Datasheet

ADE7752/ADE7752A
Rev. C | Page 20 of 24
TRANSFER FUNCTION
FREQUENCY OUTPUTS F1 AND F2
F
1–7
= 0.60 Hz, SCF = S0 = S1 = 1
V
The ADE7752 calculates the product of six voltage signals (on
current channel and voltage channel) and then low-pass filters
this product to extract real power information. This real power
information is then converted to a frequency. The frequency
information is output on F1 and F2 in the form of active high
pulses. The pulse rate at these outputs is relatively low, such as
29.32 Hz maximum for ac signals with SCF = 1; S0 = S1 = 1 (see
Table 6). This means that the frequency at these outputs is
generated from real power information accumulated over a
relatively long period of time. The result is an output frequency
that is proportional to the average real power. The averaging of
the real power signal is implicit to the digital-to-frequency
conversion. The output frequency or pulse rate is related to the
input voltage signals by the following equation:
(
)
2
71
181.6
REF
CCN
BBN
AAN
V
FIVIVIV
Freq
−
××+×+××
=
where:
Freq = the output frequency on F1 and F2 (Hz).
V
AN,
V
BN,
and V
CN
= the differential rms voltage signal on voltage
channels (V).
I
A,
I
B,
and I
C
= the differential rms voltage signal on current
channels (V).
V
REF
= the reference voltage (2.4 V ± 8%) (V).
F
1–7
= one of seven possible frequencies selected by using the
logic inputs SCF, S0, and S1 (see
Table 5).
Table 5. F
1–7
Frequency Selection
1
SCF S1 S0 F
1–7
(Hz)
0 0 0 1.27
1 0 0 1.19
0 0 1 5.09
1 0 1 4.77
0 1 0 19.07
1 1 0 19.07
0 1 1 76.29
1 1 1 0.60
1
F
1–7
is a fraction of the master clock and therefore varies if the specified
CLKIN frequency is altered.
Example 1
Thus, if full-scale differential dc voltages of +500 mV are
applied to VA, VB, VC, IA, IB, and IC, respectively (500 mV is
the maximum differential voltage that can be connected to
current and voltage channels), the expected output frequency is
calculated as follows:
AN
= V = V
BN CN
= IA = IB = IC = 500 mV dc =
0.5 V(rms of dc = dc)
V
REF
= 2.4 V (nominal reference value)
Note that if the on-chip reference is used, actual output fre-
quencies may vary from device to device due to reference
tolerance of ±8%.
Hz483.0
4.2
60.05.05.0181.6
3
2
=
×
×
×
×=Freq
Example 2
In this example, with ac voltages of ±500 mV peak applied to
the voltage channels and current channels, the expected output
frequency is calculated as follows:
()
valuereferencenominalV
AC
ICIBIAVVV
SSSCFF
REF
CN
BN
AN
V4.2
Vrms
2
5.0
peakmV500
110,Hz60.0
71
=
==
=====
=
=
=
=
−
Note that if the on-chip reference is used, actual output fre-
quencies may vary from device to device due to reference
tolerance of ±8%.
Hz24.0
4.222
6.05.05.0181.6
3
2
=
××
×
×
×
×=Freq
As can be seen from these two example calculations, the
maximum output frequency for ac inputs is always half of that
for dc input signals. The maximum frequency also depends on
the number of phases connected to the ADE7752. In a 3-phase
3-wire delta service, the maximum output frequency is different
from the maximum output frequency in a 3-phase
4-wire Wye service. The reason is that there are only two phases
connected to the analog inputs, but also that in a delta service,
the current channel input and voltage channel input of the same
phase are not in phase in normal operation.
Example 3
In this example, the ADE7752 is connected to a 3-phase 3-wire
delta service as shown in
Figure 21. The total real energy
calculation processed in the ADE7752 can be expressed as
Total Real Power = (V
A
− V
C
) × I
A
+ (V
B
− V
C
) × I
B
B B
where V
A
, V
B
, and V
C
B represent the voltage on Phase A, B, and
C, respectively. I
A
and I
B
B represent the current on Phase A and
B, respectively.