Ramsey Electronics COMPUTEMP CT255 - Manual (Page 7)

CT255
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in series with the input to reduce any noise that may be present on the
temperature sensor to increase accuracy of your reading. C3 in the feedback
branch is also used to reduce noise in the reading. In the Celsius jumper
setting, R37 and R27 are used to adjust the gain of our temperature sensor for
a Celsius reading. The gain of a non-inverting amplifier is calculated by the
formula :
A = 1 + Rf / Ri. Where Rf if the feedback resistor R27, and Ri is the input
resistor R37. (Even though the input isn’t at R37, it is considered that for
mathematical purposes).
So looking at the values in the circuit:
A = 1 + 1430 / 1000 or A = 2.43
As you can see this is very close to our requirement of 2.422, which is more
than close enough for +-1 degree C resolution.
More on the ADC
To stop the count when the output voltage of our temperature scaling amplifier
matches the count of our ripple counter DAC, we use a circuit called a
comparator. A simple op-amp makes an excellent comparator, as shown by
U1:B. When the voltage at pin 5 is above the voltage at pin 6, the output at pin
7 goes as high as possible, in this case about 3.9V. When the voltage at pin 5
is below the voltage at pin 6, the output goes as low as possible, or zero volts.
It is called a comparator because it is in reality comparing two voltages, and
giving you an output based on the difference. We employ this circuit to stop
the count in our ripple counter as soon as its output voltage surpasses the
output of our temperature sensor circuit. This allows us to take a reading from
the display, and this is what a computer would see as a value for a complete
Analog to Digital conversion!
Oh that extra count!
Ok, maybe you’re smart and figured out that for the count to stop, the output
voltage of the DAC has to surpass the output of the temperature sensing
circuit. This means that if your temperature is zero, and the count is at zero,
the output of the DAC does not quite surpass the output of the temperature
sensor. This means that the temperature reading will have to increment at
least once to have a reading. This means that your temperature readings will
at the most be high 0.5 degrees F or 0.5 degrees C depending the range you
are in. However, on average it will only be 0.25 degrees high. All ADCs have
this problem, and it is related to the bit resolution (in our case 8 bits). This is
still within +-1C accuracy.