Datasheet

AD9272
Rev. C | Page 24 of 44
LNA Noise
The short-circuit noise voltage (input-referred noise) is an impor-
tant limit on system performance. The short-circuit input-referred
noise voltage for the LNA is 0.85 nV/√Hz at a gain of 21.3 dB,
including the VGA noise at a VGA postamp gain of 27 dB. These
measurements, which were taken without a feedback resistor,
provide the basis for calculating the input noise and noise figure
(NF) performance of the configurations shown in Figure 40.
V
OUT
UNTERMINATED
+
–
LI-x
R
IN
R
S
V
OUT
RESISTIVE TERMINATION
+
–
LI-x
R
IN
R
S
R
S
V
OUT
ACTIVE IMPEDANCE MATCH
+
–
LI-x
R
IN
R
FB
R
FB
1 + A/2
R
S
R
IN
=
07029-104
Figure 40. Input Configurations
Figure 41 and Figure 42 are simulations of noise figure vs. R
S
results using these configurations and an input-referred noise
voltage of 3.8 nV/√Hz for the VGA. Unterminated (R
FB
= ∞)
operation exhibits the lowest equivalent input noise and noise
figure. Figure 42 shows the noise figure vs. source resistance
rising at low R
S
—where the LNA voltage noise is large compared
with the source noise—and at high R
S
due to the noise contribution
from R
FB
. The lowest NF is achieved when R
S
matches R
IN
.
The main purpose of input impedance matching is to improve the
transient response of the system. With resistive termination, the
input noise increases due to the thermal noise of the matching
resistor and the increased contribution of the input voltage
noise generator of the LNA. With active impedance matching,
however, the contributions of both are smaller (by a factor of
1/(1 + LNA Gain)) than they would be for resistive termination.
Figure 41 shows the relative noise figure performance. In this
graph, the input impedance was swept with R
S
to preserve the
match at each point. The noise figures for a source impedance of
50  are 7.3 dB, 4.2 dB, and 2.8 dB for the resistive termination,
active termination, and unterminated configurations, respectively.
The noise figures for 200  are 4.5 dB, 1.7 dB, and 1 dB,
respectively.
Figure 42 shows the noise figure as it relates to R
S
for various values
of R
IN
, which is helpful for design purposes.
10 100 1k
0
1.5
3.0
4.5
6.0
7.5
9.0
10.5
12.0
R
S
(Ω)
NOISE FIGURE (dB)
UNTERMINATED
RESISTIVE TERMINATION
ACTIVE TERMINATION
07029-182
Figure 41. Noise Figure vs. R
S
for Resistive Termination,
Active Termination Matched, and Unterminated Inputs, V
GAIN
= 0.8 V
10 100 1k
0
1
2
3
4
5
6
7
8
R
S
(Ω)
NOISE FIGURE (dB)
R
IN
= 50Ω
R
IN
= 75Ω
R
IN
= 100Ω
R
IN
= 200Ω
UNTERMINATED
07029-183
Figure 42. Noise Figure vs. R
S
for Various Fixed Values of R
IN
,
Active Termination Matched Inputs, V
GAIN
= 0.8 V