AD9641 Analog Devices, AD9641 Datasheet - Page 18

• Manufacturer Part Number: AD9641
• Description: 14-Bit, 80 MSPS/155 MSPS, 1.8 V Serial Output Analog-to-Digital Converter (ADC)
• Manufacturer: Analog Devices

Specifications of AD9641

• Resolution (bits): 14bit
• # Chan: 1
• Sample Rate: 80MSPS
• Interface: Ser
• Analog Input Type: Diff-Bip
• Ain Range: 1.75 V p-p
• Adc Architecture: Pipelined
• Pkg Type: CSP

AD9641
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.
For baseband applications in which SNR is a key parameter,
differential transformer coupling is the recommended input
configuration. An example is shown in Figure 50. To bias the
analog input, the voltage of VCM can be connected to the
center tap of the secondary winding of the transformer.
Table 9. Example RC Network
Frequency Range (MHz)
0 to 100
100 to 250
VIN
2V p-p
Figure 49. Differential Input Configuration Using the ADA4938-2
0.1µF
Figure 50. Differential Transformer-Coupled Configuration
76.8Ω
49.9Ω
120Ω
90Ω
0.1µF
ADA4938-2
200Ω
200Ω
R1 Series (Ω Each)
33
15
AD9641
2V p-p
R1
R1
33Ω
33Ω
C1
C2
C2
R3
15pF
5pF
R3
(see Figure 49), and the
0.1µF
15pF
R2
R2
NOTES
1. ALL INDUCTORS ARE COILCRAFT 0603CS COMPONENTS
2. FILTER VALUES SHOWN ARE FOR A 20MHz BANDWIDTH FILTER
15Ω
15Ω
P
AD8376
A
WITH THE EXCEPTION OF THE 1µH CHOKE INDUCTORS (0603LS).
CENTERED AT 140MHz.
ADA4938-2
Figure 52. Differential Input Configuration Using the AD8376
VIN+
VIN–
Figure 51. Differential Double Balun Input Configuration
VIN–
VIN+
1µH
1µH
S
ADC
ADC
C1 Differential (pF)
8.2
3.9
1000pF
1000pF
VCM
S
AVDD
VPOS
VCM
1nF
is easily
180nH
301Ω
180nH
P
Rev. B | Page 18 of 36
0.1µF
0.1µF
5.1pF
220nH
220nH
33Ω
33Ω
3.9pF
165Ω
165Ω
0.1µF
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 AD9641. For applications where
SNR is a key parameter, differential double balun coupling is the
recommended input configuration (see Figure 51). In this configu-
ration, the input is ac-coupled, and the common-mode voltage
(VCM) 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.
An alternative to using a transformer-coupled input at frequencies
in the second Nyquist zone is to use the
amplifier. An example drive circuit including a band-pass filter
is shown in Figure 52. See the
information.
VCM
15pF
1nF
R2 Series (Ω Each)
0
0
R1
R1
C2
C1
C2
R3
68nH
R3
R2
R2
AD9641
VIN+
VIN–
ADC
VCM
C2 Shunt (pF Each)
8.2
Open
AD8376
data sheet for more
AD8376
Data Sheet
R3 Shunt (Ω Each)
49.9
Open
variable gain