LT1394 Linear Technology, LT1394 Datasheet - Page 8

• Manufacturer Part Number: LT1394
• Description: 4ns/ 150MHz Dual Comparator with Independent Input/Output Supplies
• Manufacturer: Linear Technology

LT1394
circuit is the lack of feedthrough from the generator to the
comparator input. This prevents overshoot on the com-
parator input, which would give a false fast reading on
comparator response time.
To adjust the circuit for exactly 5mV overdrive, V1 is
adjusted so that the LT1394 output under test settles to
1.4V (in the linear region). Then V1 is changed by – 1V to
set overdrive to 5mV.
High Speed Design Techniques
A substantial amount of design effort has made the LT1394
relatively easy to use. It is much less prone to oscillation
than some slower comparators, even with slow input
signals. However, as with any high speed comparator,
there are a number of pitfalls which may arise because of
PC board layout and design. The most common problems
involve power supply bypassing. Bypassing is necessary
to maintain low supply impedance. DC resistance and
inductance in supply wires and PC traces can quickly build
up to unacceptable levels. This allows the supply line to
move with changing internal current levels of the con-
nected devices. This will almost always result in improper
operation. In addition, adjacent devices connected through
an unbypassed supply can interact with each other through
the finite supply impedances. Bypass capacitors furnish a
simple solution to this problem by providing a local
reservoir of energy at the device, keeping supply imped-
ances low.
Bypass capacitors should be as close as possible to the
LT1394. A good high frequency capacitor such as a 0.1 F
APPLICATIONS
8
–3V
0V
U
PULSE
IN
INFORMATION
U
50
0.1 F
–100mV
0V
W
–5V
130
400
Figure 2. Response Time Test Circuit
2N3866
750
U
25
V1**
25
10k
50
ceramic is recommended, in parallel with a larger capaci-
tor such as a 4.7 F tantalum.
Poor trace routes and high source impedances are also
common sources of problems. Be sure to keep trace
lengths as short as possible, and avoid running any output
trace adjacent to an input trace to prevent unnecessary
coupling. If output traces are longer than a few inches, be
sure to terminate them with a resistor to eliminate any
reflections that may occur. Resistor values are typically
250 to 400 . Also, be sure to keep source impedances
as low as possible, preferably 1k or less.
Crystal Oscillators
Figure 3’s circuits are crystal oscillators. In the circuit (a)
the resistors at the LT1394’s positive input set a DC bias
point. The 2k-0.068 F path sets up phase shifted feedback
and the circuit looks like a wideband unity-gain follower at
DC. The crystal’s path provides resonant positive feed-
back and stable oscillation occurs. The circuit (b) is
similar, but supports oscillation frequencies to 30MHz.
Above 10MHz, AT-cut crystals operate in overtone mode.
Because of this, oscillation can occur at multiples of the
desired frequency. The damper network rolls off gain at
high frequency, ensuring proper operation.
Switchable Output Crystal Oscillator
Figure 4 permits crystals to be electronically switched by
logic commands. This circuit is similar to the previous
examples, except that oscillation is only possible when
one of the logic inputs is biased high.
+
–
–5V
5V
LT1394
0.01 F*
0.01 F
Q
Q
** (V
* TOTAL LEAD LENGTH INCLUDING DEVICE PIN.
FET PROBE
FET PROBE
SOCKET AND CAPACITOR LEADS SHOULD BE
LESS THAN 0.5 IN. USE GROUND PLANE
OS
+ OVERDRIVE)/200
1394 F02