Datasheet

DIFFERENCE AMPLIFIER
G=2 ´
´
R
F
R
G
1+sR C
G 1
1+sR C
1 1
(1)
R
F
+V
IN
R
G+
R
G-
-V
IN
FB
R
S
20W
V
IN1
VCA821
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-
VCA821
85
80
75
70
65
60
55
50
45
40
Frequency(Hz)
Common-ModeRejectionRatio(dB)
10k 100k 1M 10M 100M
InputReferred
DIFFERENTIAL EQUALIZER
9
6
3
0
-3
-6
-9
-12
-15
-18
-21
-24
Frequency(Hz)
Gain(dB)
1M 10M 1G100M
InitialFrequencyResponse
oftheVCA821withRCLoad
EqualizedFrequencyResponse
VCA821
SBOS407B – DECEMBER 2007 – REVISED DECEMBER 2008 ....................................................................................................................................... www.ti.com
be used advantageously because its architecture
allows the application to isolate the input from the
Because both inputs of the VCA821 are
gain setting elements. Figure 77 shows an
high-impedance, a difference amplifier can be
implementation of such a configuration. The transfer
implemented without any major problem. Figure 75
function is shown in Equation 1 .
shows this implementation. This circuit provides
excellent common-mode rejection ratio (CMRR) as
long as the input is within the CMRR range of – 2.1V
to +1.6V. Note that this circuit does 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 does not limit the
bandwidth of the circuit. Figure 76 shows the
common-mode rejection ratio for this circuit
implemented in a gain of 20dB for V
G
= +2V. 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
VCA821 is not limited by common-mode input
voltage, the current flowing through R
G
, or any other
limitation described in this data sheet.
Figure 77. 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
R
L
= 75 Ω , C
L
= 100pF and wanting the VCA821 to
operate at a gain of +2V/V, which gives R
F
= R
G
=
Figure 75. Difference Amplifier
453 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 78
shows the frequency response for both the initial,
unequalized frequency response and the resulting
equalized frequency response.
Figure 76. Common-Mode Rejection Ratio
If the application requires frequency shaping (the
transition from one gain to another), the VCA821 can Figure 78. Differential Equalization of an RC Load
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