Ramsey Electronics COMPUTEMP CT255 - Manual (Page 12)

CT255
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You may notice though that the smallest step in this case is 1/256. This 8 bit
DSP is unable to accurately represent this number smaller than 3.906E-3.
Most fixed points DSPs however are 16 bit, 24 and 32 bits. 24 bits of
resolution means the smallest number we can represent becomes 1/(2^24)
which is 5.96E-8, which is considerably smaller and more accurate. Still the
math isn’t perfect, and these “steps” will accumulate over time and cause
errors. A 24 bit DSP number is usually in a 0.24 fixed point binary value, 0 bits
to the left, 24 to the right of the decimal point.
Some random parts, and what they do
C5 has been added to the output of the ladder to “smooth” the steps slightly.
U2, the 74HC393, is so fast at its job that when an output switches it causes a
slight bit of overshoot. The overshoot turned out to be about the same voltage
as a half of a degree reading, causing the reading to be incorrect sometimes.
This capacitor fixed that. Imagine sanding the top corners of all of your stairs
in a new staircase to smooth the freshly sawed edges. It looks similar to
sanded steps on an oscilloscope.
D10 is a rectifier diode. This diode converts our AC adapter input to DC, but
only in what is called a half-wave rectifier. This means that only 1/2 of a
complete AC cycle is used for powering the CT255. The CT255 does not
require much power so a full-wave bridge circuit was not necessary. A full
wave rectifier uses both halves of the AC cycle (negative and positive) for
powering the circuit.
C8 helps to average the pulses that the 1/2 rectifier creates allowing the 5 volt
regulator VR1 to work properly. VR1 smoothes out our incoming supply power
so that it is flat instead of looking like an ocean in a storm. C7 helps the
regulator VR1 work a little better, so it is included.
J1 allows you to whip up a cable with three wires and make an external
sensor. When your external sensor is plugged in, it disables the internal
sensor.