AD9649 Analog Devices, AD9649 Datasheet - Page 18

• Manufacturer Part Number: AD9649
• Description: 14-Bit, 20/40/65/80 MSPS, 1.8 V Analog-to-Digital Converter
• Manufacturer: Analog Devices

Specifications of AD9649

• Resolution (bits): 14bit
• # Chan: 1
• Sample Rate: 80MSPS
• Interface: Par
• Analog Input Type: Diff-Uni
• Ain Range: 2 V p-p
• Adc Architecture: Pipelined
• Pkg Type: CSP

AD9649
Differential Input Configurations
Optimum performance is achieved while driving the AD9649
in a differential input configuration. For baseband applications,
the AD8138, ADA4937-2, and
provide 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 AD9649 (see Figure 38), and the
driver can be configured in a Sallen-Key filter topology to
provide band limiting of the input signal.
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 39. 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
2V p-p
VIN
Figure 38. Differential Input Configuration Using the ADA4938-2
0.1µF
Figure 39. Differential Transformer-Coupled Configuration
76.8Ω
49.9Ω
120Ω
90Ω
0.1µF
ADA4938-2
ANALOG INPUT
ANALOG INPUT
200Ω
200Ω
2V p-p
ADA4938-2
33Ω
33Ω
R
R
C
10pF
C
D
0.1µF
P
0.1µF
0.1µF
A
R
D
differential drivers
0Ω
0Ω
Figure 42. Differential Input Configuration Using the AD8352
Figure 41. Differential Double Balun Input Configuration
VIN+
VIN–
VIN–
VIN+
S
R
G
16
1
2
3
4
5
ADC
ADC
S
AVDD
VCM
VCM
AD8352
V
P
CC
8, 13
14
0.1µF
Rev. 0 | Page 18 of 32
0.1µF
0.1µF
10
11
25Ω
25Ω
0.1µF
0.1µF
the true SNR performance of the AD9649. For applications above
~10 MHz where SNR is a key parameter, differential double balun
coupling is the recommended input configuration (see Figure 41).
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 42. 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
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 40 shows a typical
single-ended input configuration.
0.1µF
200Ω
200Ω
1V p-p
0.1µF
R
R
0.1µF
C
R
R
10µF
C
49.9Ω
Figure 40. Single-Ended Input Configuration
VIN+
VIN–
0.1µF
10µF
0.1µF
ADC
AVDD
VIN+
VIN–
1kΩ
1kΩ
1kΩ
1kΩ
VCM
AVDD
ADC
R Series
(Ω Each)
33
125
VCM
R
R
C
AD8352
C Differential (pF)
22
Open
VIN+
VIN–
differential driver.
ADC