Datasheet
DIFFERENTIAL EQUALIZER
G=2 ´
´
R
F
R
G
1+sR C
G 1
1+sR C
1 1
(3)
9
6
-24
Frequency(Hz)
Gain(dB)
1M 1G
3
-21
-3
0
100M10M
-18
-15
-12
-9
-6
InitialFrequencyResponse
ofVCA822withRCLoad
EqualizedFrequency
Response
DIFFERENTIAL CABLE EQUALIZER
R
F
+V
IN
R
G+
R
G-
-V
IN
FB
R
S
20W
V
IN1
VCA822
R
G
R
1
C
1
R
S
V
IN2
2.0
-1.0
Frequency(MHz)
1694FCableAttenuation(dB)
EqualizerGain(dB)
1 100
1.5
1.0
10
-0.5
0
0.5
CableAttenuations
VCA822with
Equalization
VCA822
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..................................................................................................................................... SBOS343C – SEPTEMBER 2007 – REVISED DECEMBER 2008
If the application requires frequency shaping (the
transition from one gain to another), the VCA822 can
be used advantageously because its architecture
allows the application to isolate the input from the
gain setting elements. Figure 82 shows an
implementation of such a configuration. The transfer
function is shown in Equation 3 .
Figure 83. Differential Equalization of an RC Load
A differential cable equalizer can easily be
implemented using the VCA822. An example of a
cable equalization for 100 feet of Belden Cable
1694F is illustrated in Figure 85 , with the result for
this implementation shown in Figure 84 . This
implementation has a maximum error of 0.2dB from
Figure 82. Differential Equalizer
dc to 40MHz.
This transfer function has one pole, P
1
(located at
R
G
C
1
), and one zero, Z
1
(located at R
1
C
1
). When
equalizing an RC load, R
L
and C
L
, compensate the
pole added by the load located at R
L
C
L
with the zero
Z
1
. Knowing R
L
, C
L
, and R
G
allows the user to select
C
1
as a first step and then calculate R
1
. Using
R
L
= 75 Ω , C
L
= 100pF and wanting the VCA822 to
operate at a gain of +2V/V, which gives R
F
= R
G
=
1.33k Ω , allows the user to select C
1
= 5pF to ensure
a positive value for the resistor R
1
. With all these
values known, R
1
can be calculated to be 170 Ω . The
frequency response for both the initial, unequalized
frequency response and the resulting equalized
frequency response are shown in Figure 83 .
Figure 84. Cable Attenuation versus Equalizer
Gain
Note that this implementation shows the cable
attenuation side-by-side with the equalization in the
same plot. For a given frequency, the equalization
function realized with the VCA822 matches the cable
attenuation. The circuit in Figure 85 is a driver circuit.
To implement a receiver circuit, the signal is received
differentially between the +V
IN
and – V
IN
inputs.
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