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REV. PrG 01/03
PRELIMINARY TECHNICAL DATA
ADE7754
–
13
–
VOLTAGE CHANNEL ADC
Figure 10 shows the ADC and signal processing chain for the
Input VA in voltage channel (same for VB and VC).
VAP
VN
ADC
1
x1, x2, x4
GAIN[6:5]
VA
VA
0V
0.5V
GAIN
Analog
Input Range
TO ACTIVE &
REACTIVE ENERGY
CALCULATION
-100% to +100% FS
LPF1
16
27E9h
D817h
LPF Output
word Range
TO VOLTAGE RMS AND
WAVEFORM SAMPLING
60Hz
60Hz
2838h
D7C8h
50Hz
Figure 10 – ADC and signal processing in voltage
channel
For Energy measurements, the output of the ADC (1 bit) is
passed directly to the multiplier and is not filtered. This
solution avoids a wide bits multiplier and does not affect the
accuracy of the measurement. A HPF is not required to
remove any DC offset since it is only required to remove the
offset from one channel to eliminate errors in the Power
calculation.
In the voltage channel, the samples may also be routed to the
WFORM register (WAVMODE to select VA, VB or VC and
sampling frequency). However before being passed to the
Waveform register, the ADC output is passed through a
single pole, low pass filter with a cutoff frequency of 260Hz.
The plots in Figure 11 show the magnitude and phase
response of this filter. The filter output code of any inputs of
the voltage channel swings between D70Bh (-10,485d) and
28F5h (+10,485d) for full scale sinewave inputs.
This has the effect of attenuating the signal. For example if
the line frequency is 60Hz, then the signal at the output of
LPF1 will be attenuated by 3%.
H( )
Hz
Hz
dBs
.
.
=
+
(
)
=
=
1
1
60
260
0 974
0 2
2
Note LPF1 does not affect the power calculation since it is
used only in the Waveform sample mode.
When in waveform sample mode, one of four output sample
rates can be chosen by using bits 3 and 4 of the WAVMode
register. The available output sample rates are 26.0kSPS,
13.5kSPS, 6.5kSPS or 3.3kSPS. The interrupt request
output
IRQ
signals a new sample availability by going active
low. The voltage waveform register is a 2-complement 16-bit
register. As the Waveform register is a 24-bit signed register,
the waveform data from the voltage input is located in the 16
LSB of the Waveform register. The sign of the 16-bit voltage
input value is not extended to the upper byte of the waveform
register. The upper byte is instead filled with zeros. 24-bit
waveform samples are transferred from the ADE7754 one
byte (8-bits) at a time, with the most significant byte shifted
out first. The timing is the same as that for the current
channels and is shown in Figure 9.
ZERO CROSSING DETECTION
The ADE7754 has rising edge zero crossing detection
circuits for each of voltage channels (V
AP
, V
BP
, or V
CP
).
Figure 12 shows how the zero cross signal is generated from
the output of the ADC of the voltage channel.
V
AP
, V
BP
, V
CP
V
N
ADC
PGA
1
x1, x2, x4
REFERENCE
GAIN[6:5]
V
TO
MULTIPLIER
-100% to +100% FS
V
IRQ
13 degrees @ 60Hz
1.0
ZERO
CROSS
Zero Crossing
Detection
Read RSTATUS
LPF1
f
-3dB
= 260Hz
Figure 12
–
Zero cross detection on Voltage Channel
The zero crossing interrupt is generated from the output of
LPF1. LPF1 has a single pole at 260Hz (CLKIN = 10MHz).
As a result there will be a phase lag between the analog input
signal of the voltage channel and the output of LPF1. The
phase response of this filter is shown in the Voltage channel
Sampling section of this data sheet. The phase lag response
of LPF1 results in a time delay of approximately 0.6ms (@
60Hz) between the zero crossing on the analog inputs of
Voltage channel and the falling of IRQ.
When one phase crosses zero from negative to positive values
(rising edge), the corresponding flag in the Interrupt Status
register (bit 7-9) is set to logic one. An active-low in the
IRQ
output will also appear if the corresponding ZX bit in the
Interrupt Mask register is set to logic one.
The flag in the Interrupt status register is reset to 0 when the
Interrupt status register with reset (RSTATUS) is read. Each
phase has its own interrupt flag and mask bit in the interrupt
register.
10
1
10
2
10
60°
40°
20°
0°
P
G
Frequency (Hz)
3
40
20
0
(60Hz ;
13
°
)
(60Hz ;
0.2dB)
Figure 11
–
Magnitude & Phase response of LPF1