LTC2247 Linear Technology, LTC2247 Datasheet - Page 17

• Manufacturer Part Number: LTC2247
• Description: (LTC2246 - LTC2248) 65/40/25Msps Low Power 3V ADCs
• Manufacturer: Linear Technology

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APPLICATIO S I FOR ATIO
pins are needed to reduce package inductance. Bypass
capacitors must be connected as shown in Figure 9.
Other voltage ranges in-between the pin selectable ranges
can be programmed with two external resistors as shown
in Figure 10. An external reference can be used by applying
its output directly or through a resistor divider to SENSE.
It is not recommended to drive the SENSE pin with a logic
device. The SENSE pin should be tied to the appropriate
level as close to the converter as possible. If the SENSE pin
is driven externally, it should be bypassed to ground as
close to the device as possible with a 1µF ceramic capacitor.
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 5.8dB. 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
SINUSOIDAL
Figure 11. Sinusoidal Single-Ended CLK Drive
CLOCK
INPUT
Figure 10. 1.5V Range ADC
50Ω
12k
12k
4.7µF
0.1µF
0.75V
1.5V
U
1k
1k
SENSE
U
2.2µF
1µF
NC7SVU04
V
CM
LTC2248/47/46
FERRITE
0.1µF
BEAD
W
CLK
224876 F10
LTC2248/47/46
SUPPLY
CLEAN
224876 F11
U
a low-jitter squaring circuit before the CLK pin (see
Figure 11).
The noise performance of the LTC2248/LTC2247/LTC2246
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.
Figures 12 and 13 show alternatives for converting a
differential clock to the single-ended CLK input. The use of
a transformer provides no incremental contribution to
phase noise. The LVDS or PECL to CMOS translators
provide little degradation below 70MHz, but at 140MHz
will degrade the SNR compared to the transformer solu-
tion. The nature of the received signals also has a large
Figure 12. CLK Drive Using an LVDS or PECL to CMOS Converter
LTC2248/LTC2247/LTC2246
Figure 13. LVDS or PECL CLK Drive Using a Transformer
DIFFERENTIAL
CLOCK
100Ω
INPUT
IF LVDS USE FIN1002 OR FIN1018.
FOR PECL, USE AZ1000ELT21 OR SIMILAR
4.7µF
ETC1-1T
5pF-30pF
FERRITE
BEAD
0.1µF
0.1µF
CLK
SUPPLY
CLEAN
CLK
FERRITE
LTC2248/
LTC2247/
LTC2246
BEAD
224876 F12
LTC2248/
LTC2247/
LTC2246
224876 F13
V
CM
17
224876fa