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A / D CONVERTERS - SMT
0
0 - 23
HMCAD1102
v03.0611
OCTAL 12-BIT 80 MSPS
A/D CONVERTER
pass pole with these resistors, and the values must
therefore be determined based on the requirement to
the high-pass cut-off frequency.
Note that Start Up Time from Sleep Mode and Power
Down Mode will be affected by this lter as the time
required to charge the series capacitors is dependent
on the lter cut-off frequency.
If the input signal has a long traveling distance, and the
kick-backs from the ADC are not effectively terminated
at the signal source, the input network of gure 16 can
be used. The conguration in gure 16 is designed to
attenuate the kickback from the ADC and to provide
an input impedance that looks as resistive as possible
for frequencies below Nyquist.
Figure 15. Alternative input network
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 performance
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.
Clock Input and Jitter Considerations
Typically high-speed ADCs use both clock edges to
generate internal timing signals. In HMCAD1102 only
the rising edge of the clock is used. Hence, input clock
duty cycles between 20% and 80% are acceptable.
The input clock can be supplied in a variety of formats.
The clock pins are AC-coupled internally, hence a wide
common mode voltage range is accepted. Differential
clock sources such 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 ± 0.8 Vpp. No
additional conguration is needed to set up the clock
source format.
The quality of the input clock is extremely important for
high-speed, high-resolution ADCs. The contribution to
SNR from clock jitter with a full scale signal at a given
frequency is shown in equation 1.
SNRjitter = 20 · log (2 · π · ƒIN · єt) (1)
where fIN is the signal frequency, and εt is the total
rms jitter measured in seconds. The rms jitter is the
total of all jitter sources including the clock generation
circuitry, clock distribution and internal ADC circuitry.
For applications where jitter may limit the obtainable
performance, it is of utmost importance to limit the clock
jitter. This can be obtained by using precise and stable
clock references (e.g. crystal oscillators with good jitter
specications) and make sure the clock distribution
is well controlled. It might be advantageous to use
analog power and ground planes to ensure low noise
on the supplies to all circuitry in the clock distribution.
It is of utmost importance to avoid crosstalk between
the ADC output bits and the clock and between the
analog input signal and the clock since such crosstalk
often results in harmonic distortion.
The jitter performance is improved with reduced rise
and fall times of the input clock. Hence, optimum jitter
performance is obtained with LVDS or LVPECL clock
with fast edges. CMOS and sine wave clock inputs will
result in slightly degraded jitter performance.
If the clock is generated by other circuitry, it should
be re-timed with a low jitter master clock as the last
operation before it is applied to the ADC clock input.
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responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other
rights of third parties that may result from its use. Specifications subject to change without notice. No
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