AD9251BCPZRL7 Analog Devices, Inc., AD9251BCPZRL7 Datasheet - Page 19

• Manufacturer Part Number: AD9251BCPZRL7
• Description: 14-bit, 20 Msps/40 Msps/65 Msps/80 Msps, 1.8 V Dual Analog-to-digital Converter
• Manufacturer: Analog Devices, Inc.

Preliminary Technical Data
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 33), and the
driver can be configured in a Sallen-Key filter topology to
provide band limiting of the input signal.
For baseband applications where SNR is a key parameter,
differential transformer coupling is the recommended input
configuration. An example is shown in Figure 34. To bias the
analog input, the VCM voltage can be connected to the center
tap of the secondary winding of the transformer.
VIN
0.1µF
Figure 33. Differential Input Configuration Using the ADA4938
76.8Ω
Figure 31. SNR/SFDR vs. Common-Mode Voltage,
Figure 32. SNR/SFDR vs. Common-Mode Voltage,
120Ω
90Ω
f
f
IN
IN
= 30.5 MHz, f
= 9.7 MHz, f
ADA4938
200Ω
200Ω
ADA4938-2
33Ω
33Ω
S
10pF
S
= 20 MSPS
= 80 MSPS
differential drivers provide
VIN+
VIN–
ADC
AVDD
VCM
Rev. PrH | Page 19 of 36
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 where
SNR is a key parameter, differential double balun coupling is the
recommended input configuration (see Figure 36).
An alternative to using a transformer-coupled input at frequencies
in the second Nyquist zone is to use the AD8352 differential driver.
An example is shown in Figure 37. See the
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 recommended 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
200 to 300
>300
Single-Ended Input Configuration
A single-ended input can provide adequate performance in cost
sensitive applications. In this configuration, SFDR and distortion
performance degrade due to the large input common-mode swing.
If the source impedances on each input are matched, there should
be little effect on SNR performance. Figure 35 shows a typical
single-ended input configuration.
2V p-p
1V p-p
Figure 34. Differential Transformer-Coupled Configuration
10µF
49.9Ω
49.9Ω
Figure 35. Single-Ended Input Configuration
0.1µF
0.1µF
10µF
0.1µF
AVDD
1kΩ
1kΩ
1kΩ
1kΩ
AVDD
R Series
(Ω Each)
33
33
15
15
R
R
R
R
C
C
AD8352
C Differential (pF)
15
5
5
Open
VIN+
VIN–
VIN+
VIN–
ADC
ADC
data sheet
AD9251
VCM