參數(shù)資料
型號(hào)： AD9203-EB
廠商： Analog Devices, Inc.
英文描述： 10-Bit, 40 MSPS, 3 V, 74 mW A/D Converter
中文描述： 10位，40 MSPS的，3伏，74毫瓦的A / D轉(zhuǎn)換器
文件頁(yè)數(shù)： 15/19頁(yè)
文件大小： 216K
代理商： AD9203-EB
REV. 0
AD9203
–15–
APPLICATIONS
DIRECT IF DOWN CONVERSION USING THE AD9203
Sampling IF signals above an ADC’s baseband region (i.e., dc
to FS/2) is becoming increasingly popular in communication
applications. This process is often referred to as Direct IF Down
Conversion or Undersampling. There are several potential ben-
efits in using the ADC to alias (i.e., or mix) down a narrow
band or wide band IF signal. First and foremost is the elimina-
tion of a complete mixer stage with its associated amplifiers and
filters, reducing cost and power dissipation. Second is the ability
to apply various DSP techniques to perform such functions as
filtering, channel selection, quadrature demodulation, data
reduction, detection, etc. A detailed discussion on using this
technique in digital receivers can be found in Analog Devices
Application Notes AN-301 and AN-302.
In Direct IF Down Conversion applications, one exploits the
inherent sampling process of an ADC in which an IF signal
lying outside the baseband region can be aliased back into
the baseband region in a manner similar to a mixer down-
converting an IF signal. Similar to the mixer topology, an im-
age rejection filter is required to limit other potential interfering
signals from also aliasing back into the ADC’s baseband region.
A trade-off exists between the complexity of this image rejection
filter and the ADC’s sample rate as well as dynamic range.
The AD9203 is well suited for various IF sampling applications.
The AD9203’s low distortion input SHA has a full-power band-
width extending to 130 MHz, thus encompassing many popular
IF frequencies. Only the 2 V span should be used for under-
sampling beyond 20 MHz. A DNL of
±
0.25 LSB combined
with low thermal input referred noise allows the AD9203 in the
2 V span to provide >59 dB of SNR for a baseband input sine
wave. Also, its low aperture jitter of 1.2 ps rms ensures mini-
mum SNR degradation at higher IF frequencies. In fact, the
AD9203 is capable of still maintaining 58 dB of SNR at an IF
of 70 MHz with a 2 V input span.
To maximize its distortion performance, the AD9203 should be
configured in the differential mode with a 2 V span using a
transformer. The center-tap of the transformer is biased to the
reference output of the AD9203. Preceding the AD9203 and
transformer is an optional bandpass filter as well as a gain stage.
A low Q passive bandpass filter can be inserted to reduce out-of-
band distortion and noise that lies within the AD9203’s 390 MHz
bandwidth. A large gain stage(s) is often required to compensate
for the high insertion losses of a SAW filter used for channel
selection and image rejection. The gain stage will also provide
adequate isolation for the SAW filter from the charge “kick
back” currents associated with the AD9203’s switched capacitor
input stage.
The distortion and noise performance of an ADC at the given
IF frequency is of particular concern when evaluating an ADC
for a narrowband IF sampling application. Both single tone and
dual tone SFDR vs. amplitude are very useful in assessing an
ADC’s dynamic and static nonlinearities. SNR vs. amplitude
performance at the given IF is useful in assessing the ADC’s
noise performance and noise contribution due to aperture jitter.
In any application, one is advised to test several units of the
same device under the same conditions to evaluate the given
applications sensitivity to that particular device. Figures 32 and
33 combine the dual tone SFDR as well as single tone SFDR and
SNR performances at IF frequencies of 70 MHz, and 130 MHz.
Note, the SFDR vs. amplitude data is referenced to dBFS while
the single tone SNR data is referenced to dBc. The performance
characteristics in these figures are representative of the AD9203
without any preceding gain stage. The AD9203 was operated in
the differential mode (via transformer) with a 2 V span and a
sample rate of 40 MSPS. The analog supply (AVDD) and the
digital supply (DRVDD) were set to 3.0 V.
90
S
INPUT POWER LEVEL – dB FULL SCALE
0
80
70
60
50
40
30
20
10
0
5
10
15
20
25
30
SFDR 2 TONE
SFDR 1 TONE
SNR
Figure 32. SNR/SFDR for IF @ 70 MHz (Clock = 40 MSPS)
80
S
INPUT POWER LEVEL – dB FULL SCALE
0
70
60
50
40
30
0
5
10
15
20
25
30
35
20
10
SFDR 2 TONE
SFDR 1 TONE
SNR
Figure 33. SNR/SFDR for IF @ 130 MHz (Clock = 40 MSPS)
AD9203
AINP
AINN
200
V
AVDD/2
93.1
V
50
V
BANDPASS
FILTER
MINI CIRCUITS
T4-6T
1:4
G2 = 20dB
22.1
V
50
V
G1 = 20dB
50
V
200
V
SAW FILTER
OUTPUT
Figure 31. Simplified IF Sampling Circuit
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