LTC1403 Linear Technology, LTC1403 Datasheet - Page 13

• Manufacturer Part Number: LTC1403
• Description: Serial 12-Bit/14-Bit/ 2.8Msps Sampling ADCs with Shutdown
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
CONV, without any intervening rising edges at SCK, put
the LTC1403/LTC1403A in Sleep mode and the power
drain drops from 16mW to 10 W. One or more rising
edges at SCK wake up the LTC1403/LTC1403A for opera-
tion. The internal reference (V
settle with a 10 F load. Note that, using sleep mode more
frequently than every 2ms, compromises the settled accu-
racy of the internal reference. Note that, for slower conver-
sion rates, the Nap and Sleep modes can be used for
substantial reductions in power consumption.
DIGITAL INTERFACE
The LTC1403/LTC1403A has 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 compat-
ible, if the logic swing does not exceed V
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 LTC1403/
LTC1403A until the following 16 SCK rising edges have
occurred. It is necessary to have a minimum of 16 rising
edges of the clock input SCK between rising edges of
CONV. But to obtain maximum conversion speed, it is
necessary to allow two more clock periods between con-
versions to allow 39ns of acquisition time for the internal
ADC sample-and-hold circuit. With 16 clock periods per
conversion, the maximum conversion rate is limited to
2.8Msps to allow 39ns for acquisition time. In either case,
the output data stream comes out within the first 16 clock
periods to ensure compatibility with processor serial
ports. 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 LTC1403/LTC1403A
and then buffer this signal with the appropriate number of
inverters to ensure the correct delay driving the frame
U
U
REF
) takes 2ms to slew and
W
DD
. A detailed
U
sync input of the processor serial port. It is good practice
to drive the LTC1403/LTC1403A CONV input first to avoid
digital noise interference during the sample-to-hold tran-
sition 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 corrup-
tion 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 first, 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 starts clocking out
the 12/14 data bits with the MSB sent first. A simple
approach is to generate SCK to drive the LTC1403/
LTC1403/LTC1403A
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