LTC2282
17
2282fb
APPLICATIONS INFORMATION
signals on small valued capacitors. Junction leakage will
discharge the capacitors. The specied minimum operat-
ing frequency for the LTC2282 is 1Msps.
Clock Duty Cycle Stabilizer
An optional clock duty cycle stabilizer circuit ensures
high performance even if the input clock has a non
50% duty cycle. Using the clock duty cycle stabilizer is
recommended for most applications. To use the clock
duty cycle stabilizer, the MODE pin should be connected
to 1/3VDD or 2/3VDD using external resistors.
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.
For applications where the sample rate needs to be changed
quickly, the clock duty cycle stabilizer can be disabled. If
the duty cycle stabilizer is disabled, care should be taken to
make the sampling clock have a 50% (±5%) duty cycle.
DIGITAL OUTPUTS
Table 1 shows the relationship between the analog input
voltage, the digital data bits and the overow bit.
Digital Output Buffers
Figure 14 shows an equivalent circuit for a single out-
put buffer. Each buffer is powered by OVDD and OGND,
isolated from the ADC power and ground. The additional
N-channel transistor in the output driver allows opera-
tion down to low voltages. The internal resistor in series
with the output makes the output appear as 50Ω to ex-
ternal circuitry and may eliminate the need for external
damping resistors.
As with all high speed/high resolution converters, the
digital output loading can affect the performance. The
digital outputs of the LTC2282 should drive a minimal
capacitive load to avoid possible interaction between the
digital outputs and sensitive input circuitry. For full speed
operation the capacitive load should be kept under 10pF.
Lower OVDD voltages will also help reduce interference
from the digital outputs.
Data Format
Using the MODE pin, the LTC2282 parallel digital output
can be selected for offset binary or 2’s complement for-
mat. Connecting MODE to GND or 1/3VDD selects offset
binary output format. Connecting MODE to 2/3VDD or
VDD selects 2’s complement output format. An external
resistor divider can be used to set the 1/3VDD or 2/3VDD
logic values. Table 2 shows the logic states for the
MODE pin.
Table 1. Output Codes vs Input Voltage
AIN+ – AIN–
(2V Range)
OF
D11 – D0
(Offset Binary)
D11 – D0
(2’s Complement)
>+1.000000V
+0.999512V
+0.999024V
1
0
1111 1111 1111
1111 1111 1110
0111 1111 1111
0111 1111 1110
+0.000488V
0.000000V
–0.000488V
–0.000976V
0
1000 0000 0001
1000 0000 0000
0111 1111 1111
0111 1111 1110
0000 0000 0001
0000 0000 0000
1111 1111 1111
1111 1111 1110
–0.999512V
–1.000000V
<–1.000000V
0
1
0000 0000 0001
0000 0000 0000
1000 0000 0001
1000 0000 0000
LTC2282
2282 F14
OVDD
VDD
0.1μF
43Ω
TYPICAL
DATA
OUTPUT
OGND
OVDD
0.5V
TO 3.6V
PREDRIVER
LOGIC
DATA
FROM
LATCH
OE
Figure 14. Digital Output Buffer