AD9273
Rev. B | Page 23 of 48
Active Impedance Matching
The LNA consists of a single-ended voltage gain amplifier with
differential outputs and the negative output available externally.
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 well-known technique is 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 39).
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 while Pin LG-x is 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
+
=
k
15
||
)
3
1
(
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 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 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 40 shows R
IN
vs. frequency for various values of R
FB
.
10
100
1k
100k
1M
10M
100M
INP
UT
RE
S
IS
T
A
NCE
(
Ω
)
FREQUENCY (Hz)
R
S
= 50
Ω
, R
FB
= 200
Ω
, C
SH
= 70pF
R
S
= 100
Ω
, R
FB
= 400
Ω
, C
SH
= 20pF
R
S
= 200
Ω
, R
FB
= 800
Ω
R
S
= 500
Ω
, R
FB
= 2k
Ω
07
03
0-
1
88
Figure 40. 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
Summary of Contents for AD9273
Page 47: ...AD9273 Rev B Page 46 of 48 NOTES...
