Datasheet

AD9271
Rev. B | Page 22 of 60
Because the amplifier has a gain of 6× from its input to its
differential output, it is important to note that the gain A/2 is
the gain from Pin LI-x to Pin LO-x, and it is 6 dB less than the
gain of the amplifier, or 9.6 dB (3×). The input resistance is
reduced by an internal bias resistor of 15 kΩ in parallel with the
source resistance connected to Pin LI-x, with Pin LG-x ac
grounded. Equation 2 can be used to calculate the needed R
FB
for a desired R
IN
, even for higher values of R
IN
.
Ω
+
= k15||
)31(
FB
IN
R
R
(2)
For example, to set R
IN
to 200 Ω, the value of R
FB
is 845 Ω. If the
simplified equation (Equation 2) is used to calculate R
IN
, the
value is 190 Ω, resulting in a gain error less than 0.5 dB. Some
factors, such as the presence of a dynamic source resistance,
might influence the absolute gain accuracy more significantly.
At higher frequencies, the input capacitance of the LNA needs
to be considered. The user must determine the level of
matching accuracy and adjust R
FB
accordingly.
The bandwidth (BW) of the LNA is about 70 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 42 shows R
IN
vs. frequency for various values of R
FB
.
06304-10
10
100k 1M 10M 50M
FREQUENCY (Hz)
5
100
1k
INPUT IMPEDANCE (Ω)
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 42. 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, 50 Ω, in Figure 42, R
IN
peaks at
frequencies greater than 10 MHz. This is due to the BW roll-off
of the LNA, as mentioned previously.
However, as can be seen for larger R
IN
values, parasitic capacitance
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 7
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 7. Active Termination External Component Values
LNA Gain R
IN
(Ω) R
FB
(Ω)
Minimum
C
SH
(pF) BW (MHz)
5× 50 175 90 49
6× 50 200 70 59
8× 50 250 50 73
5× 100 350 30 49
6× 100 400 20 59
8× 100 500 10 73
5× 200 700 N/A 49
6× 200 800 N/A 49
8× 200 1000 N/A 49
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 1.2 nV/√Hz or 1.4 nV/√Hz (at 15.6 dB
LNA gain), including the VGA noise. 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 43. Figure 44 and Figure 45
are simulations of noise figure vs. R
S
results using these config-
urations and an input-referred noise voltage of 4 nV/√Hz for
the VGA. Unterminated (R
FB
= ∞) operation exhibits the lowest
equivalent input noise and noise figure. Figure 45 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
.
V
OUT
UNTERMINATED
+
–
V
IN
R
IN
R
S
V
OUT
RESISTIVE TERMINATION
+
–
V
IN
R
IN
R
S
R
S
V
OUT
ACTIVE IMPEDANCE MATCH
+
–
V
IN
R
IN
R
FB
R
FB
1 + A/2
R
S
R
IN
=
06304-104
Figure 43. Input Configurations