Datasheet

AD9277
Rev. 0 | Page 24 of 48
LNA Noise
The short-circuit noise voltage (input-referred noise) is an
important limit on system performance. The short-circuit noise
voltage for the LNA is 0.75 nV/√Hz at a gain of 21.3 dB, including
the VGA noise at a VGA postamp gain of 27 dB. These measure-
ments, 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 49.
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
=
08181-043
Figure 49. Input Configurations
Figure 50 and Figure 51 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 51 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 LNA’s input
voltage noise generator. 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 50 shows the relative noise figure performance. With an
LNA gain of 21.3 dB, 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
1.0 dB, respectively.
Figure 51 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
08181-044
Figure 50. 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
08181-045
Figure 51. Noise Figure vs. R
S
for Various Fixed Values of R
IN
,
Active Termination Matched Inputs, V
GAIN
= 0.8 V