AD6659 Analog Devices, AD6659 Datasheet - Page 17

• Manufacturer Part Number: AD6659
• Description: Dual IF Receiver
• Manufacturer: Analog Devices

Specifications of AD6659

• Resolution (bits): 12bit
• # Chan: 2
• Sample Rate: 80MSPS
• Interface: Par
• Analog Input Type: Diff-Bip
• Ain Range: 2 V p-p
• Adc Architecture: Pipelined
• Pkg Type: CSP

AN-742 Application Note, the AN-827 Application Note, and the
Analog Dialogue article
Wideband A/D
information. In general, the precise values depend on the
application.
Input Common Mode
The analog inputs of the AD6659 are not internally dc-biased.
Therefore, in ac-coupled applications, the user must provide a
dc bias externally. Setting the device so that VCM = AVDD/2 is
recommended for optimum performance, but the device can
function over a wider range with reasonable performance, as
shown in Figure 27.
An on-board, common-mode voltage reference is included in
the design and is available from the VCM pin. The VCM pin
must be decoupled to ground by a 0.1 μF capacitor, as described
in the Applications Information section.
Differential Input Configurations
Optimum performance is achieved while driving the AD6659 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 AD6659 (see Figure 28), and the
driver can be configured in a Sallen-Key filter topology to
provide band limiting of the input signal.
For baseband applications below ~10 MHz where SNR is a key
parameter, differential transformer coupling is the recommended
input configuration. An example is shown in Figure 29. To bias
VIN
0.1µF
100
Figure 28. Differential Input Configuration Using the ADA4938
90
80
70
60
50
0.5
76.8Ω
Figure 27. SNR/SFDR vs. Input Common-Mode Voltage,
0.6
Converters” (Volume 39, April 2005) for more
120Ω
90Ω
INPUT COMMON-MODE VOLTAGE (V)
0.7
f
IN
= 30.5 MHz, f
ADA4938
“Transformer-Coupled Front-End for
200Ω
200Ω
ADA4938-2
0.8
SFDR (dBc)
SNR (dBFS)
33Ω
33Ω
0.9
10pF
S
= 80 MSPS
1.0
differential drivers provide
1.1
VIN–x
VIN+x
1.2
ADC
AVDD
VCM
1.3
Rev. | Page 17 of 40
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 (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 AD6659. For applications above
~10 MHz where SNR is a key parameter, differential double balun
coupling is the recommended input configuration (see Figure 31).
An alternative to using a transformer-coupled input at frequencies
in the second Nyquist zone is to use the
An example is shown in Figure 32. See the AD8352 data sheet
for more information.
In any configuration, the value of Shunt Capacitor C is dependent
on the input frequency and source impedance and may need to
be reduced or removed. Table 9 displays the suggested values to set
the RC network. However, these values are dependent on the
input signal and should be used only as a starting guide.
Table 9. Example RC Network
Frequency Range (MHz)
0 to 70
70 to 200
Single-Ended Input Configuration
A single-ended input can provide adequate performance in
cost-sensitive applications. In this configuration, SFDR and
distortion performance degrade due to the large input common-
mode swing. If the source impedances on each input are matched,
there should be little effect on SNR performance. Figure 30
shows a typical single-ended input configuration.
2V p-p
1V p-p
Figure 29. Differential Transformer-Coupled Configuration
10µF
49.9Ω
49.9Ω
Figure 30. Single-Ended Input Configuration
0.1µF
0.1µF
10µF
0.1µF
AVDD
1kΩ
1kΩ
1kΩ
1kΩ
AVDD
R Series
(Ω Each)
33
125
R
R
R
R
C
C
AD8352
C Differential (pF)
22
Open
VIN+x
VIN–x
VIN+x
VIN–x
differential driver.
ADC
ADC
AD6659
VCM