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11/30/01
SPT7750
GENERAL DESCRIPTION
The SPT7750 is one of the fastest monolithic 8-bit parallel
flash A/D converters available today. The nominal conver-
sion rate is 500 MSPS and the analog bandwidth is in
excess of 900 MHz. A major advance over previous flash
converters is the inclusion of 256 input preamplifiers
between the reference ladder and input comparators (see
block diagram). This not only reduces clock transient kick-
back to the input and reference ladder due to a low AC
beta, but also reduces the effect of the dynamic state of
the input signal on the latching characteristics of the input
comparators. The preamplifiers act as buffers and stabi-
lize the input capacitance so that it remains constant over
different input voltage and frequency ranges and therefore
makes the part easier to drive than previous flash convert-
ers. The preamplifiers also add a gain of two to the input
signal so that each comparator has a wider overdrive or
threshold range to “trip” into or out of the active state. This
gain reduces metastable states that can cause errors at
the output.
The SPT7750 has true differential analog and digital data
paths from the preamplifiers to the output buffers (Current
Mode Logic) for reducing potential missing codes while
rejecting common mode noise.
Signature errors are also reduced by careful layout of the
analog circuitry. The output drive capability of the device
can provide full ECL swings into 50
loads.
Figure 1 – SPT7750 Typical Interface Circuit
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