Datasheet

AD9272
Rev. C | Page 23 of 44
Active Impedance Matching
The LNA consists of a single-ended voltage gain amplifier with
differential outputs, and the negative output is externally
available. For example, with a fixed gain of 8× (17.9 dB), an
active input termination is synthesized by connecting a
feedback resistor between the negative output pin, LO-x, and the
positive input pin, LI-x. This is a well known technique used for
interfacing multiple probe impedances to a single system. The
input resistance is shown in Equation 1.
)
2
1(
A
R
R
FB
IN
+
=
(1)
where A/2 is the single-ended gain or the gain from the LI-x
inputs to the LO-x outputs, and R
FB
is the resulting impedance
of the R
FB1
and R
FB2
combination (see Figure 38).
Because the amplifier has a gain of 8× 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 12.1 dB (4×). 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
must be
1000 Ω. If the simplified equation (Equation 2) is used to
calculate R
IN
, the value is 188 Ω, resulting in a gain error less
than 0.6 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 must be considered. The user must determine the
level of matching accuracy and adjust R
FB
accordingly.
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 39 shows R
IN
vs. frequency for various values of R
FB
.
07029-188
10
100
1k
100k 1M 10M 100M
INPUT RESISTANCE ()
FREQUENCY (Hz)
R
S
= 50, R
FB
= 200, C
SH
= 70pF
R
S
= 100, R
FB
= 400, C
SH
= 20pF
R
S
= 200, R
FB
= 800
R
S
= 500, R
FB
= 2k
Figure 39. R
IN
vs. Frequency for Various Values of R
FB
(Effects of R
S
and C
SH
Are Also Shown)
Note that at the lowest value (50 Ω), 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
(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