Specifications

A functional diagram of a typical module is shown in Figure 2.1. It is a useful
reference that shows the data path in the module and to explain the function
of many of the module’s commands.
The first step is to acquire the sensor signal and convert it to digital data. In
Figure 2.1, all the signal conditioning circuitry has been lumped into one
block, the analog/digital converter (A/D). Autozero and autocalibration is
performed internally and is transparent to the user.
The full-scale output of the A/D converter may be trimmed using the Trim
Span (TS) command. The TS command adjusts calibration values stored
internally in the EEPROM. The TS command should only be used to trim the
accuracy of the unit with a laboratory standard reference applied to the
sensor input.
The trimmed data flows into either of two digital filters. The filter selection is
performed automatically by the microprocessor after every A/D conversion.
The filter selection depends on the difference of the current A/D output data
and the previous data stored in the output data register. If the least significant
decimal digit from the A/D differs from the old output data by more than 10
counts, the large signal filter is selected. If the change is less than 10 counts,
the small signal filter is used.
The two-filter system allows for different degrees of filtering depending on
the rate of the input change. For steady-state signals, the small-signal filter
averages out noise and small input changes to give a stable steady-state
output. The large-signal filter is activated by step changes or very noisy input
signals. The time constants for the two filters can be specified independently
with the SetUp (SU) command. The filter values are stored in nonvolatile
memory. Typically, the small-signal filter is set to a larger time constant than
the large-signal filter. This gives very good noise rejection along with fast
response to step inputs.
The modules allow user selectable output scaling in °C or °F on temperature
data. This selection is shown in Figure 2.1 as a switch following the digital
filters. The default scaling in the modules is °C, but this may be converted
to °F by feeding the data through a conversion routine. The switch position
is controlled by a bit in the setup data and may be changed with the SetUp
(SU) command. The scaling selection is nonvolatile. In non-temperature
applications, °C should always be selected.
The scaled data is summed with data stored in the Output Offset Register
to obtain the final output value. The output offset is controlled by the user and
has many purposes. The data in the Output Offset Register may be used to
trim any offsets caused by the input sensor. It may be used to null out
Chapter 2
Functional Description