AD9644 Analog Devices, AD9644 Datasheet - Page 21

• Manufacturer Part Number: AD9644
• Description: 14-Bit, 80 MSPS/155 MSPS, 1.8V Dual, Serial Output A/D Converter
• Manufacturer: Analog Devices

Specifications of AD9644

• Resolution (bits): 14bit
• # Chan: 2
• Sample Rate: 155MSPS
• Analog Input Type: Diff-Bip
• Ain Range: 1.75 V p-p
• Adc Architecture: Pipelined

Data Sheet
VIN
Differential Input Configurations
Optimum performance is achieved while driving the AD9644 in a
differential input configuration. For baseband applications, the
AD8138, ADA4937-2, and
excellent performance and a flexible interface to the ADC.
The output common-mode voltage of the ADA4938-2 is easily
set with the VCM pin of the AD9644 (see Figure 49), and 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 VCM voltage can be connected to the center
tap of the secondary winding of the transformer.
2V p-p
0.1µF
Figure 49. Differential Input Configuration Using the ADA4938-2
76.8Ω
Figure 50. Differential Transformer-Coupled Configuration
49.9Ω
120Ω
90Ω
0.1µF
ADA4938-2
200Ω
200Ω
ADA4938-2
2V p-p
R1
R1
33Ω
33Ω
C2
C1
C2
R3
15pF
5pF
0.1µF
R3
P
differential drivers provide
15pF
A
R2
R2
15Ω
15Ω
Figure 51. Differential Double Balun Input Configuration
VIN+
VIN–
S
VIN–
VIN+
ADC
S
ADC
VCM
AVDD
VCM
P
0.1µF
0.1µF
Rev. C | Page 21 of 44
33Ω
33Ω
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 AD9644. For applications in
which SNR is a key parameter, differential double balun coupling
is the recommended input configuration (see Figure 51). In this
configuration, the input is ac-coupled and the 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 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 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 50 and Figure 51.
Table 9. Example RC Network
Frequency
Range
(MHz)
0 to 100
100 to 250
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 AD8376 data sheet for more
information.
0.1µF
R1
R1
C2
C1
C2
R3
R3
R2
R2
R1
Series
(Ω)
33
15
VIN+
VIN–
C1
Differential
(pF)
8.2
3.9
ADC
VCM
R2
Series
(Ω)
0
0
AD8376
C2
Shunt
(pF)
8.2
Open
variable gain
AD9644
49.9
Open
R3
Shunt
(Ω)