Specifications

138 Control and Automation Solutions Guide
Implementing
Electronic Calibration
Digital pots, which can guarantee
50,000 write cycles, allow periodic
adjustments to occur repeatedly over
long equipment life spans. Conversely,
the best mechanical pots can support
only a few thousand adjustments.
Location exibility and size are also
advantages. Digitally adjustable pots
can be mounted on the circuit board
directly in the signal path, exactly where
they are needed. In contrast, mechanical
pots require human access, which can
necessitate placing them in nonoptimal
locations that result in long circuit traces
or with designers having to resort to
using coaxial cables to make the proper
noise-shielded connections. In sensitive
circuits, the capacitance, time delay, or
interference pickup of these connections
can reduce equipment precision.
Digital pots used in electronic calibration
schemes can be fundamental in
eliminating these types of problems.
In addition, calibration DACs (CDACs)
and calibration digital pots (CDPots)
also enable electronic trimming,
adjustment, and calibration. These
calibration-specic devices often employ
internal nonvolatile memory, which
automatically restores the calibration
setting during power-up and provides
the ability to customize the calibration
granularity to match the application.
For extra safety, one-time programmable
(OTP) CDPots are available. These
devices can permanently lock in
the calibration setting, preventing
an operator from making further
adjustments. To change the calibration
value, the device must be physically
replaced. A special variant of the OTP
CDPot always returns to its stored value
upon power-on reset, while allowing
operators to make limited adjustments
during operation at their discretion.
Leveraging Precision
Voltage References for
Digital Calibration
Sensor and voltage measurements
with precision ADCs are only as
good as the voltage reference used
for comparison. Likewise, output
control signals are only as accurate
as the reference voltage supplied to
the DAC, amplier, or cable driver.
Common power supplies are not
adequate to act as precision voltage
references. They typically are not
designed to meet the accuracy,
temperature coecients, and noise
specications needed in a voltage
reference. All voltage sources have
some imperfect specications for
power-supply rejection ratio (PSRR)
and for load regulation, but typically a
voltage reference will have very good
PSRR specications. The load range
allowed is usually far less than a power
supply’s load range, which reduces its
output voltage tolerance. No control
system can have innite gain while
remaining stable, so there will always
be some loading eect on the output
voltage of a voltage reference.
Compact, low-power, low-noise, and
low-temperature-coecient voltage
references are aordable and easy to
use. In addition, some references have
internal temperature sensors to aid in
the compensation for this environmental
variable. Voltage references with
“force” and “sense” pins further improve
accuracy by removing the slight voltage
eects of ground currents in the circuit.
In general, there are three kinds of serial
calibration voltage references (CRefs),
each of which oers unique advantages
for dierent factory applications. Having
a choice of serial voltage references
enables the designer to optimize
and calibrate with high accuracy.
The rst type of reference, a trimmable
CRef, enables a small trim range,
typically 3% to 6%. This is an advantage
for gain trim in industrial imaging
systems. For instance, coupling a video
DAC with a trimmable CRef allows the
overall system gain to be ne-tuned
by simply adjusting the CRef voltage.
The second type, an adjustable reference,
allows adjustment over a wide range
(e.g., 1V to 12V), which is advantageous
for eld devices that have wide-tolerance
sensors and that must operate on
unstable power. Some examples, such as
portable maintenance devices, may need
to operate from batteries, automotive
power, or emergency power generators.
The third type, called an E
2
Ref,
integrates memory, allowing a single-
pin command to copy any voltage
between 0.3V and (V
IN
- 0.3V) and,
then, to innitely hold that level.
E
2
Refs benet test and monitoring
instruments that need to establish a
baseline or warning-alert threshold.
Summary
Electronic and automated calibration
techniques are becoming mainstream
because they make production more
ecient and products last longer. New
products like CDACs and lower cost
precision DACs, digital pots, and CRefs
from Maxim provide an economical
way to incorporate calibration circuitry
directly into end products to minimize
downtime, reduce costs, and improve
long-term performance, even under
harsh operating conditions.