AD9272_09 Datasheet, PDF (24/44 Pages) Analog Devices – Octal LNA/VGA/AAF/ADC and Crosspoint Switch

AD9272
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.
+
LI-x
–
UNTERMINATED
R_IN
R_S
V_OUT
+
LI-x
–
RESISTIVE TERMINATION
R_IN
R_S
R_S
V_OUT
+
LI-x
–
ACTIVE IMPEDANCE MATCH
R_IN
R_S
R_FB
V_OUT
R_FB
R_IN = _{1 + A/2}
Figure 40. Input Configurations
Figure 41 and Figure 42 are simulations of noise figure vs. RS
results using these configurations and an input-referred noise
voltage of 3.8 nV/√Hz for the VGA. Unterminated (RFB = ∞)
operation exhibits the lowest equivalent input noise and noise
figure. Figure 42 shows the noise figure vs. source resistance
rising at low RS—where the LNA voltage noise is large compared
with the source noise—and at high RS due to the noise contribution
from RFB. The lowest NF is achieved when RS matches RIN.
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 RS 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 dB,
respectively.
Figure 42 shows the noise figure as it relates to RS for various values
of RIN, which is helpful for design purposes.
12.0
10.5
9.0
RESISTIVE TERMINATION
7.5
6.0
4.5
3.0
UNTERMINATED
1.5
ACTIVE TERMINATION
0
10
100
1k
R_S (Ω)
Figure 41. Noise Figure vs. RS for Resistive Termination,
Active Termination Matched, and Unterminated Inputs, VGAIN = 0.8 V
8
7
R_IN = 50Ω
R_IN = 75Ω
R_IN = 100Ω
6
R_IN = 200Ω
UNTERMINATED
5
4
3
2
1
0
10
100
1k
R_S (Ω)
Figure 42. Noise Figure vs. RS for Various Fixed Values of RIN,
Active Termination Matched Inputs, VGAIN = 0.8 V
Rev. C | Page 24 of 44