PRELIMINARY
Data Sheet
and that the bandwidth of the transformer is appropriate.
The bandwidth should exceed the sampling rate of the
ADC with at least a factor of 10. It is also important to
keep phase mismatch between the differential ADC inputs
small for good HD2 performance. This type of transformer
coupled input is the preferred configuration for high fre-
quency signals as most differential amplifiers do not have
adequate performance at high frequencies. Magnetic
coupling between the transformers and PCB traces may
impact channel crosstalk, and must hence be taken into
account during PCB layout.
If the input signal is traveling a long physical distance
from the signal source to the transformer (for example a
long cable), kick-backs from the ADC will also travel along
this distance. If these kick-backs are not terminated prop-
erly at the source side, they are reflected and will add to
the input signal at the ADC input. This could reduce the
ADC performance. To avoid this effect, the source must
effectively terminate the ADC kick-backs, or the traveling
distance should be very short. If this problem could not be
avoided, the circuit in Figure 6 can be used.
Note that startup time from Sleep Mode and Power Down
Mode will be affected by this filter as the time required
to charge the series capacitors is dependent on the filter
cut-off frequency.
If the input signal has a long traveling distance, and the
kick-backs from the ADC not are effectively terminated
at the signal source, the input network of figure 8 can
be used. The configuration in figure 8 is designed to at-
tenuate the kickback from the ADC and to provide an in-
put impedance that looks as resistive as possible for fre-
quencies below Nyquist. Values of the series inductor will
however depend on board design and conversion rate.
In some instances a shunt capacitor in parallel with the
termination resistor (e.g. 33pF) may improve ADC per-
formance further. This capacitor attenuate the ADC kick-
back even more, and minimize the kicks traveling towards
the source. However, the impedance match seen into the
transformer becomes worse.
CDK2308
Dual, 20/40/65/80MSPS, 10-bit Analog-to-Digital Converters
1:1
120nH
33
33
R
T
47
optional
R
T
68
120nH
220
pF
33
33
Figure 4. Transformer-Coupled Input
Figure 5 shows AC-coupling using capacitors. Resistors
from the CM_EXT output, RCM, should be used to bias the
differential input signals to the correct voltage. The series
capacitor, CI, form the high-pass pole with these resistors,
and the values must therefore be determined based on
the requirement to the high-pass cut-off frequency.
Figure 6. Alternative Input Network
Clock Input And Jitter Considerations
Typically high-speed ADCs use both clock edges to generate
internal timing signals. In the CDK2308 only the rising
edge of the clock is used. Hence, input clock duty cycles
between 20% and 80% is acceptable.
The input clock can be supplied in a variety of formats.
The clock pins are AC-coupled internally, and hence a wide
common mode voltage range is accepted. Differential
clock sources as LVDS, LVPECL or differential sine wave
can be connected directly to the input pins. For CMOS
inputs, the CLKN pin should be connected to ground, and
the CMOS clock signal should be connected to CLKP. For
differential sine wave clock input the amplitude must be
at least 1V
pp
.
www.cadeka.com
Rev 0.1.1
pF
Figure 5. AC-Coupled Input
©2008 CADEKA Microcircuits LLC
12
CADEKA [ CADEKA MICROCIRCUITS LLC. ]
