Changes To Active Impedance Matching Section And Figure 40 - Analog Devices AD9273 Manual

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.
R
=
FB
R
IN
1 ( A +
) 2
where A/2 is the single-ended gain or the gain from the LI-x
inputs to the LO-x outputs, and R
of the R and R combination (see Figure 39).
FB1 FB2
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
for a desired R , even for higher values of R
IN
R
= Ω
FB
R || 15 k
IN +
1 ( ) 3
For example, to set R to 200 Ω, the value of R
IN
1000 Ω. If the simplified equation (Equation 2) is used to
calculate R , the value is 188 Ω, resulting in a gain error less
IN
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
The bandwidth (BW) of the LNA is greater than 100 MHz.
Ultimately, the BW of the LNA limits the accuracy of the
synthesized R . For R = R
IN IN S
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
Figure 40 shows R vs. frequency for various values of R
IN
is the resulting impedance
FB
.
IN
must be
FB
accordingly.
FB
up to about 200 Ω, the best match
limit the BW at higher frequencies.
FB
1k
R
R
R
100
R
(1)
10
100k
Figure 40. R
Note that at the lowest value (50 Ω), R
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
starts rolling off the signal BW before the LNA can produce
peaking. C
SH
FB
not be used for values of R
lists the recommended values for R
(2)
C is needed in series with R
FB
and Pin LI-x are unequal.
Table 7. Active Termination External Component Values
LNA Gain
(dB)
15.6
17.9
21.3
15.6
17.9
21.3
15.6
17.9
21.3
.
Rev. B | Page 23 of 48
= 500Ω, R = 2kΩ
S FB
= 200Ω, R = 800Ω
S FB
= 100Ω, R = 400Ω, C = 20pF
S FB SH
= 50Ω, R = 200Ω, C = 70pF
S FB SH
1M 10M
FREQUENCY (Hz)
vs. Frequency for Various Values of R
IN
(Effects of R and C Are Also Shown)
S SH
peaks at frequencies
IN
values, parasitic capacitance
IN
further degrades the match; therefore, C
that are greater than 100 Ω. Table 7
IN
and C
FB
because the dc levels at Pin LO-x
FB
Minimum
R (Ω) R (Ω) C
IN FB
50 200 90
50 250 70
50 350 50
100 400 30
100 500 20
100 700 10
200 800 N/A
200 1000 N/A
200 1400 N/A
AD9273
100M
FB
should
SH
in terms of R .
SH IN
(pF) BW (MHz)
SH
57
69
88
57
69
88
72
72
72