Datasheet

AD9279
Rev. 0 | Page 24 of 44
shows an example of how to select R
FB1
and R
FB2
for 66 Ω, 100 Ω,
and 200 Ω input impedance for LNA gain = 21.3 dB (12×).
Table 9. Active Termination Example for LNA Gain = 21.3 dB,
R
FB1
= 700 Ω, R
FB2
= 1400 Ω
Register
0x2C
Value R
s
(Ω)
LO-x
Switch
LOSW-x
Switch R
FB
(Ω)
R
IN
(Ω)
(Eq. 1)
00
(default)
100 On Off R
FB1
100
01 50 On On R
FB1
||R
FB2
66
10 200 Off On R
FB2
200
11 N/A Off Off ∞ ∞
The bandwidth (BW) of the LNA is greater than 100 MHz.
Ultimately, the BW of the LNA limits the accuracy of the
synthesized R
IN
. For R
IN
= R
S
up to about 200 Ω, the best match
is between 100 kHz and 10 MHz, where the lower frequency
limit is determined by the size of the ac coupling capacitors, and
the upper limit is determined by the LNA BW. Furthermore, the
input capacitance and R
S
limit the BW at higher frequencies.
Figure 46 shows R
IN
vs. frequency for various values of R
FB
.
09423-040
10
100
1k
100k 1M 10M 100M
INPUT RESISTANCE (Ω)
FREQUENCY (Hz)
R
S
= 50Ω, R
FB
= 200Ω, C
SH
= 70pF
R
S
= 100Ω, R
FB
= 400Ω, C
SH
= 20pF
R
S
= 200Ω, R
FB
= 800Ω
R
S
= 500Ω, R
FB
= 2kΩ
Figure 46. R
IN
vs. Frequency for Various Values of R
FB
(Effects of R
SH
and C
SH
Are Also Shown)
Note that, at the lowest value of R
IN
(50 Ω), R
IN
peaks at frequencies
greater than 10 MHz. This is due to the BW roll-off of the LNA.
However, as can be seen for larger R
IN
values, parasitic capaci-
tance starts rolling off the signal BW before the LNA can produce
peaking. C
SH
further degrades the match; therefore, C
SH
should
not be used for values of R
IN
that are greater than 100 Ω.
Table 10 lists the recommended values for R
FB
and C
SH
in terms
of R
IN
.
C
FB
is needed in series with R
FB
because the dc levels at Pin LO-x
and Pin LI-x are unequal.
Table 10. Active Termination External Component Values
LNA Gain
(dB) R
IN
(Ω) R
FB
(Ω)
Minimum
C
SH
(pF) BW (MHz)
15.6 50 200 90 57
17.9 50 250 70 69
21.3 50 350 50 88
15.6 100 400 30 57
17.9 100 500 20 69
21.3 100 700 10 88
15.6 200 800 N/A 72
17.9 200 1000 N/A 72
21.3 200 1400 N/A 72
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 47.
V
OUT
UNTERMINATED
+
–
LI-x
R
IN
R
S
V
OUT
SHUNT TERMINATION
+
–
LI-x
R
IN
R
S
R
S
V
OUT
ACTIVE TERMINATION
+
–
LI-x
R
IN
R
FB
R
FB
1 + A/2
R
S
R
IN
=
09423-041
Figure 47. Input Configurations
Figure 48 and Figure 49 are simulations of noise figure vs. R
S
results using these configurations and an input-referred noise
voltage of 2.5 nV/√Hz for the VGA. Unterminated (R
FB
= ∞)
operation exhibits the lowest equivalent input noise and noise
figure. Figure 49 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
.