Specifications

136 Control and Automation Solutions Guide
Test Equipment
Calibration
While calibration of the end product is
required to establish its performance,
the production test equipment used
to calibrate it must, of course, also
be operating within its specications
(Figure 3). This calibration is maintained
with more accurate test equipment
and reference standards used only
occasionally for this purpose. Eventually
these standards must also be calibrated.
As one moves further back in the chain,
the equipment gets more accurate and
more sensitive, usually by an order of
magnitude or at least 4:1 at each stage,
so it must be treated with more care to
avoid “knocking” it out of calibration.
Today test equipment is being built
with new techniques that reduce or
eliminate calibration expenses or
downtime. These techniques, called
electronic or automated calibration, use
self-calibration or digital calibration.
Benets of Automated
Calibration
Automated calibration can reduce cost
in many areas. It does this by removing
manufacturing tolerances, allowing
the use of less expensive components,
reducing test time, improving reliability,
increasing customer satisfaction,
reducing customer returns, lowering
warranty costs, and increasing
the speed of product delivery.
Automated Calibration
Characteristics
Automated calibration is built around
circuitry that is designed into the end
equipment for the explicit purpose of
maintaining calibration. This circuitry
can take a variety of forms and
functions. For example, this circuitry
could utilize digital communication
between the end equipment and a
remote host or a factory test system.
Once communication is established,
the end equipment uploads data to
the host and then through commands
and downloaded data, the host
calibrates the end equipment’s circuit
parameters. Or, the circuitry could be
completely internal to the equipment
itself. In this latter case, the circuitry
might measure an imbedded precision
component, such as a precision resistor
or voltage reference, to allow adjustment
and verication of the accuracy of
the signal chain components.
Testing and calibration generally
fall into three broad areas:
1. Production-line final-test calibration
2. Periodic self-testing
3. Continuous monitoring and
readjustment
Automated and electronic calibration
can be cost eective in each area.
Final-Test Calibration
When a circuit is developed in the lab,
typically 20 to 50 devices are prototyped
and tested. All signal levels are
measured, and variances and tolerance
margins are noted. However, when the
product goes into production, hundreds
of thousands or even millions of devices
are built and they do not receive the
same level of testing for proper signal
levels, variances, and tolerance margins.
All practical components, both
mechanical and electronic, have
manufacturing tolerances. The more
relaxed the tolerance, the more
aordable the component. When
components are assembled into a
system, the individual tolerances
accumulate to create a total system
error tolerance. When thousands
of devices are manufactured, the
errors can multiply so that a properly
manufactured product may not work.
If this happens enough to reduce
yields, then protability is aected.
Through the proper design of trim,
adjustment, and calibration circuits, it
is possible to correct for the worst-case
tolerance stackups, thereby ensuring
that a higher percentage of products
can be made to meet specications
upon exiting the assembly line.
Final-test calibration corrects for these
errors. Multiple adjustments may be
required to calibrate the device under
test (DUT) to meet specications.
For example, suppose the design
engineers nd that they can use ±5%
resistors and a low-cost op amp because
their Monte Carlo testing shows that
even under worst-case tolerance
stackup, the use of two low-cost digital
potentiometers (pots) for oset and
gain can calibrate out all the variation
from the components chosen. They also
see that to eliminate the adjustability
altogether, they would have to use
expensive tight tolerance resistors and a
precision op amp. With this knowledge,
they decide to use the circuits as-is
and to simply adjust the oset and
span (gain) during nal test to meet
system specications. By using digital
pots instead of mechanical pots, they
avoid using human labor to make the
adjustments.
Periodic Self-Testing
Environmental inuences in the eld can
create a need for test and calibration.
Figure 3. An oscilloscope has many functions to be calibrated
in the instrument and on the probe itself. Voltage probes
usually have a compensation adjustment for proper frequency
response. A true square wave is generated by the scope to test
this probe setting. Internal self-tests and self-calibrations are
common, but use of known good external standards is still
periodically needed for calibration certification.