AD9642 Analog Devices, AD9642 Datasheet - Page 18

• Manufacturer Part Number: AD9642
• Description: 14-Bit, 170 MSPS/210 MSPS/250 MSPS, 1.8 V Analog-to-Digital Converter (ADC)
• Manufacturer: Analog Devices

Specifications of AD9642

• Resolution (bits): 14bit
• # Chan: 1
• Sample Rate: 250MSPS
• Interface: LVDS
• Analog Input Type: Diff-Bip,Diff-Uni
• Ain Range: 1.75 V p-p
• Adc Architecture: Pipelined
• Pkg Type: CSP

AD9642
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, connect the VCM voltage 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 AD9642. For applications where
SNR is a key parameter, differential double balun coupling is
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. This resistor
compensates 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
2V p-p
Figure 48. Differential Transformer-Coupled Configuration
49.9Ω
0.1µF
2V p-p
R1
R1
C2
C1
C2
R3
0.1µF
R3
P
A
R2
R2
Figure 50. Differential Double Balun Input Configuration
VIN+
VIN–
S
0.1µF
ADC
S
VCM
P
0.1µF
0.1µF
Rev. 0 | Page 18 of 28
33Ω
33Ω
adjusted or some components may need to be removed. Table 9
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 9 are for each
R1, R2, C2, and R3 component shown in Figure 48 and Figure 50.
Table 9. 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
(DVGA) provides good performance for driving the AD9642.
Figure 49 shows an example of the
through a band-pass antialiasing filter.
0.1µF
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.
AD8375
Figure 49. Differential Input Configuration Using the
R1
R1
C2
C1
C2
R3
1µH
1µH
R3
1000pF
1000pF
R1
Series
(Ω)
33
15
R2
R2
VPOS
1nF
VIN+
VIN–
180nH
180nH
301Ω
5.1pF
C1
Differential
(pF)
8.2
3.9
AD8375
ADC
0.1µF
220nH
220nH
3.9pF
®
VCM
0603CS COMPONENTS WITH THE
165Ω
165Ω
digital variable gain amplifier
AD8375
R2
Series
(Ω)
0
0
15pF
VCM
1nF
driving the
68nH
C2
Shunt
(pF)
15
8.2
2.5kΩ║2pF
AD8375
AD9642
AD9642
R3
Shunt
(Ω)
49.9
49.9