EVAL-ADE7759E View Datasheet(PDF) - Analog Devices
Part Name
Description
MFG CO.
'EVAL-ADE7759E' PDF : 32 Pages View PDF
ADE7759
isolated interface to external calibration equipment. Figure 38
illustrates the Energy-to-Frequency conversion in the ADE7759.
The Energy-to-Frequency conversion is accomplished by accumu-
lating the Active power signal in a 24-bit register. An output pulse
is generated when there is a zero to one transition on the MSB
(most significant bit) of the register. Under steady load conditions
the output frequency is proportional to the Active Power. The output
frequency at CF, with full-scale ac signals on Channel 1 and Chan-
nel 2 and CFDEN = 000h, CFNUM = 000h, and APGAIN = 000h,
is approximately 5.593 kHz. This can be calculated as follows:
With the Active Power Gain register set to 000h, the average
value of the instantaneous power signal (output of LPF2) is
CCCDh or 52,429 decimal. An output frequency is generated
on CF when the MSB in the energy to frequency register (24 bits)
toggles, i.e., when the register accumulates 223. This means the
register is updated 223/CCCDh times (or 159.999 times). Since
the update rate is 4/CLKIN or 1.1175 µs, the time between
MSB toggles (CF pulses) is given as:
159.999 × 1.1175 µs = 1.78799 × 10–4 s(5592.86 Hz)
Equation 8 gives an expression for the output frequency at the
Energy-to-Frequency (ETF) output with the contents of CFDEN
and CFNUM registers are both zero.
Average LPF2 Output × CLKIN
ETF Output (Hz) =
225
(8)
This output frequency is easily scaled by a pair of Calibration
Frequency Divider registers (CFDEN[11:0] and CFNUM[11:0]).
These frequency scaling registers are 12-bit registers that can
scale the output frequency by 1 to 212. The output frequency is
given by the expression below.
CF(Hz) = ETF Output (Hz) × CFNUM [11:0] + 1
CFDEN [11:0] + 1
(9)
For example, if the CF output frequency is 5.59286 kHz while
the contents of CFNUM and CFDEN are zero, the CF output
frequency can be set to 25 Hz by writing 8 BDh (2237 in decimal) to
the CFDEN register and 00Ah (10 in decimal) to the CFNUM
register. Note that the CFNUM and CFDEN registers are meant
only to scale down the frequency from the ETF output. Therefore,
the content of CFDEN should always be set no less than that of
the CFNUM register, i.e., the maximum output frequency from
CF pin will never exceed that of the ETF output. The power-up
default value for CFDEN is 3Fh and CFNUM is 0h.
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 section. Equation 3 gives
an expression for the instantaneous power signal. This is filtered
by LPF2, which has a magnitude response given by Equation 10.
H( f ) =
1
1 + f /8.9 Hz
(10)
The Active Power signal (output of LPF2) can be rewritten as
( ) p(t)
=
VI
−
1 +
2
VI
fl / 8.9
Hz
cos
4πfl t
(11)
where fl is the line frequency (e.g., 60 Hz)
From Equation 6
( ) ( ) E(t) =VIt
−
4πfl
VI
1 + 2 fl / 8.9 Hz
sin
4πfl t
(12)
From Equation 12 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 39. The Active Energy calculation is shown
by the dashed straight line and is equal to V × I × 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
frequencies. The ripple will get larger as a percentage of the fre-
quency at larger loads and higher output frequencies. The reason
is that at higher output frequencies the integration 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.
15
APOS [15:0]
0
SIGN 26 25 24 23 22 21 20 2–1 2–2 2–3 2–4 2–5 2–6 2–7 2–8
LPF2
+
23
20
+
WAVEFORM [23:0]
0
ACTIVE POWER OFFSET
CALIBRATION
ACTIVE POWER
SIGNAL – P
11
CFNUM [11:0]
0
ENERGY-TO-FREQUENCY
23
MSB
TRANSITION
0
+
+
11
CFDEN [11:0]
0
CF
Figure 38. Energy-to-Frequency Conversion
REV. 0
–23–
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