AD9231 查看數據表（PDF） - Analog Devices
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AD9231 Datasheet PDF : 37 Pages
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AD9231
Input Common Mode
The analog inputs of the AD9231 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 39 and Figure 40.
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.
100
SFDR (dBc)
90
80
SNR (dBFS)
70
60
50
0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3
INPUT COMMON-MODE VOLTAGE (V)
Figure 39. SNR/SFDR vs. Input Common-Mode Voltage,
fIN = 32.1 MHz, fS = 80 MSPS
100
SFDR (dBc)
90
80
SNR (dBFS)
70
60
50
0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3
INPUT COMMON-MODE VOLTAGE (V)
Figure 40. SNR/SFDR vs. Input Common-Mode Voltage,
fIN = 10.3 MHz, fS = 20 MSPS
Differential Input Configurations
Optimum performance is achieved while driving the AD9231 in a
differential input configuration. For baseband applications, the
AD8138, ADA4937-2, and ADA4938-2 differential drivers provide
excellent performance and a flexible interface to the ADC.
Data Sheet
The output common-mode voltage of the ADA4938-2 is easily
set with the VCM pin of the AD9231 (see Figure 41), and the
driver can be configured in a Sallen-Key filter topology to
provide band limiting of the input signal.
VIN
76.8Ω
200Ω
33Ω
90Ω
VIN–x AVDD
0.1µF
ADA4938 10pF
120Ω
33Ω
200Ω
ADC
VIN+x VCM
Figure 41. Differential Input Configuration Using the ADA4938-2
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 42. 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
49.9Ω
VIN+x
R
C
ADC
R
VIN–x VCM
0.1µF
Figure 42. Differential Transformer-Coupled Configuration
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 AD9231. For applications above
~10 MHz where SNR is a key parameter, differential double balun
coupling is the recommended input configuration (see Figure 44).
An alternative to using a transformer-coupled input at frequencies
in the second Nyquist zone is to use the AD8352 differential driver.
An example is shown in Figure 45. 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)
R Series
(Ω Each)
0 to 70
33
70 to 200
125
C Differential (pF)
22
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Rev. B | Page 20 of 36
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