ADE7759 Analog Devices, ADE7759 Datasheet - Page 22

• Manufacturer Part Number: ADE7759
• Description: Active Energy Metering IC with di/dt Sensor Interface
• Manufacturer: Analog Devices

ADE7759
As shown, the fastest integration time will occur when the Active
Power Gain register is set to maximum full scale, i.e., 7FFh.
Note that the energy register contents will roll over to full-scale
negative (80,0000,0000h) and continue increasing in value when
the power or energy flow is positive—see Figure 37. Conversely,
if the power is negative, the energy register would underflow to
full-scale positive (7F, FFFF, FFFFh) and continue decreasing
in value. By using the Interrupt Enable register, the ADE7759
can be configured to issue an interrupt (IRQ) when the Active
Energy register is half-full (positive or negative) or when an
over/underflow occurs.
Integration Time under Steady Load
As mentioned in the last section, the discrete time sample period
(T) for the accumulation register is 1.1 µs (4/CLKIN). With
full-scale sinusoidal signals on the analog inputs, digital integrator
turned off, and the Active Power Gain register set to 000h, the
average word value from LPF2 is CCCD—see Figures 34 and 35.
The maximum value that can be stored in the Active Energy
3F,FFFF,FFFFh
7F,FFFF,FFFFh
80,0000,0000h
00,0000,0000h
40,0000,0000
AENERGY [39:0]
CURRENT CHANNEL
VOLTAGE CHANNEL
5.8s
11.5s
23s
15
SIGN 2
T
TIME – sec
20
6
2
CLKIN
APGAIN = 7FFh
APGAIN = 000h
APGAIN = 800h
5
4
TIME – nT
2
LPF2
4
2
3
ACTIVE POWER
SIGNAL = P
2
2
WAVEFORM
REGISTER
VALUES
APOS [15:0]
2
+
1
register before it overflows is 2
integration time under these conditions is calculated as follows:
POWER OFFSET CALIBRATION
The ADE7759 also incorporates an Active Power Offset register
(APOS[15:0]). This is a signed two’s complement 16-bit register
that can be used to remove offsets in the active power calculation—
see Figure 36. 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.
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
to store in the Waveform register is CCCDh (52,429 in decimal)
when inputs on Channels 1 and 2 are both at full scale and the
digital integrator is turned off. At –60 dB down on Channel 1 (1/1000
of the Channel 1 full-scale input), the average word value outputs
from LPF2 is 52.429 (52,429/1,000). One 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 1/256 LSB of the Waveform register, hence the power
offset correction resolution is 0.007%/LSB (1.9%/256) at –60 dB.
When the digital integrator is turned on, the resolution of the
LSB varies slightly with the line frequency.
ENERGY-TO-FREQUENCY CONVERSION
ADE7759 also provides energy to frequency conversion for cali-
bration purposes. After initial calibration at manufacturing, 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 pro-
portional to the energy or active power under steady load conditions.
This output frequency can provide a simple, single-wire, optically
2
0
+
2
–1
Time
23
39
2
–2
2
–3
=
7
2
–4
F FFFF FFFFh
WAVEFORM [24:0]
WAVEFORM REGISTER VALUES ARE
ACCUMULATED (INTEGRATED) IN
THE ACTIVE ENERGY REGISTER
AENERGY [39:0]
,
2
–5
CCCDh
2
–6
2
,
–7
39
2
–8
or 7F,FFFF,FFFFh. Therefore the
0
×
1 1
.
0
0
µ
s
=
11 53
+
+
.
sec
onds