Datasheet

AD9277
Rev. 0 | Page 23 of 48
Low value feedback resistors and the current-driving capability
of the output stage allow the LNA to achieve a low input-
referred noise voltage of 0.75 nV/√Hz (at a gain of 21.3 dB).
This is achieved with a current consumption of only 27 mA per
channel (80 mW). On-chip resistor matching results in precise
single-ended gains, which are critical for accurate impedance
control. The use of a fully differential topology and negative
feedback minimizes distortion. Low second-order harmonic
distortion is particularly important in second harmonic ultra-
sound imaging applications. Differential signaling enables
smaller swings at each output, further reducing third-order
harmonic distortion.
Active Impedance Matching
The LNA consists of a single-ended voltage gain amplifier with
differential outputs and the negative output 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 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.
R
FB
is the resulting impedance of the R
FB1
and R
FB2
combination
(see Figure 47).
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 that it is 6 dB less
than the gain of the amplifier, or 11.9 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
required 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 of less than 0.6 dB.
Some factors, such as the presence of a dynamic source resistance,
may 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 48 shows R
IN
vs. frequency for various values of R
FB
.
08181-042
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 48. 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 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,
as mentioned previously.
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 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