LTC2222/LTC2223
21
22223fb
APPLICATIONS INFORMATION
Figure 11. Transformer Driven ENC+/ENC–
In applications where jitter is critical (high input frequen-
cies) take the following into consideration:
1. Differential drive should be used.
2. Use as large an amplitude as possible; if transformer
coupled use a higher turns ratio to increase the
amplitude.
3. If the ADC is clocked with a sinusoidal signal, lter the
encode signal to reduce wideband noise.
4. Balance the capacitance and series resistance at both
encode inputs so that any coupled noise will appear at
both inputs as common mode noise. The encode inputs
have a common mode range of 1.1V to 2.5V. Each input
may be driven from ground to VDD for single-ended
drive.
Maximum and Minimum Encode Rates
The maximum encode rate for the LTC2222/LTC2223 is
105Msps (LTC2222) and 80Msps (LTC2223). For the
ADC to operate properly, the encode signal should have a
50% (±5%) duty cycle. Each half cycle must have at least
4.5ns (LTC2222) or 5.9ns (LTC2223) for the ADC internal
circuitry to have enough settling time for proper operation.
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.
An optional clock duty cycle stabilizer circuit can be used
if the input clock has a non 50% duty cycle. This circuit
uses the rising edge of the ENC+ pin to sample the analog
input. The falling edge of ENC+ is ignored and the internal
falling edge is generated by a phase-locked loop. The input
clock duty cycle can vary from 20% to 80% 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 a hundred
clock cycles for the PLL to lock onto the input clock. To
use the clock duty cycle stabilizer, the MODE pin should be
connected to 1/3VDD or 2/3VDD using external resistors.
Input Range
The input range can be set based on the application.
The 2V input range will provide the best signal-to-noise
performance while maintaining excellent SFDR. The 1V
input range will have better SFDR performance, but the
SNR will degrade by 5dB. See the Typical Performance
Characteristics section.
Driving the Encode Inputs
The noise performance of the LTC2222/LTC2223 can
depend on the encode signal quality as much as on the
analog input. The ENC+/ENC– inputs are intended to be
driven differentially, primarily for noise immunity from
common mode noise sources. Each input is biased
through a 6k resistor to a 1.6V bias. The bias resistors
set the DC operating point for transformer coupled drive
circuits and can set the logic threshold for single-ended
drive circuits.
Any noise present on the encode signal will result in ad-
ditional aperture jitter that will be RMS summed with the
inherent ADC aperture jitter.
VDD
LTC2222/LTC2223
22223 F11
VDD
ENC–
ENC+
1.6V BIAS
1:4
0.1μF
CLOCK
INPUT
50Ω
6k
TO INTERNAL
ADC CIRCUITS