AD9273BSVZ-25 Analog Devices Inc, AD9273BSVZ-25 Datasheet - Page 24

• Manufacturer Part Number: AD9273BSVZ-25
• Description: 12Bit 25 MSPS Octal ADC
• Manufacturer: Analog Devices Inc
• Type: Crosspoint Switchr

Specifications of AD9273BSVZ-25

• Resolution (bits): 12 b
• Sampling Rate (per Second): 25M
• Data Interface: Serial
• Voltage Supply Source: Single Supply
• Voltage - Supply: 1.7 V ~ 3.6 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 100-TQFP Exposed Pad, 100-eTQFP, 100-HTQFP, 100-VQFP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

AD9273
LNA Noise
The short-circuit noise voltage (input-referred noise) is an impor-
tant limit on system performance. The short-circuit input-referred
noise voltage for the LNA is 1.4 nV/√Hz at a gain of 21.3 dB,
including the VGA noise at a VGA postamp gain of 27 dB. These
measurements, which were taken without a feedback resistor,
provide the basis for calculating the input noise and noise figure
(NF) performance of the configurations shown in Figure 41.
Figure 42 and Figure 43 are simulations of noise figure vs. source
resistance (R
referred noise voltage of 6 nV/√Hz for the VGA. Unterminated
(R
and noise figure. Figure 43 shows the noise figure vs. R
low R
source noise—and at high R
R
The main purpose of input impedance matching is to improve the
transient response of the system. 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 (by a factor of
1/(1 + LNA Gain)) than they would be for resistive termination.
FB
FB
. The lowest NF is achieved when R
= ∞) operation exhibits the lowest equivalent input noise
S
—where the LNA voltage noise is large compared with the
S
) results using these configurations and an input-
LI-x
LI-x
LI-x
+
–
+
–
+
–
Figure 41. Input Configurations
ACTIVE IMPEDANCE MATCH
R
R
R
RESISTIVE TERMINATION
S
S
S
R
UNTERMINATED
IN
=
R
R
R
S
1 + A/2
IN
IN
IN
due to the noise contribution from
R
FB
R
S
R
FB
S
matches R
V
V
V
OUT
OUT
OUT
IN
.
S
rising at
Rev. B | Page 24 of 48
Figure 42 shows the relative noise figure performance. In this
graph, the input impedance was swept with R
match at each point. The noise figures for a source impedance of
50 Ω are 7.3 dB, 4.2 dB, and 2.8 dB for the resistive termination,
active termination, and unterminated configurations, respectively.
The noise figures for 200 Ω are 4.5 dB, 1.7 dB, and 1.0 dB,
respectively.
Figure 43 shows the noise figure as it relates to R
of R
Active Termination Matched, and Unterminated Inputs, V
IN
, which is helpful for design purposes.
12.0
10.5
12.0
10.5
9.0
7.5
6.0
4.5
3.0
1.5
9.0
7.5
6.0
4.5
3.0
1.5
Figure 43. Noise Figure vs. R
0
0
10
10
Figure 42. Noise Figure vs. R
Active Termination Matched Inputs, V
UNTERMINATED
UNTERMINATED
R
R
R
R
IN
IN
IN
IN
S
R
R
= 200Ω
= 100Ω
= 75Ω
= 50Ω
for Various Fixed Values of R
100
100
S
S
S
(Ω)
(Ω)
for Shunt Termination,
SHUNT TERMINATION
ACTIVE
TERMINATION
GAIN
S
to preserve the
S
= 0.8 V
for various values
GAIN
= 0.8 V
IN
,
1k
1k