Datasheet

DIFFERENTIAL EQUALIZER
G=2 ´
´
R
F
R
G
1+sR C
G 1
1+sR C
1 1
(3)
9
6
3
0
-3
-6
-9
-12
-15
-18
-21
-24
Frequency(Hz)
Gain(dB)
1M 10M 1G100M
InitialFrequencyResponse
oftheVCA824withRCLoad
EqualizedFrequencyResponse
DIFFERENTIAL CABLE EQUALIZER
R
F
+V
IN
R
G+
R
G-
-V
IN
FB
R
S
20W
V
IN1
VCA824
R
G
R
1
C
1
R
S
V
IN2
2.0
1.5
1.0
0.5
0
-0.5
-1.0
Frequency(MHz)
1694FCableAttenuation(dB)
EqualizerGain(dB)
1 10 100
CableAttenuation
VCA824Equalization
VCA824
www.ti.com ....................................................................................................................................... SBOS394C – NOVEMBER 2007 – REVISED DECEMBER 2008
If the application requires frequency shaping (the
transition from one gain to another), the VCA824 can
be used advantageously because its architecture
allows the application to isolate the input from the
gain setting elements. Figure 80 shows an
implementation of such a configuration. The transfer
function is shown in Equation 3 .
Figure 81. Differential Equalization of an RC Load
A differential cable equalizer can easily be
implemented using the VCA824. An example of a
cable equalization for 100 feet of Belden Cable
1694F is illustrated in Figure 83 , with Figure 82
showing the result for this implementation. This
implementation has a maximum error of 0.2dB from
Figure 80. Differential Equalizer
dc to 70MHz.
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 VCA824 to
operate at a gain of +2V/V, which gives R
F
= R
G
=
453k Ω , allows the user to select C
1
= 15.5pF to
ensure a positive value for the resistor R
1
. With all
these values known, to achieve greater than 300MHz
bandwidth, R
1
can be calculated to be 20 Ω . Figure 81
shows the frequency response for both the initial,
unequalized frequency response and the resulting
equalized frequency response.
Figure 82. 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 VCA824 matches the cable
attenuation. The circuit in Figure 83 is a driver circuit.
To implement a receiver circuit, the signal is received
differentially between the +V
IN
and – V
IN
inputs.
Copyright © 2007 – 2008, Texas Instruments Incorporated Submit Documentation Feedback 21
Product Folder Link(s): VCA824