AD9649 Datasheet, PDF (18/32 Pages) Analog Devices – 14-Bit, 20/40/65/80 MSPS, 1.8 V Analog-to-Digital Converter

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 ADA4938-2 differential drivers
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.
VIN
76.8Ω
0.1µF
200Ω
33Ω
90Ω
10pF
ADA4938-2
120Ω
33Ω
200Ω
VIN– _AVDD
ADC
VIN+ ^VCM
Figure 38. Differential Input Configuration Using the ADA4938-2
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.
2V p-p
49.9Ω
VIN+
R
C
ADC
R
VIN–
VCM
0.1µF
Figure 39. Differential Transformer-Coupled Configuration
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 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 AD8352 differential driver.
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)
R Series
(Ω Each)
0 to 70
33
70 to 200
125
C Differential (pF)
22
Open
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.
1V p-p
49.9Ω
10µF
10µF
AVDD
1kΩ
R
0.1µF _1kΩ
AVDD
C
1kΩ _R
0.1µF 1kΩ
VIN+
ADC
VIN–
Figure 40. Single-Ended Input Configuration
2V p-p
0.1µF
P_A
SS
0.1µF
25Ω
P
25Ω
0.1µF
R
C
0.1µF
R
VIN+
ADC
VIN–
VCM
Figure 41. Differential Double Balun Input Configuration
0.1µF
ANALOG INPUT
0Ω ₁₆
1
2
C_D
ANALOG INPUT
R_D
R_{G 3}
4
5
0.1µF ^0Ω
V_CC
8, 13
11
AD8352
10
14
0.1µF
0.1µF
0.1µF
0.1µF
R
200Ω
200Ω
C
R
0.1µF
VIN+
ADC
VIN–
VCM
Figure 42. Differential Input Configuration Using the AD8352
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