LT1016
APPLICATIONS INFORMATION
Other common problems include different measurement
results using various pieces of test equipment, inability
to make measurement connections to the circuit without
inducing spurious responses and dissimilar operation
between two “identical” circuits. If the components used
in the circuit are good and the design is sound, all of the
above problems can usually be traced to failure to pro-
vide a proper circuit “environment.” To learn how to do
this requires studying the causes of the aforementioned
difficulties.
By far the most common error involves power supply
bypassing. Bypassing is necessary to maintain low sup-
ply 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 as internal
current levels of the devices connected to it change. This
will almost always cause unruly operation. In addition,
several devices connected to an unbypassed supply can
“communicate” through the finite supply impedances,
causing erratic modes. Bypass capacitors furnish a simple
way to eliminate this problem by providing a local reser-
voir of energy at the device. The bypass capacitor acts
like an electrical flywheel to keep supply impedance low
at high frequencies. The choice of what type of capaci-
tors to use for bypassing is a critical issue and should be
approached carefully. An unbypassed LT1016 is shown
responding to a pulse input in Figure 3. The power supply
the LT1016 sees at its terminals has high impedance at
high frequency. This impedance forms a voltage divider
with the LT1016, allowing the supply to move as internal
conditions in the comparator change. This causes local
feedback and oscillation occurs. Although the LT1016
responds to the input pulse, its output is a blur of 100MHz
oscillation.
Always use bypass capacitors.
TEST CIRCUIT
7404
PULSE
GENERATOR
10
TRACE A
5V/DIV
1k
OUTPUTS
+
LT1016
TRACE B
5V/DIV
TRACE C
5V/DIV
10ns/DIV
1016 F02
–
V
REF
Figure 2. LT1016 vs a TTL Gate
2V/DIV
100ns/DIV
1016 F03
Figure 3. Unbypassed LT1016 Response
Rev D
8
For more information
LINER [ LINEAR TECHNOLOGY ]
