LTC2355-14 Linear Technology, LTC2355-14 Datasheet - Page 14

• Manufacturer Part Number: LTC2355-14
• Description: (LTC2355-12/-14) 3.5Msps Sampling ADCs
• Manufacturer: Linear Technology

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LTC2355-12/LTC2355-14
APPLICATIO S I FOR ATIO
The Typical Application Figure on page 16 shows a circuit
for level-shifting and squaring the output from an RF
signal generator or other low-jitter source. A single D-type
flip flop is used to generate the CONV signal to the
LTC2355-12/LTC2355-14. Re-timing the master clock
signal eliminates clock jitter introduced by the controlling
device (DSP, FPGA, etc.) Both the inverter and flip flop
must be treated as analog components and should be
powered from a clean analog supply.
Serial Clock Input (SCK)
The rising edge of SCK advances the conversion process
and also udpates each bit in the SDO data stream. After
CONV rises, the third rising edge of SCK starts clocking out
the 12/14 data bits with the MSB sent first. A simple
approach is to generate SCK to drive the LTC2355-12/
LTC2355-14 first and then buffer this signal with the
appropriate number of inverters to drive the serial clock
input of the processor serial port. Use the falling edge of
the clock to latch data from the Serial Data Output (SDO)
into your processor serial port. The 14-bit serial data will
be received right justified, in a 16-bit word with 16 or more
clocks per frame sync. It is good practice to drive the
LTC2355-12/LTC2355-14 SCK input first to avoid digital
noise interference during the internal bit comparison
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decision by the internal high speed comparator. Unlike the
CONV input, the SCK input is not sensitive to jitter because
the input signal is already sampled and held constant.
Serial Data Output (SDO)
Upon power-up, the SDO output is automatically reset to
the high impedance state. The SDO output remains in high
impedance until a new conversion is started. SDO sends
out 12/14 bits in the output data stream beginning at the
third rising edge of SCK after the rising edge of CONV. SDO
is always in high impedance mode when it is not sending
out data bits. Please note the delay specification from SCK
to a valid SDO. SDO is always guaranteed to be valid by the
next rising edge of SCK. The 16-bit output data stream is
compatible with the 16-bit or 32-bit serial port of most
processors.
Loading on the SDO line must be minimized. SDO can
directly drive most fast CMOS logic inputs directly. How-
ever, the general purpose I/O pins on many programmable
logic devices (FPGAs, CPLDs) and DSPs have excessive
capacitance. In these cases, a 100Ω resistor in series with
SDO can isolate the input capacitance of the receiving
device. If the receiving device has more than 10pF of input
capacitance or is located far from the LTC2355-12/
LTC2355-14, an NC7SVU04P5X inverter can be used to
provide more drive.
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