Datasheet

G=2 ´
´
R
F
R
G
1+sR C
G 1
1+sR C
1 1
R
F
+V
IN
R
G+
R
G-
-V
IN
FB
R
S
20W
V
IN1
VCA820
R
G
R
1
C
1
R
S
V
IN2
R
F
+V
IN
R
G+
R
G-
-V
IN
FB
R
G
R
S
R
S
20W
V
IN+
V
IN-
VCA820
95
90
40
Frequency(Hz)
Common-ModeRejectionRatio(dB)
100k 100M
85
45
75
80
10M1M
50
55
60
65
70
Input-Referred
VCA820
SBOS395C –OCTOBER 2007–REVISED OCTOBER 2009............................................................................................................................................ www.ti.com
DIFFERENCE AMPLIFIER DIFFERENTIAL EQUALIZER
Because both inputs of the VCA820 are If the application requires frequency shaping (the
high-impedance, a difference amplifier can be transition from one gain to another), the VCA820 can
implemented without any major problem. This be used advantageously because its architecture
implementation is shown in Figure 77. This circuit allows the application to isolate the input from the
provides excellent common-mode rejection ratio gain setting elements. Figure 79 shows an
(CMRR) as long as the input is within the CMRR implementation of such a configuration. The transfer
range of –2.1V to +1.6V. Note that this circuit does function is shown in Equation 1.
not make use of the gain control pin, V
G
. Also, it is
recommended to choose R
S
such that the pole
formed by R
S
and the parasitic input capacitance (1)
does not limit the bandwidth of the circuit. The
common-mode rejection ratio for this circuit
implemented in a gain of 20dB for V
G
= +2V is shown
in Figure 78. Note that because the gain control
voltage is fixed and is normally set to +2V, the
feedback element can be reduced in order to
increase the bandwidth. When reducing the feedback
element make sure that the VCA820 is not limited by
common-mode input voltage, the current flowing
through R
G
, or any other limitation described in this
data sheet.
Figure 79. Differential Equalizer
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
Figure 77. Difference Amplifier
R
L
= 75Ω, C
L
= 100pF and wanting the VCA820 to
operate at a gain of +2V/V, which gives R
F
= R
G
=
1.33kΩ, 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 170Ω. The
frequency response for both the initial, unequalized
frequency response and the resulting equalized
frequency response are illustrated in Figure 80.
Figure 78. Common-Mode Rejection Ratio
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