# LM35 1V-5V Output from 30°C to 100°C (OpAmp Config)

Discussion in 'General Electronics Chat' started by Eduardo Aguirre, Aug 30, 2015.

1. ### Eduardo Aguirre Thread Starter New Member

Aug 30, 2015
1
0
Hello guys! I was wondering if you could help me with a class assignment. Right now I am learning about OpAmps configurations, I am trying to modify an output signal coming from a LM35 in order to achieve a 1V at 30°C and 5V at 100°C output.

I am not sure if I am doing this right so your help will be appreciated.

The LM35 Datasheet shows that the Volt/Temp output works with a 10mV / °C linearity. That means that at 30°C Vout = 300mV. So first I need to add 700mV to get 1V at 30°C (I know I can do that just adding a constant input of 700mV from another source).

But here's the problem, so far I know that between 30°C and 100°C I have a 70°C gap, and also from 1V to 5V there's a 4V gap which means I must add a (4/70) factor for every °C which gives:

For me it seems like I need a variable Gain to get something like: 1V + n(4/70)=Vout where n is the gain and varies from 1 to 70, and I dont know if a configuration like that is possible.

Another solution might be just multiplying each 10mV for a 4/70 gain but as input I'll just have values like 310mV, 320mV, 330mV and the linearity will be lost ( is there a way to just take that 10mV difference per °C?)

I am pretty sure there's an easier way to do it but I've been struggling with this, and that's why I am asking for help. Thank you!

2. ### atferrari AAC Fanatic!

Jan 6, 2004
2,839
942
Hola Eduardo,

Usually, it is always the same: you scale the difference between extreme inputs to get the desired difference between extreme outputs, and THEN you move them (offset bah) a certain value.

BTW, before doing any further calculation do yourself a favor and RE-READ the datasheet.

Keep in mind these words: linearity, absolute, Kelvin and lost. Then, go for numbers using the magic one.

Show your work as tidy as possible.

3. ### dl324 Distinguished Member

Mar 30, 2015
4,991
1,077
This should probably be posted in the Homework forum.

What are your constraints? Are you to use only the LM35 and an opamp?

4. ### crutschow Expert

Mar 14, 2008
16,545
4,459
Close, but no cigar.
You don't need (or want to try to design) a circuit with variable gain.
It just needs a fixed gain with an offset.

Here's some additional info to help get the ball rolling:

The LM35 output voltage changes 10mV * (100-30) = 700mV from 30°C to 100°C.
For that you want a voltage change of 5V - 1V = 4V at the op amp output.
So what op amp gain do you need for that?

Since you also want a LM35 output of 300mV to equal 1V at the opamp output then you will need to add/subtract an offset.
So, with the op amp gain that you calculated above, what offset is needed to give a 1V output at 30°C?

The rest of your assignment is to design the op amp circuit to do that.