
21
LTC1282
U
S
A
O
PPLICATI
WU
U
I FOR ATIO
Figure 25.
Time Measurement with the LTC1282
MUXing with CD4051
The high input impedance of the LTC1282 provides an
easy, cheap, fast, and accurate way to multiplex many
channels of data through one converter. Figure 24 shows
a low cost CD4051, one of the most common multiplex-
ers connected to the LTC1282. The LTC1282’s input
draws no DC input current so it can be accurately driven
by the unbuffered MUX. The CD4520 counter increments
the MUX channel after each sample is taken.
100ps Resolution
Time Measurement with LTC1282
Figure 25 shows a circuit that precisely measures the
difference in time between two events. It has a 400ns full
scale and 100ps resolution. The start signal releases the
ramp generator made up of the PNP current source and
the 500pF capacitor. The circuit ramps until the stop
signal shuts off the current source. The final value of the
ramp represents the time between the start and stop
events. The LTC1282 digitizes this final value and out-
puts the digital data.
Figure 24. MUXing the LTC1282 with CD4051
VSS GND
AIN
REFOUT
VDD
LTC1282
CS
RD BUSY
12-BIT
DATA
OUTPUT
10
F
2N2369
20k
200k
250pF
POLYSTYRENE
430
2N5771
65
3.3V
74HC03
65
45.3
45.3
1N457
LM134
D
CLK
Q
CLR
D
CLK
Q
CLR
1k
5V
START
↑
5V
STOP
↑
DATA
LATCH
SIGNAL
10
F
1k
5V
100k
1N4148
0.001
F
10k
1N457
74HC74
10pF
1k
1N4148
100pF
10k
LTC1282 F25
VSS
AB C
8 INPUT
CHANNELS
±1.25V
INPUT
VARIES
NO
BUFFER
REQUIRED
D11
D0
CS
RD
BUSY
LTC1282
AIN
P
OR
DSP
ENABLE
RESET
Q2
Q1
Q0
CD4520
COUNTER
LTC1282 F24
3V
–3V
CD4051