AD8331ARQ-REEL Analog Devices Inc, AD8331ARQ-REEL Datasheet - Page 26

• Manufacturer Part Number: AD8331ARQ-REEL
• Description: IC VGA SINGLE W/PREAMP 20-SSOP
• Manufacturer: Analog Devices Inc
• Series: X-AMP®r
• Type: Variable Gain Amplifierr

Specifications of AD8331ARQ-REEL

• Rohs Status: RoHS non-compliant
• Design Resources: Interfacing the High Frequency AD8331 to AD9215 (CN0096)
• Applications: Signal Processing
• Mounting Type: Surface Mount
• Package / Case: 20-QSOP
• For Use With: AD8331-EVALZ - BOARD EVAL FOR AD8331

AD8331/AD8332/AD8334
Active Impedance Matching
The LNA supports active impedance matching through an external
shunt feedback resistor from Pin LON to Pin INH. The input
resistance, R
ended gain of 4.5, and 6 kΩ is the unterminated input impedance.
C
and Pin INH are unequal. Expressions for choosing R
of R
Information section. C
at higher frequencies, where the loop gain is diminished, and
prevent peaking. Frequency response plots of the LNA are shown
in Figure 23 and Figure 24. The bandwidth is approximately
130 MHz for matched input impedances of 50 Ω to 200 Ω and
declines at higher source impedances. The unterminated
bandwidth (when R
Each output can drive external loads as low as 100 Ω in addition
to the 100 Ω input impedance of the VGA (200 Ω differential).
Capacitive loading up to 10 pF is permissible. All loads should
be ac-coupled. Typically, Pin LOP output is used as a single-ended
driver for auxiliary circuits, such as those used for Doppler
ultrasound imaging. Pin LON drives R
differential external circuit can be driven from the two outputs
in addition to the active feedback termination. In both cases,
important stability considerations discussed in the Applications
Information section should be carefully observed.
The impedance at each LNA output is 5 Ω. A 0.4 dB reduction
in open circuit gain results when driving the VGA, and a 0.8 dB
reduction results with an additional 100 Ω load at the output.
The differential gain of the LNA is 6 dB higher. If the load is less
than 200 Ω on either side, a compensating load is recommended
on the opposite output.
LNA Noise
The input-referred voltage noise sets an important limit on
system performance. The short-circuit input voltage noise of
the LNA is 0.74 nV/√Hz or 0.82 nV/√Hz (at maximum gain),
including the VGA noise. The open circuit, current noise is
2.5 pA/√Hz. These measurements, taken without a feedback
resistor, provide the basis for calculating the input noise and
noise figure performance of the configurations in Figure 75.
Figure 76 and Figure 77 show simulations extracted from these
results and the 4.1 dB noise figure (NF) measurement with the
input actively matched to a 50 Ω source. Unterminated (R
operation exhibits the lowest equivalent input noise and noise
figure. Figure 76 shows the noise figure vs. source resistance,
rising at low R
to the source noise, and again at high R
The VGA input-referred voltage noise of 2.7 nV/√Hz is
included in all of the curves.
IZ
is needed in series with R
IN
R
and for choosing C
IN
=
1
IN
R
+
, is given in Equation 5, where A is the single-
IZ
S
A
, where the LNA voltage noise is large compared
6
IZ
kΩ
= ∞) is approximately 80 MHz.
SH
=
and the ferrite bead enhance stability
IZ
33
6
are found in the Applications
kΩ
IZ
kΩ
because the dc levels at Pin LON
×
+
R
R
IZ
IZ
IZ
S
. Alternatively, a
due to current noise.
IZ
IZ
in terms
= ∞)
Rev. G | Page 26 of 56
(5)
Figure 77. Noise Figure vs. R
7
6
5
4
3
2
1
0
7
6
5
4
3
2
1
0
50
50
SIMULATION
INCLUDES NOISE OF VGA
(SIMULATED RESULTS)
V
V
V
Figure 76. Noise Figure vs. R
Active Match, and Unterminated Inputs
IN
IN
IN
ACTIVE IMPEDANCE MATCH - R
+
–
+
–
+
–
Figure 75. Input Configurations
100
100
R
R
R
RESISTIVE TERMINATION
R
S
S
S
R
R
S
R
IN
IN
for Various Fixed Values of R
IN
R
IN
R
= 100Ω
UNTERMINATED
IN
= 50Ω
= 200Ω
IZ
= 75Ω
RESISTIVE TERMINATION
=
R
R
R
=
ACTIVE IMPEDANCE MATCH
IN
IN
IN
1 + 4.5
UNTERMINATED
∞
R
INCLUDES NOISE OF VGA
IZ
R
R
R
(R
S
S
S
S
(Ω)
(Ω)
= R
R
IZ
S
IN
for Resistive,
)
S
= R
V
V
V
OUT
OUT
OUT
IN
IN
, Actively Matched
1k
1k