VCA2611 View Datasheet(PDF) - TEXAS ADVANCED OPTOELECTRONIC SOLUTIONS
Part Name
Description
MFG CO.
'VCA2611' PDF : 19 Pages View PDF
RS
LNPIN
RIN
RIN =
RF
1+A
= RS
RS
RS
RF
A
Active Feedback
A
Conventional Cable Termination
FIGURE 5. Configurations for Active Feedback and Conven-
tional Cable Termination.
the user-selected gain of the LNP, and RIN is the resulting
amplifier input impedance with active feedback. In this case,
unlike the conventional termination above, both the signal
voltage and the RS noise are attenuated by the same factor
of 2 (6dB) before being re-amplified by the A gain setting.
This avoids the extra 3dB degradation due to the square-root
effect described earlier, the key advantage of the active
termination technique.
This previous explanation ignored the input noise contribu-
tion 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
VCA2616 and VCA2611 user to compare the achievable
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.
VCA NOISE = 3.8nV√Hz, LNP GAIN = 20dB
14
LNP Noise
nV/√Hz
12
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.
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.
A switch, controlled by the FBSWCNTL signal on pin 45,
enables the user to reduce the feedback resistance by
adding an additional parallel component, connected between
the LNPINP 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.
RF
CF
0.001µF
CC
RS
VCM
44pF
200kΩ
Buffer
44pF
Gain
Stage
(VCA) LNP
Buffer
200kΩ
VCM
LNPOUTN
LNPOUTP
FIGURE 8. Low-Frequency LNP Time Constants.
12
VCA2616, VCA2611
www.ti.com
SBOS234D
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