LTC2249 Linear Technology, LTC2249 Datasheet - Page 13

• Manufacturer Part Number: LTC2249
• Description: 80Msps Low Power 3V ADC
• Manufacturer: Linear Technology

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APPLICATIO S I FOR ATIO
Input Range
The input range can be set based on the application. The
2V input range will provide the best signal-to-noise perfor-
mance while maintaining excellent SFDR. The 1V input
range will have better SFDR performance, but the SNR will
degrade by 5.7dB. See the Typical Performance Charac-
teristics section.
Driving the Clock Input
The CLK input can be driven directly with a CMOS or TTL
level signal. A sinusoidal clock can also be used along with
a low-jitter squaring circuit before the CLK pin (see
Figure 11).
TIE TO V
TIE TO V
RANGE = 2 • V
CM
0.5V < V
DD
FOR 2V RANGE;
FOR 1V RANGE;
2.2µF
1µF
1µF
1.5V
SENSE
SENSE
Figure 9. Equivalent Reference Circuit
< 1V
FOR
Figure 10. 1.5V Range ADC
12k
12k
0.75V
1.5V
U
2.2µF
SENSE
0.1µF
REFH
REFL
V
CM
SENSE
U
2.2µF
1µF
LTC2249
V
CONTROL
CM
DETECT
4Ω
RANGE
AND
LTC2249
1.5V BANDGAP
DIFF AMP
W
REFERENCE
2249 F10
INTERNAL ADC
HIGH REFERENCE
INTERNAL ADC
LOW REFERENCE
1V
BUFFER
0.5V
U
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2249 F09
The noise performance of the LTC2249 can depend on the
clock signal quality as much as on the analog input. Any
noise present on the clock signal will result in additional
aperture jitter that will be RMS summed with the inherent
ADC aperture jitter.
In applications where jitter is critical, such as when digitiz-
ing high input frequencies, use as large an amplitude as
possible. Also, if the ADC is clocked with a sinusoidal
signal, filter the CLK signal to reduce wideband noise and
distortion products generated by the source.
Maximum and Minimum Conversion Rates
The maximum conversion rate for the LTC2249 is 80Msps.
For the ADC to operate properly, the CLK signal should
have a 50% (±5%) duty cycle. Each half cycle must have
at least 5.9ns for the ADC internal circuitry to have enough
settling time for proper operation.
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 CLK pin to sample the analog
input. The falling edge of CLK is ignored and the internal
falling edge is generated by a phase-locked loop. The input
clock duty cycle can vary from 40% to 60% 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 1/3V
SINUSOIDAL
Figure 11. Sinusoidal Single-Ended CLK Drive
CLOCK
INPUT
50Ω
4.7µF
0.1µF
DD
or 2/3V
1k
1k
NC7SVU04
DD
using external resistors.
FERRITE
0.1µF
BEAD
CLK
LTC2249
SUPPLY
CLEAN
LTC2249
2249 F11
13
2249f