VCA2613Y BURR-BROWN [Burr-Brown Corporation], VCA2613Y Datasheet - Page 8

• Manufacturer Part Number: VCA2613Y
• Description: Dual, VARIABLE GAIN AMPLIFIER with Low-Noise Preamp
• Manufacturer: BURR-BROWN [Burr-Brown Corporation]

FIGURE 5. Configurations for Active Feedback and Conven-
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 50
figure but also the greatest advantage of the active feedback
method.
FIGURE 6. Noise Figure for Active Termination.
8
9
8
7
6
5
4
3
2
1
0
0
R
R
100 200
S
S
tional Cable Termination.
VCA NOISE = 3.8nV Hz, LNP GAIN = 20dB
R
R
cable termination, showing the worst noise
IN
IN
=
1 + A
300
R
F
Source Impedance ( )
= R
400
S
LNP
500
Conventional Cable Termination
IN
R
600 700 800 900 1000
S
Active Feedback
LNP Noise
R
A
A
nV/ Hz
6.0E-10
8.0E-10
1.0E-09
1.2E-09
1.4E-09
1.6E-09
1.8E-09
2.0E-09
F
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FIGURE 7. Noise Figure for Conventional Termination.
A switch, controlled by the FBSW
enables the user to reduce the feedback resistance by
adding an additional parallel component, connected between
the LNP
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 1 .
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
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
FIGURE 8. Low Frequency LNP Time Constants.
0.001 F
14
12
10
R
C
C
8
6
4
2
0
S
C
F
0
IN
P and SWFB terminals. The two different values of
F
C
100 200
C
(VCA) LNP
VCA NOISE = 3.8nV Hz, LNP GAIN = 20dB
time constant be set to about 5 s.
Stage
Gain
300
Source Impedance ( )
44pF
44pF
400
V
V
CM
CM
R
500
F
1M
1M
600 700
CNTL
Buffer
Buffer
signal on pin 45,
LNP Noise
800
nV/ Hz
6.0E-10
8.0E-10
1.0E-09
1.2E-09
1.4E-09
1.6E-09
1.8E-09
2.0E-09
VCA2613
900 1000
SBOS179D
LNP
LNP
OUT
OUT
F
. To
N
P