AD9251 Analog Devices, AD9251 Datasheet - Page 21

• Manufacturer Part Number: AD9251
• Description: 14-Bit, 20 MSPS/40 MSPS/65 MSPS/80 MSPS, 1.8 V Dual Analog-to-Digital Converter
• Manufacturer: Analog Devices

Specifications of AD9251

• Resolution (bits): 14bit
• # Chan: 2
• Sample Rate: 80MSPS
• Interface: Par
• Analog Input Type: Diff-Uni
• Ain Range: (2Vref) p-p,2 V p-p
• Adc Architecture: Pipelined
• Pkg Type: CSP

Input Common Mode
The analog inputs of the AD9251 are not internally dc-biased.
Therefore, in ac-coupled applications, the user must provide a
dc bias externally. Setting the device so that VCM = AVDD/2 is
recommended for optimum performance, but the device can
function over a wider range with reasonable performance, as
shown in Figure 39 and Figure 40.
An on-board, common-mode voltage reference is included in
the design and is available from the VCM pin. The VCM pin
must be decoupled to ground by a 0.1 μF capacitor, as described
in the Applications Information section.
Differential Input Configurations
Optimum performance is achieved while driving the AD9251 in a
differential input configuration. For baseband applications, the
AD8138, ADA4937-2, and
excellent performance and a flexible interface to the ADC.
The output common-mode voltage of the ADA4938-2 is easily
set with the VCM pin of the AD9251 (see Figure 41), and the
driver can be configured in a Sallen-Key filter topology to
provide band limiting of the input signal.
100
100
90
80
70
60
50
90
80
70
60
50
0.5
0.5
Figure 39. SNR/SFDR vs. Input Common-Mode Voltage,
Figure 40. SNR/SFDR vs. Input Common-Mode Voltage,
0.6
0.6
INPUT COMMON-MODE VOLTAGE (V)
INPUT COMMON-MODE VOLTAGE (V)
0.7
0.7
f
f
IN
IN
= 32.1 MHz, f
= 10.3 MHz, f
ADA4938-2
0.8
0.8
SFDR (dBc)
SNR (dBFS)
SFDR (dBc)
SNR (dBFS)
0.9
0.9
S
S
= 80 MSPS
= 20 MSPS
1.0
1.0
differential drivers provide
1.1
1.1
1.2
1.2
1.3
1.3
Rev. A | Page 21 of 36
For baseband applications below ~10 MHz where SNR is a key
parameter, differential transformer-coupling is the recommended
input configuration. An example is shown in Figure 42. To bias
the analog input, the VCM voltage can be connected to the
center tap of the secondary winding of the transformer.
The signal characteristics must be considered when selecting
a transformer. Most RF transformers saturate at frequencies
below a few megahertz (MHz). Excessive signal power can also
cause core saturation, which leads to distortion.
At input frequencies in the second Nyquist zone and above, the
noise performance of most amplifiers is not adequate to achieve
the true SNR performance of the AD9251. For applications above
~10 MHz where SNR is a key parameter, differential double balun
coupling is the recommended input configuration (see Figure 44).
An alternative to using a transformer-coupled input at frequencies
in the second Nyquist zone is to use the
An example is shown in Figure 45. See the AD8352 data sheet
for more information.
In any configuration, the value of Shunt Capacitor C is dependent
on the input frequency and source impedance and may need to
be reduced or removed. Table 9 displays the suggested values to set
the RC network. However, these values are dependent on the
input signal and should be used only as a starting guide.
Table 9. Example RC Network
Frequency Range (MHz)
0 to 70
70 to 200
2V p-p
VIN
Figure 41. Differential Input Configuration Using the ADA4938-2
0.1µF
Figure 42. Differential Transformer-Coupled Configuration
76.8Ω
49.9Ω
120Ω
90Ω
0.1µF
ADA4938
200Ω
200Ω
R Series
(Ω Each)
33
125
33Ω
33Ω
R
R
10pF
C
AD8352
C Differential (pF)
22
Open
VIN–x
VIN+x
VIN+x
VIN–x
differential driver.
ADC
ADC
AD9251
AVDD
VCM
VCM