![](http://datasheet.mmic.net.cn/310000/AD9245_datasheet_16239698/AD9245_15.png)
AD9245
to the largest input span of 2 V p-p. The relative SNR degradation
is 3 dB when changing from 2 V p-p mode to 1 V p-p mode.
03583-B
-014
AD9245
VIN+
VIN–
AVDD
AGND
33
33
10pF
49.9
1k
1k
0.1
μ
F
2V p-p
The SHA may be driven from a source that keeps the signal
peaks within the allowable range for the selected reference volt-
age. The minimum and maximum common-mode input levels
are defined as
2
VREF
VCM
MIN
=
(
)
2
VREF
AVDD
VCM
MAX
+
=
The minimum common-mode input level allows the AD9245 to
accommodate ground referenced inputs.
Although optimum performance is achieved with a differential
input, a single-ended source may be applied to VIN+ or VIN–.
In this configuration, one input accepts the signal, while the
opposite input should be set to midscale by connecting it to an
appropriate reference. For example, a 2 V p-p signal may be
applied to VIN+ while a 1 V reference is applied to VIN–. The
AD9245 then accepts an input signal varying between 2 V and
0 V. In the single-ended configuration, distortion performance
may degrade significantly as compared to the differential case.
However, the effect is less noticeable at lower input frequencies.
Differential Input Configurations
As previously detailed, optimum performance is achieved while
driving the AD9245 in a differential input configuration. For
baseband applications, the AD8138 differential driver provides
excellent performance and a flexible interface to the ADC. The
output common-mode voltage of the AD8138 is easily set to
AVDD/2, and the driver can be configured in a Sallen Key filter
topology to provide band limiting of the input signal.
AD9245
VIN+
VIN–
AGND
AVDD
1V p-p
49.9
523
1k
1k
0.1
μ
F
33
33
20pF
499
499
499
AD8138
03583-B
-013
Figure 28. Differential Input Configuration Using the AD8138
At input frequencies in the second Nyquist zone and above, the
performance of most amplifiers is not adequate to achieve the
true performance of the AD9245. This is especially true in IF
undersampling applications where frequencies in the 70 MHz to
100 MHz range are being sampled. For these applications,
differential transformer coupling is the recommended input
configuration. The value of the shunt capacitor is dependent on
the input frequency and source impedance and should be
reduced or removed. An example is shown in F
.
igure 29
Figure 29. Differential Transformer-Coupled Configuration
The signal characteristics must be considered when selecting
a transformer. Most RF transformers saturate at frequencies
below a few MHz, and excessive signal power can also cause
core saturation, which leads to distortion.
Single-Ended Input Configuration
A single-ended input may provide adequate performance in
cost-sensitive applications. In this configuration, there is a
degradation in SFDR and distortion performance due to the
large input common-mode swing (see F
the source impedances on each input are matched, there should
be little effect on SNR performance. F
single-ended input configuration.
). However, if
details a typical
igure 13
igure 30
Figure 30. Single-Ended Input Configuration
03583-B-015
AD9245
VIN+
VIN–
AVDD
AGND
2V p-p
33
33
20pF
49.9
1k
1k
0.33
μ
F
10
μ
F
0.1
μ
F
1k
1k
+
CLOCK INPUT CONSIDERATIONS
Typical high speed ADCs use both clock edges to generate a
variety of internal timing signals, and as a result may be sensitive
to clock duty cycle. Commonly a 5% tolerance is required on the
clock duty cycle to maintain dynamic performance characteris-
tics. The AD9245 contains a clock duty cycle stabilizer (DCS) that
retimes the nonsampling edge, providing an internal clock signal
with a nominal 50% duty cycle. This allows a wide range of clock
input duty cycles without affecting the performance of the
AD9245. As shown in Figure 21, noise and distortion perform-
ance is nearly flat for a 30% to 70% duty cycle with the DCS on.
The duty cycle stabilizer uses a delay-locked loop (DLL) to
create the nonsampling edge. As a result, any changes to the
sampling frequency require approximately 100 clock cycles to
allow the DLL to acquire and lock to the new rate.
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