Specifications

Related functions
Trim, calibrate, and adjust
144 Maxim Industrial Solutions
from the gross weight is necessary to
accurately measure the net weight of
the material on the scale. Because the
weight of the package may change
over time due to manufacturing
variation or a change of vendors,
it is desirable to update the tare or
container weight from time to time.
Another example is using a switch to
short an amplifier input to ground
to measure offset voltage. This could
be done during power-on self-test to
compensate for component aging.
Alternatively, it can be performed
periodically to compensate for
temperature-induced drift. If the
temperature drift is predictable and
repeatable, a microprocessor can aid
testing by measuring temperature
and controlling the calibration device
in an open-loop manner.
System gain errors can be calibrated
by switching a known signal into
the equipment at an early stage and
measuring the output level. This is
done at power-up or periodically
during lulls in operation.
Enabling accurate automated
adjustments with calibration
DACs and pots
Calibration digital-to-analog converters
(CDACs) and calibration digital
pots (CDPots) share some unique
attributes that enable trimming,
adjustment, and calibration. The
first advantage is internal nonvola-
tile memory, which automatically
restores the calibration setting
during power-up. Figure 1 illustrates
a second advantage: the ability to
customize the calibration granularity
and location for industrial safety.
Ordinary DACs allow a single
reference voltage (V
REF
) to be
applied; this reference voltage
usually becomes the highest DAC
setting. The lowest DAC setting is a
fixed voltage, typically ground. For a
near-center adjustment, much of this
range between V
REF
and ground must
be ignored and not used, since the
available step size is evenly distrib-
uted over the range. For example,
with V
REF
set to 4V, a 10-bit DAC
yields a step size of 0.0039V per step.
It is critical in industrial equipment to
remove all safety-related errors.
Removing the unused adjustment
range eliminates any possibility that the
circuit could be grossly misadjusted.
The CDAC and CDPot allow both
the top and bottom DAC voltage
to be set to arbitrary voltages, thus
removing excess adjustment range.
In Figure 1, a low value of 1V and
a high value of 2V are selected as
examples. To achieve a 0.0039V step
size over the 1V to 2V range, only an
8-bit device is needed, which saves
cost. Additionally, this increases
safety by removing any possibility
that the circuit could be misadjusted.
The high and low voltages for the
CDAC are arbitrary and, therefore,
can be centered wherever the
circuit calibration is required. If the
tolerance analysis for the circuit
indicates that a range of 1.328V to
1.875V is needed for calibration, it
can be accommodated. The 256-step
device would yield a granularity of
0.00214V. Thus, the granularity of the
adjustment can be optimized for the
specific application.
Reducing cost and improving
accuracy by replacing
mechanical trims with all-
electronic equivalents
Digitally controlled adjustable
devices offer several advantages
over mechanical devices in industrial
systems. The largest advantage is
lower cost. ATE can perform calibra-
tion precisely time after time, thereby
eliminating the considerable costs
associated with error-prone manual
adjustments. Also, digital pots allow
periodic testing to occur more
ORDINARY DAC
10 BITS, 1024 STEPS
4V
GROUND
0.0039V
per step
REFIN
10-BIT
DAC
FB
OUT
CALIBRATION DAC
REFHI
REFLO
8-BIT
DAC
OUT
2V
1V
8 BITS, 256 STEPS
Ground
4V
2V
1V
Figure 1. Comparing the calibration range of an ordinary DAC to a CDAC.