LTC2205-14 LINER [Linear Technology], LTC2205-14 Datasheet - Page 20

• Manufacturer Part Number: LTC2205-14
• Description: 14-Bit, 65Msps ADC
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LTC2205-14
Maximum and Minimum Encode Rates
The maximum encode rate for the LTC2205-14 is 65Msps.
For the ADC to operate properly the encode signal should
have a 50% (±2.5%) duty cycle. Achieving a precise 50%
duty cycle is easy with differential sinusoidal drive using
a transformer or using symmetric differential logic such
as PECL or LVDS. When using a single-ended ENCODE
signal asymmetric rise and fall times can result in duty
cycles that are far from 50%.
An optional clock duty cycle stabilizer can be used if the
input clock does not have a 50% duty cycle. This circuit
uses the rising edge of ENC pin to sample the analog input.
The falling edge of ENC is ignored and an internal falling
edge is generated by a phase-locked loop. The input clock
duty cycle can vary from 30% to 70% and the clock duty
cycle stabilizer will maintain a constant 50% internal duty
cycle. If the clock is turned off for a long period of time,
the duty cycle stabilizer circuit will require one hundred
clock cycles for the PLL to lock onto the input clock. To
use the clock duty cycle stabilizer, the MODE pin must be
connected to 1/3V
20
Figure 10. ENC Drive Using a CMOS to PECL Translator
MC100LVELT22
V
THRESHOLD
Figure 9. Single-Ended ENC Drive,
Not Recommended for Low Jitter
D0
DD
= 1.6V
3.3V
or 2/3V
0.1µF
Q0
Q0
130Ω
83Ω
1.6V
DD
3.3V
using external resistors.
ENC
ENC
+
–
130Ω
ENC
ENC
83Ω
LTC2205-14
+
–
LTC2205-14
220514 F09
220514 F10
The lower limit of the LTC2205-14 sample rate is determined
by droop of the sample and hold circuits. The pipelined
architecture of this ADC relies on storing analog signals on
small valued capacitors. Junction leakage will discharge
the capacitors. The specifi ed minimum operating frequency
for the LTC2205-14 is 1Msps.
DIGITAL OUTPUTS
Digital Output Buffers
Figure 11 shows an equivalent circuit for a single output
buffer. Each buffer is powered by OV
from the ADC power and ground. The additional N-channel
transistor in the output driver allows operation down to
low voltages. The internal resistor in series with the output
eliminates the need for external damping resistors.
As with all high speed/high resolution converters, the
digital output loading can affect the performance. The
digital outputs of the LTC2205-14 should drive a minimum
capacitive load to avoid possible interaction between the
digital outputs and sensitive input circuitry. The output
should be buffered with a device such as a ALVCH16373
CMOS latch. For full speed operation the capacitive load
should be kept under 10pF. A resistor in series with the
output may be used but is not required since the ADC has
a series resistor of 33 on chip.
Lower OV
from the digital outputs.
LTC2205-14
LATCH
FROM
DATA
Figure 11. Equivalent Circuit for a Digital Output Buffer
PREDRIVER
LOGIC
DD
V
DD
voltages will also help reduce interference
V
DD
DD
OV
and OGND, isolated
DD
33Ω
220514 F11
OV
OGND
DD
TYPICAL
DATA
OUTPUT
220514fa
0.1µF
0.5V
TO 3.6V