# Voltage scaling for Temperature

#### SoundguyAL

Joined Nov 4, 2020
27
HI All,
I'm trying to figure out how to design the circuit to scale voltage from a TMP01 from Vptat to F.
Also, the circuit in the datasheet uses 2 opamps. I know it can be done with 1 using differential inputs.
I have reduced the equation and now I need to figure out to calculate R1, R2, R3, R4
On the TMP01 Vref = 2.5V.
The conversion for Vptat to 10mV/F is:
Vptat*3.6 - 4.6

So at room temperature, and as per the datasheet, 1.49*3.6 - 4.6 = .764 (76.4F)

Just to prove the eqn...
Boiling point = 1.86575
1.86575*3.6-4.6 = 2.1167 (212F)

and freezing
1.36575*3.6-4.6 = .3167 (32F)

The transfer function is:

I need some tutelage on how to tackle finding the R values.
Do you just set 1 or more R values at say 100K and solve?

Whats the approach?
I really appreciate some knowledge here!

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#### jpanhalt

Joined Jan 18, 2008
11,087
Welcome to AAC, Saoundguy.

Isn't calculation of the divider resistors covered on page 9 of the datasheet or thereabouts?

#### SoundguyAL

Joined Nov 4, 2020
27
Welcome to AAC, Saoundguy.

Isn't calculation of the divider resistors covered on page 9 of the datasheet or thereabouts?
Thanks jpanhalt!

No, those calculations are making the high and low temp set points for the comparator.
I'm trying to scale the output on pin 5.

#### SoundguyAL

Joined Nov 4, 2020
27
I drew the opamp wrong.... Vref and Vptat are reversed.
And I didn't see how to edit the original post.

Here is the way it's supposed to be:

#### jpanhalt

Joined Jan 18, 2008
11,087
Are these the equations you need:

Source: National Semiconductor AN-31 (old). National Semi is currently owned by TI. I suspect the same or similar AN is on its site.