Ramsey Electronics COMPUTEMP CT255 - Manual (Page 9)

CT255
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to add 32. We do this by switching in R22 into the circuit, which combined with
R23 adds in the 32 offset that we need. The problem however is this acts like
a voltage divider on the output of U1:A our scaling amplifier, so now we have
to adjust the non-inverting amplifier’s gain to compensate.
First off we need to set the zero point of the sensor to a count of 32. This
requires us to have a voltage on the opamp pin 6 of 12.16mV * 32 * 2. (Don’t
forget to scale by 2 for our 1/2 degree steps!) This means we need pin 6 to be
778mV when the output of U1:A is 0.0V.
We do this with our voltage divider of R22 and R23. We know our supply
voltage is close to 5.0V from our regulator VR1, so we choose a random
resistor for R22 within the 1K to 10K range. In this case we choose 10K. Now
we can find R23 using the formula:
Vout = Vin * R23 / (R22 + R23) or 0.778 = 5.0 * R23 / (10000 + R23)
Rearranging we get:
R23 = Vout * R22 / (Vin—Vout) or R23 = 0.778 * 10000 / (5.0—0.778)
So R23 = 1.84K. The closest 1% tolerant resistor is then 1.82K.
Now we have to find out what it takes for 100.0 degrees Fahrenheit to have a
proper count of 100.0 on the display. For this we need a voltage of 12.16mV *
100 * 2 or 2.432 volts. This will have to be at pin 6 of U1:B, not pin 1 of U1:A.
So how do we get that?
First we have to work backwards from pin 6 of U1:B. Here we will insert our
2.432 volts. This means the voltage at pin 1 of U1:A has to be found by taking
the 5.0 volts on R22 on the header side and the 2.432 volts at the pin 6 side
into account to find what the voltage needs to be at the pin 1 of U1:A side
needs to be to make the numbers work. This can be found by looking at the
current through R22, which is found through Ohms law by:
I(R22) = (5.0V—2.432V) / 10000 =
So I(R22) = 0.256mA
Then using this current to find the voltage across R23.
So V(R23) = 1.82K * 0.256mA or 0.467 volts across R23.
This does not take into account current into pin 6 of U1:B, which could effect
these levels considerably if it is large enough. We have designed in a nice rail
to rail opamp to reduce this to a minimum. This means the output of U1:A
needs to be 2.432—0.467 or 1.965 volts at 100.0 degrees Fahrenheit.