Datasheet
17
LTC1744
1744f
APPLICATIO S I FOR ATIO
WUU
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Any noise present on the encode signal will result in
additional aperture jitter that will be RMS summed with the
inherent ADC aperture jitter.
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, filter 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 1.8V to
V
DD
. Each input may be driven from ground to V
DD
for
single-ended drive.
1744 F08a
ENC2V
V
THRESHOLD
= 2V
ENC
0.1µF
LTC1744
1744 F08b
ENC
ENC
130Ω
3.3V
3.3V
130Ω
D0
Q0
Q0
MC100LVELT22
LTC1744
83Ω83Ω
Figure 8a. Single-Ended ENC Drive,
Not Recommended for Low Jitter Figure 8b. ENC Drive Using a CMOS-to-PECL Translator
Maximum and Minimum Encode Rates
The maximum encode rate for the LTC1744 is 50Msps. For
the ADC to operate properly the ENCODE signal should
have a 50% (±5%) duty cycle. Each half cycle must have
at least 9.5ns 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. When using a single-ended
ENCODE signal asymmetric rise and fall times can result
in duty cycles that are far from 50%.
At sample rates slower than 50Msps the duty cycle can
vary from 50% as long as each half cycle is at least 9.5ns.
The lower limit of the LTC1744 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 specified minimum operating fre-
quency for the LTC1744 is 1Msps.