AD8332 AD [Analog Devices], AD8332 Datasheet - Page 19

• Manufacturer Part Number: AD8332
• Description: Ultralow Noise VGAs with Preamplifier and Programmable RIN
• Manufacturer: AD [Analog Devices]

Figure 62. Noise Figure vs. R
7
6
5
4
3
2
1
0
7
6
5
4
3
2
1
0
50
50
V
V
V
IN
IN
IN
SIMULATION
ACTIVE IMPEDANCE MATCH –R
+
–
+
–
+
–
SIMULATION
Figure 61. Noise Figure vs. R
Active Matched, and Unterminated Inputs
INCLUDES NOISE OF VGA
R
R
R
RESISTIVE TERMINATION
S
S
S
100
R
Figure 60. Input Configurations
100
IN
UNTERMINATED
R
=
R
R
R
IN
ACTIVE IMPEDANCE MATCH
S
1 + 4.5
IN
IN
IN
for Various Fixed Values of R
INCLUDES NOISE OF VGA
R
R
RESISTIVE TERMINATION
= 50 Ω
100 Ω
200 Ω
FB
70 Ω
FB
R
UNTERMINATED
= ∞
S
R
S
R
R
(R
( Ω )
FB
S
S
( Ω )
= R
IN
S
= R
)
S
for Resistive,
V
V
V
OUT
OUT
IN
OUT
IN
, Actively Matched
1k
1k
Rev. C | Page 19 of 32
The primary purpose of input impedance matching is to
improve the system transient response. With resistive
termination, the input noise increases due to the thermal noise
of the matching resistor and the increased contribution of the
LNA’s input voltage noise generator. With active impedance
matching, however, the contributions of both are smaller than
they would be for resistive termination by a factor of 1/(1 +
LNA Gain ). Figure 61 shows their relative noise figure (NF)
performance. In this graph, the input impedance has been swept
with R
for a source impedance of 50 Ω are 7.1 dB, 4.1 dB, and 2.5 dB,
respectively, for the resistive, active, and unterminated
configurations. The noise figures for 200 Ω are 4.6 dB, 2.0 dB,
and 1.0 dB, respectively.
Figure 62 is a plot of the NF versus R
which is helpful for design purposes. The plateau in the NF for
actively matched inputs mitigates source impedance variations.
For comparison purposes, a preamp with a gain of 19 dB and
noise spectral density of a 1.0 nV/√Hz, combined with a VGA
with 3.75 nV/√Hz, would yield a noise figure degradation of
approximately 1.5 dB (for most input impedances), significantly
worse than the AD8332 performance.
The equivalent input noise of the LNA is the same for single-
ended and differential output applications. The LNA noise
figure improves to 3.5 dB at 50 Ω without VGA noise, but this
is exclusive of noise contributions from other external circuits
connected to LOP. A series output resistor is usually
recommended for stability purposes, when driving external
circuits on a separate board (see the Applications section). In
low noise applications, a ferrite bead is even more desirable.
VARIABLE GAIN AMPLIFIER
The differential X-AMP VGA provides precise input
attenuation and interpolation. It has a low input-referred noise
of 2.7 nV/√Hz and excellent gain linearity. A simplified block
diagram is shown in Figure 63.
GAIN
VIN
VIP
g
S
m
to preserve the match at each point. The noise figures
6dB
R
GAIN INTERPOLATOR
(BOTH CHANNELS)
2R
Figure 63. Simplified VGA Schematic
S
48dB
for various values of R
AD8331/AD8332
POST-AMP
POST-AMP
IN
,