ADE7753
–22–REV. PrC 01/02
PRELIMINARY TECHNICAL DATA
POWER OFFSET CALIBRATIONPOWER OFFSET CALIBRATION
POWER OFFSET CALIBRATIONPOWER OFFSET CALIBRATION
POWER OFFSET CALIBRATION
The ADE7753 also incorporates an Active Power Offset
register (APOS[15:0]). This is a signed 2’s complement 16-
bit register which can be used to remove offsets in the active
power calculation—see Figure 33. An offset may exist in the
power calculation due to cross talk 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.
Two hundred fifty six LSBs (APOS=0100h) written to the
Active Power Offset register are equivalent to 1 LSB in the
Waveform Sample register. Assuming the average value
outputs from LPF2 is CCCCDh (838,861 in Decimal) when
inputs on Channels 1 and 2 are both at full-scale. At -60dB
down on Channel 1 (1/1000 of the Channel 1 full-scale
input), the average word value outputs from LPF2 is 838.861
(838,861/1,000). 1 LSB in the LPF2 output has a measure-
ment error of 1/838.861 × 100% = 0.119% of the average
value. The Active Power Offset register has a resolution
equal to 1/256 LSB of the Waveform register, hence the
power offset correction resolution is 0.00047%/LSB (0.119%/
256) at -60dB.
ENERGY TO FREQUENCY CONVERSIONENERGY TO FREQUENCY CONVERSION
ENERGY TO FREQUENCY CONVERSIONENERGY TO FREQUENCY CONVERSION
ENERGY TO FREQUENCY CONVERSION
ADE7753 also provides energy to frequency conversion for
calibration purposes. After initial calibration at manufactur-
ing, 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 which 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 37 illustrates the
Energy-to-Frequency conversion in the ADE7753.
CF
0
11
CFNUM[11:0]
Energy
0
11
CFDEN[11:0]
DFC
023
AENERGY[23:0]
Figure 37– ADE7753 Energy to Frequency Conversion
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+1)/(CFNUM+1)
number of pulses are generated at the DFC output. Under
steady load conditions the output frequency is proportional
to the Active Power.
The maximum output frequency, with AC input signals at
full-scale and CFNUM=00h & CFDEN=00h, is approxi-
mately 5.8 kHz.
The ADE7753 incorporates two registers, CFNUM[11:0]
and CFDEN[11:0], to set the CF frequency. These are
unsigned 12-bit registers which can be used to adjust the CF
frequency to a wide range of values. These frequency scaling
registers are 12-bit registers which can scale the output
frequency by 1/2
12
to 1 with a step of 1/2
12
.
If the value zero is written to any of these registers, the value
one would be applied to the register. The ratio (CFNUM+1)/
(CFDEN+1) should be smaller than one to assure proper
operation. If the ratio of the registers (CFNUM+1)/
(CFDEN+1) is greater than one, the register values would
be adjust to a ratio (CFNUM+1)/(CFDEN+1) of one.
For example if the output frequency is 1.562kHz while the
contents of CFDIV are zero (000h), then the output frequency
can be set to 6.1Hz by writing FFh to the CFDEN register.
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. Equation 3
gives an expression for the instantaneous power signal. This
is filtered by LPF2 which has a magnitude response given by
Equation 9.
2
2
9.8
1
1
)
(
f
fH
+
=
(9)
The Active Power signal (output of LPF2) can be rewritten
as.
()
tf
f
VI
VItp l
l
π
4cos
9.8
2
1
)( 2⋅
+
−=
(10)
where f
l
is the line frequency (e.g., 60Hz)
From Equation 6
l
l
π
π
4sin9.8214)( 2⋅
+−= (11)
From Equation 11 it can be seen that there is a small ripple
in the energy calculation due to a sin(2ωt) component. This
is shown graphically in Figure 38. The Active Energy
calculation is shown by the dashed straight line and is equal
to V x I x t. The sinusoidal ripple in the Active Energy
calculation is also shown. Since the average value of a
sinusoid is zero, this ripple will not contribute to the energy
calculation over time. However, the ripple can be observed
in the frequency output, especially at higher output frequen-
cies. 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 frequencies the integra-
tion 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
reduce the ripple. Also averaging the output frequency by
using a longer gate time for the counter will achieve the same
results.