VCA2613 Datasheet, PDF (8/16 Pages) Burr-Brown (TI) – Dual, VARIABLE GAIN AMPLIFIER with Low-Noise Preamp

R_S
LNP_IN
R_IN
R_IN =
R_F
1+A
= R_S
R_S
R_S
R_F
A
Active Feedback
A
Conventional Cable Termination
FIGURE 5. Configurations for Active Feedback and Conven-
tional Cable Termination.
of two (6dB) before being re-amplified by the A gain setting.
This avoids the extra 3dB degradation due to the square-root
effect described above, the key advantage of the active
termination technique.
As mentioned above, the previous explanation ignored the
input noise contribution of the LNP itself. Also, the noise
contribution of the feedback resistor must be included for a
completely correct analysis. The curves given in Figures 6
and 7 allow the VCA2613 user to compare the achievable
noise figure for active and conventional termination methods.
The left-most set of data points in each graph give the results
for typical 50Ω cable termination, showing the worst noise
figure but also the greatest advantage of the active feedback
method.
VCA NOISE = 3.8nV√Hz, LNP GAIN = 20dB
14
LNP Noise
12
nV/√Hz
6.0E-10
8.0E-10
10
1.0E-09
1.2E-09
8
1.4E-09
1.6E-09
1.8E-09
6
2.0E-09
4
2
0
0 100 200 300 400 500 600 700 800 900 1000
Source Impedance (Ω)
FIGURE 7. Noise Figure for Conventional Termination.
A switch, controlled by the FBSW_CNTL signal on pin 45,
enables the user to reduce the feedback resistance by
adding an additional parallel component, connected between
the LNP_INP and SWFB terminals. The two different values of
feedback resistance will result in two different values of
active-feedback input resistance. Thus, the active-feedback
impedance can be optimized at two different LNP gain
settings. The switch is connected at the buffered output of
the LNP and has an ON resistance of approximately 1Ω.
When employing active feedback, the user should be careful
to avoid low-frequency instability or overload problems. Fig-
ure 8 illustrates the various low-frequency time constants.
Referring again to the input resistance calculation of Equa-
tion (3), and considering that the gain term A falls off below
3.6kHz, it is evident that the effective LNP input impedance
will rise below 3.6kHz, with a DC limit of approximately R_F. To
avoid interaction with the feedback pole/zero at low frequen-
cies, and to avoid the higher signal levels resulting from the
rising impedance characteristic, it is recommended that the
external R_FC_C time constant be set to about 5µs.
VCA NOISE = 3.8nV√Hz, LNP GAIN = 20dB
9
8
LNP Noise
nV/√Hz
7
6.0E-10
8.0E-10
6
1.0E-09
5
1.2E-09
1.4E-09
4
1.6E-09
3
1.8E-09
2.0E-09
2
1
0
0 100 200 300 400 500 600 700 800 900 1000
Source Impedance (Ω)
FIGURE 6. Noise Figure for Active Termination.
R_F
C_F
0.001µF
C_C
R_S
V_CM
44pF
1MΩ
Buffer
44pF
Gain
Stage
(VCA) LNP
1MΩ
Buffer
V_CM
LNP_OUTN
LNP_OUTP
FIGURE 8. Low Frequency LNP Time Constants.
8
VCA2613
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