Temperature measurement

[tottekarlsson]

Hi,
I'm planning to measure temperature using a LM35 temperature sensor and a 8bit AD card.

https://www.electro-tech-online.com/custompdfs/2011/01/LM35.pdf

I want to have good precision in the measurement which only need to be in a narrow temperature interval, say 30C +/-5C

The lm35 sensor gives 10 mV/C. The AD measure device have 8bit resolution in the range 0-5V.
If I want to look at the above range, the voltage change will be 100 mV.

My question is, how to make an electronic circuit that can 'zoom' up the x to 100mV to be in the range of the AD, 0-5V? x would be a offset voltage that depends on where the temperature range 'starts'.

Thanks,
Totte

ps: also, if I want more precision than what the lm35 can give, what should one use?

 

[ericgibbs]

Hi,
I'm planning to measure temperature using a LM35 temperature sensor and a 8bit AD card.

https://www.electro-tech-online.com/custompdfs/2011/01/LM35-1.pdf

I want to have good precision in the measurement which only need to be in a narrow temperature interval, say 30C +/-5C

The lm35 sensor gives 10 mV/C. The AD measure device have 8bit resolution in the range 0-5V.
If I want to look at the above range, the voltage change will be 100 mV.

My question is, how to make an electronic circuit that can 'zoom' up the x to 100mV to be in the range of the AD, 0-5V? x would be a offset voltage that depends on where the temperature range 'starts'.

Thanks,
Totte

ps: also, if I want more precision than what the lm35 can give, what should one use?

hi,
You can use a OPA to amplify and offset the LM35 signal to give 0v thru +5V for 25C thru 35C, but it will not improve the accuracy of the LM35 signal only the resolution.

 

[tottekarlsson]

thanks for your reply,
Any suggestion on such a circuit using an opamp? I'm newbie on opamps
I hope to be able to 'switch' between a few intervals, with a switch.

Thanks,
ps:To make really accurate temp. measurement one can use crystals (only heard about it, don't know how exactly).

 

[MikeMl]

Here is an opamp circuit that does what you asked for: I used LTSpice to iterate on the resistor values until I fit the ends of the V(AN0) vs temperature line to go through zero and 5V, respectively.

Note that this circuit uses the ADC Reference voltage (5.000V) as the source of the offset, which means it should be relatively immune to errors induced by drift in the supply. I.e., the circuit is ratiometric with respect to the 5V supply.

 

[tottekarlsson]

Great!
I'll try to build the circuit and get back with the result,

Thanks!
totte

 

[tripping]

ps: also, if I want more precision than what the lm35 can give, what should one use?

The MCP9700 TO-92 Temp Sensor Analog gives you a better error band +-2%
And is 10mV / °C