AD9643 Analog Devices, AD9643 Datasheet - Page 24

• Manufacturer Part Number: AD9643
• Description: 14-Bit, 170/210/250 MSPS, 1.8 V Dual Analog-to-Digital Converter (ADC)
• Manufacturer: Analog Devices

Specifications of AD9643

• Resolution (bits): 14bit
• # 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

AD9643
differential drivers provide excellent performance and a flexible
interface to the ADC.
The output common-mode voltage of the
set with the VCM pin of the AD9643 (see Figure 47), 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 48. 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. 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 AD9643. For applications where
SNR is a key parameter, differential double balun coupling is
VIN
2V p-p
0.1µF
Figure 47. Differential Input Configuration Using the ADA4930-2
76.8Ω
Figure 48. Differential Transformer-Coupled Configuration
49.9Ω
120Ω
90Ω
0.1µF
ADA4930-2
200Ω
200Ω
2V p-p
R1
R1
33Ω
C2
C1
C2
33Ω
33Ω
R3
15pF
5pF
0.1µF
R3
P
A
15pF
R2
R2
15Ω
15Ω
ADA4930-2
33Ω
Figure 50. Differential Double Balun Input Configuration
VIN+
VIN–
S
VIN–
VIN+
ADC
S
ADC
VCM
AVDD
is easily
VCM
0.1µF
P
0.1µF
0.1µF
0.1µF
Rev. B | Page 24 of 36
33Ω
33Ω
0.1µF
the recommended input configuration (see Figure 50). In this
configuration, the input is ac-coupled and the VCM voltage 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 fre-
quency 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 50.
Table 10. Example RC Network
Frequency
Range
(MHz)
0 to 100
100 to 300
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 AD9643.
Figure 49 shows an example of the AD8376 driving the AD9643
through a band-pass antialiasing filter.
NOTES
1. ALL INDUCTORS ARE COILCRAFT
2. FILTER VALUES SHOWN ARE FOR A 20MHz BANDWIDTH FILTER
EXCEPTION OF THE 1µH CHOKE INDUCTORS (COIL CRAFT 0603LS).
CENTERED AT 140MHz.
AD8376
R1
R1
Figure 49. Differential Input Configuration Using the AD8376
C2
C1
C2
R3
1µH
1µH
AD8375
R3
R1
Series
(Ω)
33
15
R2
R2
1000pF
1000pF
VPOS
1nF
33Ω
or
VIN+
VIN–
180nH
180nH
301Ω
5.1pF
AD8376
C1
Differential
(pF)
8.2
3.9
ADC
220nH
220nH
3.9pF
®
VCM
0603CS COMPONENTS WITH THE
digital variable gain amplifier
0.1µF
165Ω
165Ω
R2
Series
(Ω)
0
0
15pF
VCM
1nF
68nH
Data Sheet
C2
Shunt
(pF)
15
8.2
2.5kΩ║2pF
AD9643
R3
Shunt
(Ω)
49.9
49.9