If you
follow these simple rules, your lab experience will be a happy one.
When building a new circuit.
- Print out the data sheets for the integrated
circuits you will use. Make sure you know
which pins get connected to the power and
ground, and that you understand the absolute
maximum ratings on all pins.
- Keep your workspace and project tidy. Use
short wires, color code when possible. If
you are not neat, your circuit will not work.
- When building your circuit, remember that
the word circuit implies a closed
loop. If your circuit is connected to the
positive output of a power supply, but the
circuit ground is not connected to the ground
on the power supply, there is no return path
for the circuit. Conservation laws should
tell you something is wrong; more current
flows into the circuit than comes back out.
- Another word for ground is common. This
should imply that all grounds must be connected
in order for the ground to provide
a common reference voltage for the whole
circuit. Sometimes circuits have separate
analog and digital grounds, but ultimately
there is always a DC connection (e.g., through
an inductor). Connecting to case ground
is usually a good idea, but not necessary.
- Use bypass capacitors of between 0.1 µF
and a few µF from power to ground,
as close as possible to all DC power levels
on every IC on your board.
- Before turning on your circuit, check for
continuity between the power supply and power
pins on the ICs. Also check for continuity
of ground between the ground pins on the
ICs and the supply ground. Check for shorts
between power and ground. Make sure that
all power and ground pins are connected for
all ICs.
- Put your finger on one or more of the ICs
as you switch on the power supplies for the
first time. Ideally all supplies should be
switched on or off simultaneously. If the
IC gets hot, turn off all power immediately.
- If you pass this smoke test, use
a Volt-Ohm meter to measure the voltage on
the pins of the ICs.
- Using a high impedance scope probe, trace
the signal through the circuit from input
to output, one step at a time.
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