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AD7750
–9–
REV. 0
DE T E RMINING T HE OUT PUT FRE QUE NCIE S OF T HE
AD7750
F
OUT
, F1 and F2 are the frequency outputs of the AD7750. T he
output frequencies of the AD7750 are a multiple of a binary
fraction of the master clock frequency CLK IN. T his binary
fraction of the master clock is referred to as F
MAX
in this data
sheet. F
MAX
can have one of two values, F
MAX 1
and F
MAX 2
,
depending on which mode of operation the AD7750 is in. T he
operating modes of the AD7750 are selected by the logic inputs
FS, S2 and S1. T he table below outlines the F
MAX
frequencies
and the transfer functions for the various operating modes of the
AD7750.
NOT ES
1
T he variable k
is proportional to the product of the rms differential input voltages on Channel 1 and Channel 2 (V
1
and V
2
).
k
= (1.32
×
V
×
V
×
Gain)/V
2
Applies to F
only. T he variable k
is proportional to the instantaneous differential input voltage on Channel 1 (FS = 0, S1 = 1, S0 = 1) or the instantaneous differ-
ential voltage on Channel 2 (FS = 1, S1 = 1, S0 = 1), i.e., Channel Monitor Mode.
k
= (0.81
×
V)/V
REF
V = V
1
×
Gain
or
V = V
2
×
2
NOT E: V
1
and V
2
here refer to the instantaneous differential voltage on Channel 1 or Channel 2, not the rms value.
2
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1
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2
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=
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1
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1
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F
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2
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1
,
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F
1
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F
±
1
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F
3
1
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k
±
2
F
1
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]
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F
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2
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±
2
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1
2
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F
±
2
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]
F
F
2
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A
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2
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=
=
8
1
2
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6
5
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1
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]
F
F
2
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2
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1
=
=
8
1
2
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A
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6
6
1
1
0
.
d
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F
o
t
0
2
X
A
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F
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t
0
[
3
2
X
A
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]
F
F
2
X
A
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2
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N
I
H
K
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C
1
=
=
8
1
2
X
A
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6
7
2
1
1
1
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F
2
X
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F
±
2
X
A
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F
1
2
X
A
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k
±
2
F
2
X
A
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]
F
F
2
X
A
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2
z
N
I
H
K
L
C
1
=
=
8
1
2
X
A
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6
Mode Description (T able I)
T he section of T able I labeled Mode Description summarizes
the functional modes of the AD7750. T he AD7750 has two
basic modes of operation, i.e., four and two quadrant multiplica-
tion. T he diagram in Figure 14 is a graphical representation of
the transfer functions for two and four quadrant multiplication.
Four Quadrant Multiplication (Modes 0, 3, 4 and 7)
When the AD7750 is operating in its four quadrant multiplica-
tion mode the output pulse frequency on F1, F2 and F
OUT
contains both sign and magnitude information. T he magni-
tude information is indicated by the output frequency variation
(k.F
MAX
) from a center frequency (F
MAX
). T he sign informa-
tion is indicated by the sign of the frequency variation around
F
MAX
. For example if the output frequency is equal to F
MAX
–
k.F
MAX
then the magnitude of the product is given by k.F
MAX
and it has a negative sign.
T wo Quadrant Multiplication (Modes 1, 2, 5 and 6)
When operating in this mode the output pulse frequency only
contains magnitude information. Again as in the case of four
quadrant multiplication the magnitude information is included
in the output frequency variation (k.F
MAX
). However, in this
mode the zero power frequency is 0 Hz, so the output frequency
variation is from 0 Hz to (k.F
MAX
) Hz. Also note that a no-load
threshold and the reverse polarity indicator are implemented in
these modes see No Load T hreshold and Reverse Polarity
Indicator sections. T hese modes are the most suitable for a
Class 1 meter implementation.
Channel Monitor Modes (Modes 3 and 7)
In this mode of operation the F
OUT
pulse frequency does not
give product information. When FS = 0, the F
OUT
output fre-
quency gives sign and magnitude information about the volt-
age on Channel 1. When FS = 1 the F
OUT
output frequency
gives sign and magnitude information about the voltage on
Channel 2.
Note the F1, F2 pulse outputs still continue to give power
information.
T able I. Operating Mode