LTC1407/LTC1407A
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APPLICATIONS INFORMATION
DIGITAL INTERFACE
The LTC1407/LTC1407A have a 3-wire SPI (Serial Protocol
Interface) interface. The SCK and CONV inputs and SDO
output implement this interface. The SCK and CONV inputs
accept swings from 3V logic and are TTL compatible, if the
logic swing does not exceed V
DD
. A detailed description
of the three serial port signals follows:
Conversion Start Input (CONV)
The rising edge of CONV starts a conversion, but subse-
quent rising edges at CONV are ignored by the LTC1407/
LTC1407A until the following 32 SCK rising edges have
occurred. The duty cycle of CONV can be arbitrarily chosen
to be used as a frame sync signal for the processor serial
port. A simple approach to generate CONV is to create a
pulse that is one SCK wide to drive the LTC1407/LTC1407A
and then buffer this signal to drive the frame sync input
of the processor serial port. It is good practice to drive
the LTC1407/LTC1407A CONV input fi rst to avoid digital
noise interference during the sample-to-hold transition
triggered by CONV at the start of conversion. It is also good
practice to keep the width of the low portion of the CONV
signal greater than 15ns to avoid introducing glitches in
the front end of the ADC just before the sample-and-hold
goes into Hold mode at the rising edge of CONV.
Minimizing Jitter on the CONV Input
In high speed applications where high amplitude sinewaves
above 100kHz are sampled, the CONV signal must have
as little jitter as possible (10ps or less). The square wave
output of a common crystal clock module usually meets
this requirement easily. The challenge is to generate a CONV
signal from this crystal clock without jitter corruption from
other digital circuits in the system. A clock divider and
any gates in the signal path from the crystal clock to the
CONV input should not share the same integrated circuit
with other parts of the system. As shown in the interface
circuit examples, the SCK and CONV inputs should be
driven fi rst, with digital buffers used to drive the serial port
interface. Also note that the master clock in the DSP may
already be corrupted with jitter, even if it comes directly
from the DSP crystal. Another problem with high speed
processor clocks is that they often use a low cost, low
speed crystal (i.e., 10MHz) to generate a fast, but jittery,
phase-locked-loop system clock (i.e., 40MHz). The jitter
in these PLL-generated high speed clocks can be several
nanoseconds. Note that if you choose to use the frame
sync signal generated by the DSP port, this signal will
have the same jitter of the DSP’s master clock.
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 sends out two
sets of 12/14 data bits, with the MSB sent fi rst. A simple
approach is to generate SCK to drive the LTC1407/LTC1407A
fi rst 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 justifi ed,
in two 16-bit words with 32 or more clocks per frame
sync. It is good practice to drive the LTC1407/LTC1407A
SCK input fi rst to avoid digital noise interference during
the internal bit comparison 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 two sets of 12/14 bits in the output data stream after
the third rising edge of SCK after the start of conversion
with the rising edge of CONV. The two 12-/14-bit words
are separated by two clock cycles in high impedance
mode. Please note the delay specifi cation from SCK to a
valid SDO. SDO is always guaranteed to be valid by the
next rising edge of SCK. The 32-bit output data stream is
compatible with the 16-bit or 32-bit serial port of most
processors.
