AD9613 Analog Devices, AD9613 Datasheet - Page 24

• Manufacturer Part Number: AD9613
• Description: 12-bit, 170/210/250 MSPS, 1.8 V Dual Analog-to-Digital Converter (ADC)
• Manufacturer: Analog Devices

Specifications of AD9613

• Resolution (bits): 12bit
• # Chan: 2
• Sample Rate: 250MSPS
• Interface: LVDS,Par
• Analog Input Type: Diff-Bip
• Ain Range: 1.75 V p-p
• Adc Architecture: Pipelined
• Pkg Type: CSP

AD9613
Differential Input Configurations
Optimum performance is achieved while driving the AD9613
in a differential input configuration. For baseband applications, the
AD8138, ADA4937-2, ADA4938-2, and
drivers provide excellent performance and a flexible interface to
the ADC.
The output common-mode voltage of the ADA4930-2 is easily
set with the VCM pin of the AD9613 (see Figure 47), and the
driver can be configured in a Sallen-Key filter topology to
provide band limiting of the input signal.
VIN
For baseband applications where SNR is a key parameter,
differential transformer coupling is the recommended input
configuration. An example is shown in Figure 48. To bias the
analog input, the VCM voltage can be connected to the center
tap of the secondary winding of the transformer.
Table 10. Example RC Network
Frequency Range (MHz)
0 to 100
100 to 300
2V p-p
0.1µF
Figure 47. Differential Input Configuration Using the ADA4938-2
76.8Ω
Figure 48. Differential Transformer-Coupled Configuration
49.9Ω
120Ω
90Ω
0.1µF
ADA4930-2
200Ω
200Ω
2V p-p
33Ω
R1
R1
33Ω
33Ω
C2
C1
C2
R3
15pF
R1 Series (Ω)
33
15
5pF
0.1µF
R3
P
A
15pF
R2
R2
ADA4930-2
15Ω
15Ω
33Ω
Figure 49. Differential Double Balun Input Configuration
VIN+
VIN–
S
VIN+
VIN–
0.1µF
ADC
S
ADC
differential
VCM
C1 Differential (pF)
8.2
3.9
AVDD
VCM
P
0.1µF
0.1µF
0.1µF
Rev. B | Page 24 of 36
33Ω
33Ω
0.1µF
The signal characteristics must be considered when selecting
a transformer. Most RF transformers saturate at frequencies
below a few megahertz. 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 AD9613. For applications where
SNR is a key parameter, differential double balun coupling is
the recommended input configuration (see Figure 49). In this
configuration, the input is ac-coupled, and the CML is provided
to each input through a 33 Ω resistor. These resistors compensate
for losses in the input baluns to provide a 50 Ω impedance to
the driver.
In the double balun and transformer configurations, the value
of the input capacitors and resistors is dependent on the input
frequency and source impedance. Based on these parameters,
the value of the input resistors and capacitors may need to be
adjusted or some components may need to be removed. Table 10
displays recommended values to set the RC network for different
input frequency ranges. However, these values are dependent
on the input signal and bandwidth and should be used only as
a starting guide. Note that the values given in Table 10 are for
each R1, R2, C2, and R3 component shown in Figure 48 and
Figure 49.
An alternative to using a transformer-coupled input at frequencies
in the second Nyquist zone is to use an amplifier with variable
gain. The
(DVGAs) provides good performance for driving the AD9613.
Figure 50 shows an example of the AD8376 driving the AD9613
through a band-pass antialiasing filter.
R2 Series (Ω)
0
0
R1
R1
C2
C1
C2
R3
AD8375
R3
R2
R2
33Ω
or
VIN+
VIN–
AD8376
ADC
C2 Shunt (pF)
15
8.2
VCM
digital variable gain amplifier
0.1µF
Data Sheet
R3 Shunt (Ω)
49.9
49.9