VCA820IDGSR Datasheet, PDF (20/37 Pages) Texas Instruments – Wideband, 40dB Adjust Range, Linear in dB VARIABLE GAIN AMPLIFIER

VCA820
SBOS395C – OCTOBER 2007 – REVISED OCTOBER 2009............................................................................................................................................ www.ti.com
DIFFERENCE AMPLIFIER
Because both inputs of the VCA820 are
high-impedance, a difference amplifier can be
implemented without any major problem. This
implementation is shown in Figure 77. 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. 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.
V_IN+
V_IN-
R_S
R_G
R_S
R_F
+V_IN
R_{G+ FB}
VCA820
R_G-
-V_IN
20W
Figure 77. Difference Amplifier
95
90
85
80
75
70
65
60
55
50
45
Input-Referred
40
100k
1M
10M
Frequency (Hz)
100M
DIFFERENTIAL EQUALIZER
If the application requires frequency shaping (the
transition from one gain to another), the VCA820 can
be used advantageously because its architecture
allows the application to isolate the input from the
gain setting elements. Figure 79 shows an
implementation of such a configuration. The transfer
function is shown in Equation 1.
G=2´
R_F
R_G
´
1 + sR_GC₁
1 + sR₁C₁
(1)
V_IN1
V_IN2
+V_IN
R_F
R_S
R_G+
R₁
R_G
FB
VCA820
C₁
R_G-
-V_IN
20W
R_S
Figure 79. Differential Equalizer
This transfer function has one pole, P₁ (located at
R_GC₁), and one zero, Z₁ (located at R₁C₁). When
equalizing an RC load, R_L and C_L, compensate the
pole added by the load located at R_LC_L with the zero
Z₁. Knowing R_L, C_L, and R_G allows the user to select
C₁ as a first step and then calculate R₁. Using
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 C₁ = 5pF to ensure
a positive value for the resistor R₁. With all these
values known, R₁ 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
20
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