LT1016
APPLICATIO S I FOR ATIO
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 provide
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 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 as internal current
levels of the devices connected to it change. This will
almost always cause unruly operation. In addition, several
TEST CIRCUIT
7404
PULSE
GENERATOR
1k
OUTPUTS
10Ω
TRACE A
5V/DIV
+
LT1016
–
V
REF
Figure 2. LT1016 vs a TTL Gate
2V/DIV
Figure 3. Unbypassed LT1016 Response
8
U
devices connected to an unbypassed supply can “commu-
nicate” through the finite supply impedances, causing
erratic modes. Bypass capacitors furnish a simple way to
eliminate this problem by providing a local reservoir 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 capacitors 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 fre-
quency. This impedance forms a voltage divider with the
LT1016, allowing the supply to move as internal condi-
tions in the comparator change. This causes local feed-
back and oscillation occurs. Although the LT1016
responds to the input pulse, its output is a blur of 100MHz
oscillation.
Always use bypass capacitors.
TRACE B
5V/DIV
TRACE C
5V/DIV
10ns/DIV
1016 F02
W
U U
100ns/DIV
1016 F03
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