PRELIMINARY
Data Sheet
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CDK2308
Dual, 20/40/65/80MSPS, 10-bit Analog-to-Digital Converters
N-13
N-13
N-12
N-11
N-10
N-9
N-8
CLK_EXT
Figure 1. Timing Diagram
Recommended Usage
Analog Input
The analog inputs to the CDK2308 is a switched capacitor
track-and-hold amplifier optimized for differential opera-
tion. Operation at common mode voltages at mid sup-
ply is recommended even if performance will be good for
the ranges specified. The CM_EXT pin provides a voltage
suitable as common mode voltage reference. The internal
buffer for the CM_EXT voltage can be switched off, and
driving capabilities can be changed by using the CM_EXT-
BC control input.
Figure 2 shows a simplified drawing of the input network.
The signal source must have sufficiently low output imped-
ance to charge the sampling capacitors within one clock
cycle. A small external resistor (e.g. 22Ω) in series with
each input is recommended as it helps reducing transient
currents and dampens ringing behavior. A small differential
shunt capacitor at the chip side of the resistors may be
used to provide dynamic charging currents and may im-
prove performance. The resistors form a low pass filter
with the capacitor, and values must therefore be deter-
mined by requirements for the application.
DC-Coupling
Figure 3 shows a recommended configuration for DC-
coupling. Note that the common mode input voltage must
be controlled according to specified values. Preferably, the
CM_EXT output should be used as a reference to set the
common mode voltage.
The input amplifier could be inside a companion chip or
it could be a dedicated amplifier. Several suitable single
ended to differential driver amplifiers exist in the market.
The system designer should make sure the specifications
of the selected amplifier is adequate for the total system,
and that driving capabilities comply with the CDK2308
input specifications.
pF
Rev 0.1.1
Figure 3. DC-Coupled Input
Detailed configuration and usage instructions must be
found in the documentation of the selected driver.
AC-Coupling
A signal transformer or series capacitors can be used to
make an AC-coupled input network. Figure 4 shows a
recommended configuration using a transformer. Make
sure that a transformer with sufficient linearity is selected,
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Figure 2. Input Configuration
©2008 CADEKA Microcircuits LLC
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