Datasheet
R
2
1.33kW
+V
IN
R
G+
R
G-
-V
IN
R
9
1.27kW
C
9
10 Fm
R
8
50W
R
5
50W
R
1
20W
V
IN
R
10
75W
VCA822
R
21
8.7kW
C
5
1.42pF
R
17
17.5kW
C
6
120nF
R
18
40kW
C
7
100nF
V
OUT
V
G DC
=+1V
75 LoadW
V
G
GND
V
REF
FB
V
OUT
VOLTAGE-CONTROLLED LOW-PASS FILTER
G
2 R Cp
2
f =
8
(5)
V
OUT
V
IN
R
2
R
1
1
1+s
R C
2
G
= - ´
(4)
- £0.8V V 0.8V
G
£
(6)
R
F
1kW
+V
IN
R
G+
R
G-
-V
IN
FB
R
G
200W
20W
V
OUT
VCA822
Out
V
G
OPA690
24pF
C
V
IN
R
332W
2
R
332W
1
24pF
50W
V +1
G
2
R
F
R
G
G=2 ´ ´
(7)
VCA822
SBOS343C – SEPTEMBER 2007 – REVISED DECEMBER 2008 .....................................................................................................................................
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Figure 85. Differential Cable Equalizer
this circuit performs as if the amplifier were replaced
by a short circuit. Visually replacing the amplifier by a
short leaves a simple voltage-feedback amplifier with
In the circuit of Figure 86 , the VCA822 serves as the
a feedback resistor bypassed by a capacitor.
variable-gain element of a voltage-controlled
Replacing this gain with a variable gain, G, the pole
low-pass filter. This section discusses how this
can be written as shown in Equation 5 :
implementation expands the circuit voltage swing
capability over that normally achieved with the
equivalent multiplier implementation. The circuit
control voltage, V
G
, is calculated as according to the
Because the VCA822 is most linear in the midrange,
simplified relationship in Equation 4 :
the median of the adjustable pole should be set at V
G
= 0V (see Figure 26 , Figure 46 , Figure 67 , and
Equation 6 ). Selecting R
1
= R
2
= 332 Ω , and targeting
a median frequency of 10MHz, the capacitance (C) is
24pF. Because the OPA690 was selected for the
circuit of Figure 86 , and in order to limit peaking in
the OPA690 frequency response, a capacitor equal to
C was added on the inverting mode to ground. This
architecture has the effect of setting the
high-frequency noise gain of the OPA690 to +2V/V,
ensuring stability and providing flat frequency
response.
Once the median frequency is set, the maximum and
minimum frequencies can be determined by using V
G
= – 0.8V and V
G
= +0.8V in the gain equation of
Equation 7 . Note that this is a first-order analysis and
does not take into consideration the open-loop gain
Figure 86. Voltage-Control Low-Pass Filter
limitation of the OPA690.
The response control results from amplification of the
feedback voltage applied to R
2
. First, consider the
case where the VCA822 produces G = 1V/V. Then
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