175 µs = 1.78799 × 10
. The output frequency is given in Equation 9.
(
=
VIt
=
VI
=
) =
–
1
–



=
+
Average LPF Output CLKIN



4
f
1 2
Frequency CFDIV
/ .
×
+
8 9
1
π
×
fl
Hz
CFDIV
VI
/ .
fl
23
8 9
(
/CCCDh times (or 159.999 times). Since
1 2
VI
+
(
Hz
2
2
fl
25
+
/ .



1
8 9
cos (
=
Hz
×
4 π
) 0
)
× ×



–4
sin (
s = 5592.86 Hz.
fl t
ADE7756
4
×
×
23
)
π
. This means
×
fl
×
t
)
(10)
(11)
(12)
(8)
(9)