Temperature Sensor TMP275 Temperature Data Format question: -0.25 deg c

Cyndy Tee

Refer to datasheet, Table 5: Temperature Data Format.

From the Digital Output column, it looks like first 8 bits are integral part of the temperature, and last 4 bits are the fractional part .

Using the above format for positive temperature, all values can be converted from binary to deg c without problem.

However, I do not get the FFC at all.

According to the table, 1111 1111 1100 = FFC = -0.25 deg c.

for integral part, apply 2's compliment to 11111111 = -1.

Why table shows that is -0???????

Thank you.

Cyndy

over 12 years ago

0 Abhishek Muppiri over 12 years ago

TI__Expert 7945 points

Hello Cindy,

When you do a 2's compliment you need to perform it on the entire 12bit value. Resolution of this part is 0.0625degC (in data sheet page 3 table). Which means every LSB change in the 12bit binaly value represents a 0.0625degC change in the temperature.

Lets apply this on 0.25 --> 0000 0000 0100 (as per table) --> Convert this to decimal--> Answer is 4. Therefore wn we multiply 4 with resolution --> 4*0.0625 = 0.25degC Agreed so far?

Now, lets move one step further and take your example:

-0.25degC = 1111 1111 1100 (FFC)--> 2s compliment is(1s compliment+1) : (0000 0000 0011+ 1)=>0000 0000 0100 -->This is nothing but 0.25degC.

Did you understand my explanation? Apply the above logic to other values in the table so you can understand better.

Let me know if you still have trouble with this...

Best Regards,

Abhi Muppiri

Applications Engineer

AIP- Sensing Products

Texas Instruments

0 Cyndy Tee over 12 years ago in reply to Abhishek Muppiri

Prodigy 20 points

Yes, I understand now.

Thanks for quick reply, that's very helpful.

0 Abhishek Muppiri over 12 years ago in reply to Cyndy Tee