ade7169f16 Analog Devices, Inc., ade7169f16 Datasheet - Page 57

• Manufacturer Part Number: ade7169f16
• Description: Single-phase Energy Measurement Ic With 8052 Mcu, Rtc And Lcd Driver
• Manufacturer: Analog Devices, Inc.

Preliminary Technical Data
The average reactive power over an integral number of lines (n)
is given in Equation 26.
where:
T is the line cycle period.
q is referred to as the reactive power.
Note that the reactive power is equal to the dc component of the
instantaneous reactive power signal q(t) in Equation 25. This is
the relationship used to calculate reactive power in the
ADE7169F16. The instantaneous reactive power signal q(t) is
generated by multiplying Voltage and Current channels. In this
case, the phase of Current channel is shifted by +90°. The dc
component of the instantaneous reactive power signal is then
extracted by a low-pass filter in order to obtain the reactive
power information – see Figure 45.
In addition, the phase shifting filter has a non-unity magnitude
response. Because the phase-shift filter has a large attenuation at
high frequency, the reactive power is primarily for the
calculation at line frequency. The effect of harmonics is largely
ignored in the reactive power calculation. Note that because of
the magnitude characteristic of the phase shifting filter, the
weight of the reactive power is slightly different from the active
power calculation – see Energy register scaling.
The frequency response of the LPF in the reactive signal path is
identical to that of the LPF2 used in the average active power
calculation. Since LPF2 does not have an ideal “brick wall”
frequency response—see Figure 38, the reactive power signal
has some ripple due to the instantaneous reactive power signal.
This ripple is sinusoidal and has a frequency equal to twice the
line frequency. Because the ripple is sinusoidal in nature, it is
removed when the reactive power signal is integrated to
calculate energy—see the Reactive Power Calculation section.
The reactive power signal can be read from the waveform
register by setting the WAVMODE register (0x0D) and setting
the WFSM bit in the Interrupt Enable Register 3 SFR
(MIRQENH, 0xDB). Like the current and voltage channels
waveform sampling modes, the waveform date is available at
sample rates of 27.9 kSPS, 14 kSPS, 7 kSPS, or
3.5 kSPS.
Reactive power gain calibration
Figure 45 shows the signal processing chain for the reactive
power calculation in the ADE7169F16. As explained, the
reactive power is calculated by low-pass filtering the
instantaneous reactive power signal. Note that when reading the
waveform samples from the output of LPF2, the gain of the
q(t) = VI sin (  ) + VI sin
Q
nT
1
nT
0
q
) (
t
dt
VI
sin(
2 ( t
)
)
Rev. PrD | Page 57 of 140
(26)
reactive energy can be adjusted by using the multiplier and var
gain register (VARGAIN[11:0]). The gain is adjusted by writing
a twos complement 12-bit word to the var gain register.
Equation 11 shows how the gain adjustment is related to the
contents of the watt gain register:
The resolution of the VARGAIN register is the same as the
WGAIN register – see Active power gain calibration section.
VARGAIN can be used to calibrate the reactive power (or
energy) calculation in the ADE7169F16.
Reactive power offset calibration
The ADE7169F16 also incorporates a reactive power offset
register (VAROS[15:0]). This is a signed twos complement 16-
bit register that can be used to remove offsets in the reactive
power calculation—see Figure 45. An offset could exist in the
reactive power calculation due to crosstalk between channels on
the PCB or in the IC itself. The offset calibration allows the
contents of the reactive power register to be maintained at 0
when no power is being consumed.
The 256 LSBs (VAROS = 0x100) written to the reactive power
offset register are equivalent to 1 LSB in the waveform sample
register.
Sign of Reactive Power Calculation
Note that the average reactive power is a signed calculation. The
phase shift filter has –90° phase shift when the integrator is
enabled, and +90° phase shift when the integrator is disabled.
Table 41 summarizes the relationship between the phase differ-
ence between the voltage and the current and the sign of the
resulting VAR calculation.
Table 41. Sign of Reactive Power Calculation
Angle
Between 0° to 90°
Between –90° to 0°
Between 0° to 90°
Between –90° to 0°
Reactive power sign detection
The ADE7169F16 detects a change of sign in the reactive power.
The VARSIGN flag in the Interrupt Status Register 1 SFR
(MIRQSTL, 0xDC) record when a change of sign according to
bit VARSIGN in the ACCMODE register (0x0F) has occurred.
If the VARSIGN bit is set in the Interrupt Enable Register 1 SFR
(MIRQENL, 0xD9), the 8052 core has a pending ADE
interrupt. The ADE interrupt stays active until the VARSIGN
status bit is cleared—see Energy measurement interrupts
section.
When VARSIGN in the ACCMODE register (0x0F) is cleared
Output
VARGAIN
Integrator
Off
Off
On
On
Re
active
Power
1
Sign
Positive
Negative
Positive
Negative
ADE7169F16
VARGAIN
2
12
(11)