LT1720 Linear Technology, LT1720 Datasheet - Page 19

• Manufacturer Part Number: LT1720
• Description: 4ns/ 150MHz Dual Comparator with Independent Input/Output Supplies
• Manufacturer: Linear Technology

APPLICATIONS
You don’t need expensive equipment to confirm the actual
overall performance of this circuit. All you need is a
respectable waveform generator (capable of >~100kHz), a
splitter, a variety of cable lengths and a 20MHz or 60MHz
oscilloscope. Split a single pulse source into different
cable lengths and then into the delay detector, feeding the
longer cable into the Y input (see Figure 15). A 6 foot cable
length difference will create a ~9.2ns delay (using 66%
propagation speed RG-58 cable), and should result in
2V
0V
Figure 15. RG-58 Cable with Velocity of Propogation = 66%;
Delay at Y = (n – 1) • 1.54ns
Figure 14. Log Pulse Stretcher Output Pulse vs Input Pulse
1 FOOT CABLE
NANOSECOND
INPUT RANGE
14
12
10
8
6
4
2
0
SPLITTER
1
U
10
MEASURED
INFORMATION
t
PULSE
U
100
(ns)
n FOOT CABLE
CIRCUIT OF
1000
FIGURE 12
EQUATION 1
X
Y
W
1720/21 F14
L
10000
OUTPUT RANGE
MICROSECOND
(SEE TEXT)
U
t
OUT
1720/21 F15
easily measured 1.70 s output pulses. A 12 foot cable
length difference will result in ~18.4ns delay and 2.07 s
output pulses. The difference in the two output pulse
widths is the per-octave response of your circuit (see
equation (3)). Shorter cable length differences can be
used to get a plot of circuit performance down to 1.5ns (if
any), which can then later be used as a lookup reference
when you have moved from quantifying the circuit to using
the circuit. (Note there is a slight aberration in perfor-
mance below 10ns. See Figure 14.) As a final check, feed
the circuit with identical cable lengths and check that it is
not producing any output pulses.
10ns Triple Overlap Generator
The circuit of Figure 16 utilizes an LT1721 to generate
three overlapping outputs whose pulse edges are sepa-
rated by 10ns as shown. The time constant is set by the RC
network on the output of comparator A. Comparator B and
D trip at fixed percentages of the exponential voltage decay
across the capacitor. The 4.22k feed-forward to the C
comparator’s inverting input keeps the delay differences
the same in each direction despite the exponential nature
of the RC network’s voltage.
There is a 15ns delay to the first edge in both directions,
due to the 4.5ns delay of two LT1721 comparators, plus
6ns delay in the RC network. This starting delay is short-
ened somewhat if the pulse was shorter than 40ns be-
cause the RC network will not have fully settled; however,
the 10ns edge separations stay constant.
The values shown utilize only the lowest 75% of the supply
voltage span, which allows it to work down to 2.7V supply.
The delay differences grow a couple nanoseconds from 5V
to 2.7V supply due to the fixed V
as a percentage at low supply voltage. To keep this effect
to a minimum, the 1k pull-up on comparator A provides
equal loading in either state.
Fast Waveform Sampler
Figure 17 uses a diode-bridge-type switch for clean, fast
waveform sampling. The diode bridge, because of its
inherent symmetry, provides lower AC errors than other
semiconductor-based switching technologies. This cir-
cuit features 20dB of gain, 10MHz full power bandwidth
LT1720/LT1721
OL
/V
OH
drops which grow
19