AD9634 Analog Devices, AD9634 Datasheet - Page 20

• Manufacturer Part Number: AD9634
• Description: 12-Bit, 170 MSPS/210 MSPS/250 MSPS, 1.8 V Analog-to-Digital Converter
• Manufacturer: Analog Devices

Specifications of AD9634

• Resolution (bits): 12bit
• # Chan: 1
• Sample Rate: 250MSPS
• Interface: LVDS
• Analog Input Type: Diff-Uni
• Ain Range: 1.75 V p-p
• Adc Architecture: Pipelined
• Pkg Type: CSP

AD9634
Differential Input Configurations
Optimum performance can be achieved when driving the
AD9634
applications, the AD8138, ADA4937-1, and
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
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, connect the VCM voltage to the center tap of the
secondary winding of the transformer.
Table 9. Example RC Network
Frequency Range (MHz)
0 to 100
100 to 300
0.1µF
Figure 47. Differential Input Configuration Using the
76.8Ω
in a differential input configuration. For baseband
120Ω
90Ω
ADA4930-1
200Ω
200Ω
2V p-p
AD9634
R1 Series (Ω)
33
15
33Ω
33Ω
15pF
5pF
0.1µF
(see Figure 47), and the
P
A
15pF
15Ω
15Ω
ADA4930-1
ADA4930-1
Figure 49. Differential Double Balun Input Configuration
S
VIN–
VIN+
ADA4930-1
S
0.1µF
ADC
C1 Differential (pF)
8.2
3.9
AVDD
VCM
is easily
P
0.1µF
0.1µF
Rev. 0 | Page 20 of 32
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 AD9634. 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 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
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 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 the
R1, R2, R3, C1, and C2 components shown in Figure 49.
R2 Series (Ω)
0
0
2V p-p
R1
R1
Figure 48. Differential Transformer-Coupled Configuration
C2
C1
C2
R3
49.9Ω
R3
R2
R2
0.1µF
VIN+
VIN–
C2 Shunt (pF)
15
8.2
ADC
0.1µF
VCM
R1
R1
C1
C2
C2
R3
R3
R2
R2
R3 Shunt (Ω)
49.9
49.9
VIN+
VIN–
0.1µF
ADC
VCM