Analog Devices AD9273, Manual

Page: 25 / 49

AD9273
Rev. B | Page 24 of 48
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 1.4 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 41.
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
=
0
70
30
-10
4
Figure 41. Input Configurations
Figure 42 and Figure 43 are simulations of noise figure vs. source
resistance (R
S
) results using these configurations and an input-
referred noise voltage of 6 nV/√Hz for the VGA. Unterminated
(R
FB
= ∞) operation exhibits the lowest equivalent input noise
and noise figure. Figure 43 shows the noise figure vs. R
S
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 42 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 Ω 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 Ω are 4.5 dB, 1.7 dB, and 1.0 dB,
respectively.
Figure 43 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
)
NO
IS
E
F
IG
UR
E
(
d
B)
07
03
0-
18
2
UNTERMINATED
SHUNT TERMINATION
ACTIVE
TERMINATION
Figure 42. Noise Figure vs. R
S
for Shunt Termination,
Active Termination Matched, and Unterminated Inputs, V
GAIN
= 0.8 V
10 100 1k
0
1.5
3.0
4.5
6.0
7.5
9.0
10.5
12.0
R
S
(
Ω
)
NO
IS
E
F
IG
UR
E
(
d
B)
07
03
0-
18
3
UNTERMINATED
R
IN
= 200
Ω
R
IN
= 100
Ω
R
IN
= 75
Ω
R
IN
= 50
Ω
Figure 43. Noise Figure vs. R
S
for Various Fixed Values of R
IN
,
Active Termination Matched Inputs, V
GAIN
= 0.8 V