LTC2241-12 Linear Technology, LTC2241-12 Datasheet - Page 22

• Manufacturer Part Number: LTC2241-12
• Description: 210Msps ADC
• Manufacturer: Linear Technology

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APPLICATIO S I FOR ATIO
LTC2241-12
Output Enable
The outputs may be disabled with the output enable pin,
OE. In CMOS or LVDS output modes OE high disables all
data outputs including OF and CLKOUT. The data access
and bus relinquish times are too slow to allow the outputs
to be enabled and disabled during full speed operation. The
output Hi-Z state is intended for use during long periods
of inactivity.
The Hi-Z state is not a truly open circuit; the output pins that
make an LVDS output pair have a 20k resistance between
them. Therefore in the CMOS output mode, adjacent data
bits will have 20k resistance in between them, even in the
Hi-Z state.
Sleep and Nap Modes
The converter may be placed in shutdown or nap modes
to conserve power. Connecting SHDN to GND results in
normal operation. Connecting SHDN to V
results in sleep mode, which powers down all circuitry
including the reference and typically dissipates 1mW. When
exiting sleep mode it will take milliseconds for the output
data to become valid because the reference capacitors have
to recharge and stabilize. Connecting SHDN to V
to GND results in nap mode, which typically dissipates
28mW. In nap mode, the on-chip reference circuit is kept
on, so that recovery from nap mode is faster than that from
sleep mode, typically taking 100 clock cycles. In both sleep
and nap mode all digital outputs are disabled and enter the
Hi-Z state.
GROUNDING AND BYPASSING
The LTC2241-12 requires a printed circuit board with a clean
unbroken ground plane. A multilayer board with an inter-
nal ground plane is recommended. Layout for the printed
circuit board should ensure that digital and analog signal
lines are separated as much as possible. In particular, care
should be taken not to run any digital signal alongside an
analog signal or underneath the ADC.
High quality ceramic bypass capacitors should be used at
the V
pins. Bypass capacitors must be located as close to the pins
22
DD
, OV
DD
, V
CM
U
, REFHA, REFHB, REFLA and REFLB
U
W
DD
and OE to V
U
DD
and OE
DD
as possible. Of particular importance are the capacitors
between REFHA and REFLB and between REFHB and
REFLA. These capacitors should be as close to the device
as possible (1.5mm or less). Size 0402 ceramic capacitors
are recommended. The 2.2µF capacitor between REFHA and
REFLA can be somewhat further away. The traces connect-
ing the pins and bypass capacitors must be kept short and
should be made as wide as possible.
The LTC2241-12 differential inputs should run parallel and
close to each other. The input traces should be as short as
possible to minimize capacitance and to minimize noise
pickup.
HEAT TRANSFER
Most of the heat generated by the LTC2241-12 is transferred
from the die through the bottom-side exposed pad and
package leads onto the printed circuit board. For good
electrical and thermal performance, the exposed pad should
be soldered to a large grounded pad on the PC board. It is
critical that all ground pins are connected to a ground plane
of sufficient area.
Clock Sources for Undersampling
Undersampling is especially demanding on the clock
source and the higher the input frequency, the greater the
sensitivity to clock jitter or phase noise. A clock source that
degrades SNR of a full-scale signal by 1dB at 70MHz will
degrade SNR by 3dB at 140MHz, and 4.5dB at 190MHz.
In cases where absolute clock frequency accuracy is
relatively unimportant and only a single ADC is required,
a canned oscillator from vendors such as Saronix or
Vectron can be placed close to the ADC and simply
connected directly to the ADC. If there is any distance to
the ADC, some source termination to reduce ringing that
may occur even over a fraction of an inch is advisable. You
must not allow the clock to overshoot the supplies or
performance will suffer. Do not filter the clock signal with
a narrow band filter unless you have a sinusoidal clock
source, as the rise and fall time artifacts present in typical
digital clock signals will be translated into phase noise.
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