Specifications

Related functions
Trim, calibrate, and adjust
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frequently or over longer equipment
lifespans, since they can guarantee
50,000 writing cycles. The best
mechanical pots can support only a
few thousand adjustments.
Location flexibility and size are other
advantages compared to mechanical
pots. Digitally adjustable pots can
be mounted on the circuit board
directly in the signal path, exactly
where they are needed. In contrast,
mechanical pots may require human
access, which can necessitate long
circuit traces or coaxial cables. In
sensitive circuits, the capacitance,
time delay, or interference pickup of
these cables can reduce equipment
performance.
Digital pots also maintain their
calibration values better over
time, whereas mechanical pots
can continue to experience small
movements even after they are
sealed. The wiper will move as the
wiper spring relaxes, when the pot is
temperature cycled, or when the pot
is subjected to shipping vibration.
Calibration values stored in digital
pots are not affected by these factors.
A one-time programmable (OTP)
CDPot can be used for extra safety. It
permanently locks in the calibration
setting, preventing an operator
from making further adjustments.
To change the calibration value,
one must physically replace the OTP
CDPot. 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 analog-to-digital
converters (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, amplifier, or cable driver.
Common power supplies are not
adequate to act as precision voltage
references. Typical power supplies
are only five to ten percent accurate;
they change with load and line
changes; and they tend to be noisy.
Compact, low-power, low-noise, and
low-temperature-coefficient voltage
references are affordable and easy
to use. In addition, some references
have internal temperature sensors to
aid in the tracking of environmental
variations.
In general, there are three kinds of
serial calibration voltage references
(CRefs), each of which offers unique
advantages for different factory
applications. Having a choice of
serial voltage references enables the
designer to optimize and calibrate his
exact circuits.
The first type of reference enables a
small trim range, typically three to six
percent; 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 fine-tuned
by simply adjusting the CRef voltage.
The second type is an adjustable
reference that allows adjustment
over a wide range (e.g., 1V to 12V),
which is advantageous for field
devices that have wide-tolerance
sensors and that must operate on
unstable power. Portable mainte-
nance devices may need to operate
from batteries, automotive power,
or emergency power generators.
The third type, called an E
2
CRef, inte-
grates memory, allowing a single-pin
command to copy any voltage
between 0.3V and [V
IN
- 0.3V] and,
then, to infinitely hold that level.
E
2
CRefs benefit test and monitoring
instruments that need to establish a
baseline or warning-alert threshold.
Figure 2 illustrates the production
advantages of using an E
2
CRef. In this
example, a power-supply manufac-
turer uses an E
2
CRef to create an
affordable power supply that stores
the setting established during final
production test. The manufacturer
builds a generic power supply and
places it into a holding inventory.
When a customer order is received,
the output voltage is adjusted by an
automated test system before the
order is shipped.
The power-supply manufacturer
leverages final-test calibration to
provide two real benefits. First, he
reduces cost by using individual
components with relaxed tolerances,
as the final-test calibration corrects
for cumulative errors. Second,
he provides faster delivery to the
customer by making custom adjust-
ments to a standard product.
Figure 2. Illustrating the manufacturing benefits of using an E
2
CRef.
UNITS ARE
ASSEMBLED
TESTED UNITS ARE PLACED
INTO A HOLDING INVENTORY
CUSTOMER ORDERS 10K
1.35V V
OUT
SUPPLIES
UNITS ARE PULLED FROM
INVENTORY AND V
OUT
IS ADJUSTED
UNITS ARE IMMEDIATELY
SHIPPED TO THE CUSTOMER