ade7518 Analog Devices, Inc., ade7518 Datasheet - Page 49

• Manufacturer Part Number: ade7518
• Description: Single-phase Energy Measurement Ic With 8052 Mcu, Rtc, And Lcd Driver
• Manufacturer: Analog Devices, Inc.

Active Power Gain Calibration
Figure 52 shows the signal processing chain for the active
power calculation in the ADE7518. As explained previously,
the active power is calculated by filtering the output of the
multiplier with a low-pass filter. Note that when reading the
waveform samples from the output of LPF2, the gain of the
active energy can be adjusted by using the multiplier and watt
gain register (WGAIN[11:0]). The gain is adjusted by writing
a twos complement 12-bit word to the watt gain register.
Equation 10 shows how the gain adjustment is related to the
contents of the watt gain register.
For example, when 0x7FF is written to the watt gain register, the
power output is scaled up by 50% (0x7FF = 2047d, 2047/2
Similarly, 0x800 = −2048d (signed, twos complement) and
power output is scaled by –50%. Each LSB scales the power
output by 0.0244%. The minimum output range is given when
the watt gain register contents are equal to 0x800 and the
maximum range is given by writing 0x7FF to the watt gain
register. This watt gain register can be used to calibrate the
active power (or energy) calculation in the ADE7518.
Active Power Offset Calibration
The ADE7518 also incorporates an active power offset register
(WATTOS[15:0]). It is a signed, twos complement, 16-bit register
that can be used to remove offsets in the active power calculation
(see Figure 49). An offset can exist in the power calculation due
to crosstalk between channels on the PCB or in the IC itself. The
offset calibration allows the contents of the active power register
to be maintained at 0 when no power is being consumed.
The 256 LSBs (WATTOS = 0x0100) written to the active power
offset register are equivalent to 1 LSB in the waveform sample
register. Assuming the average value, output from LPF2 is
0xCCCCD (838,861d) when inputs on the voltage and current
channels are both at full scale. At −60 dB below full scale on the
current channel (1/1000 of the current channel full-scale input),
the average word value output from LPF2 is 838.861
Output
–12
–16
–20
–24
–4
–8
0
1
WGAIN
Figure 51. Frequency Response of LPF2
3
=
⎛
⎜ ⎜
⎝
Active
FREQUENCY (Hz)
Power
10
×
⎧
⎨
⎩
1
+
30
WGAIN
2
12
⎫
⎬
⎭
⎞
⎟ ⎟
⎠
12
100
= 0.5).
Rev. 0 | Page 49 of 128
(10)
(838,861/1000). One LSB in the LPF2 output has a measurement
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. Therefore, the power offset correction
resolution is 0.000464%/LSB (0.119%/256) at −60 dB.
Active Power Sign Detection
The ADE7518 detects a change of sign in the active power. The
APSIGN flag in the Interrupt Status 1 SFR (MIRQSTL, 0xDC)
records when a change of sign has occurred according to Bit
APSIGN in the ACCMODE register (0x0F). If the APSIGN flag
is set in the Interrupt Enable 1 SFR (MIRQENL, 0xD9), the
8052 core has a pending ADE interrupt. The ADE interrupt
stays active until the APSIGN status bit is cleared (see the
Energy Measurement Interrupts section).
When APSIGN in the ACCMODE register (0x0F) is cleared
(default), the APSIGN flag in the Interrupt Status 1 SFR
(MIRQSTL, 0xDC) is set when a transition from positive to
negative active power has occurred.
If APSIGN in the ACCMODE register (0x0F) is set, the
APSIGN flag in the MIRQSTL SFR is set when a transition
from negative to positive active power occurs.
Active Power No Load Detection
The ADE7518 includes a no load threshold feature on the active
energy that eliminates any creep effects in the meter. The part
accomplishes this by not accumulating energy if the multiplier
output is below the no load threshold. When the active power is
below the no load threshold, the APNOLOAD flag in the Interrupt
Status 1 SFR (MIRQSTL, 0xDC) is set. If the APNOLOAD bit is
set in the Interrupt Enable 1 SFR (MIRQENL, 0xD9), the 8052 core
has a pending ADE interrupt. The ADE interrupt stays active
until the APNOLOAD status bit is cleared (see the Energy
Measurement Interrupts section).
The no load threshold level is selectable by setting the
APNOLOAD bits in the NLMODE register (0x0E). Setting
these bits to 0b00 disables the no load detection, and setting
them to 0b01, 0b10, or 0b11 sets the no load detection threshold
to 0.015%, 0.0075%, or 0.0037%, respectively, of the multiplier’s
full-scale output frequency. The IEC 62053-21 specification
states that the meter must start up with a load equal to or less
than 0.4% I
output frequency of the multiplier.
P
, which translates to 0.0167% of the full-scale
ADE7518