AD9276-80KITZ Analog Devices Inc, AD9276-80KITZ Datasheet - Page 24

• Manufacturer Part Number: AD9276-80KITZ
• Description: 80MSPS ADC Converter Evaluation Board
• Manufacturer: Analog Devices Inc

Specifications of AD9276-80KITZ

• Silicon Manufacturer: Analog Devices
• Application Sub Type: ADC
• Kit Application Type: Data Converter
• Silicon Core Number: AD9276
• Number Of Adc's: 1
• Number Of Bits: 12
• Sampling Rate (per Second): 80M
• Data Interface: Serial, SPI™
• Inputs Per Adc: 1 Differential
• Input Range: *
• Power (typ) @ Conditions: 195mW @ 40MSPS
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 85°C
• Utilized Ic / Part: AD9276
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

AD9276
LNA Noise
The short-circuit noise voltage (input-referred noise) is an
important limit on system performance. The short-circuit noise
voltage for the LNA is 0.75 nV/√Hz at a gain of 21.3 dB, including
the VGA noise at a VGA postamp gain of 27 dB. These measure-
ments, 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 49.
Figure 50 and Figure 51 are simulations of noise figure vs. R
results using these configurations and an input-referred noise
voltage of 3.8 nV/√Hz for the VGA. Unterminated (R
operation exhibits the lowest equivalent input noise and noise
figure. Figure 51 shows the noise figure vs. source resistance
rising at low R
with the source noise—and at high R
from 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 match-
ing 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
. The lowest NF is achieved when R
S
—where the LNA voltage noise is large compared
LI-x
LI-x
LI-x
+
–
+
–
+
–
Figure 49. Input Configurations
ACTIVE IMPEDANCE MATCH
R
R
R
RESISTIVE TERMINATION
S
S
S
R
UNTERMINATED
IN
=
R
R
R
1 + A/2
IN
IN
IN
R
FB
R
S
S
R
due to the noise contribution
FB
V
V
V
S
OUT
OUT
OUT
matches R
FB
= ∞)
IN
.
S
Rev. 0 | Page 24 of 48
Figure 50 shows the relative noise figure performance. With an
LNA gain of 21.3 dB, the input impedance was swept with R
preserve the 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 51 shows the noise figure as it relates to R
values of R
12.0
10.5
Termination Matched, and Unterminated Inputs, V
9.0
7.5
6.0
4.5
3.0
1.5
Figure 50. Noise Figure vs. R
8
7
6
5
4
3
2
1
0
Figure 51. Noise Figure vs. R
0
10
10
UNTERMINATED
IN
Active Termination Matched Inputs, V
, which is helpful for design purposes.
R
R
R
R
UNTERMINATED
IN
IN
IN
IN
= 50Ω
= 75Ω
= 100Ω
= 200Ω
RESISTIVE TERMINATION
S
S
for Resistive Termination, Active
R
for Various Fixed Values of R
R
100
S
100
S
(Ω)
(Ω)
ACTIVE TERMINATION
GAIN
= 0.8 V
S
GAIN
for various
= 0.8 V
IN
1k
,
1k
S
to