Temperature sensor. Тake voltage difference between two points, but respect to the ground.

Thread Starter

rashev

Joined Feb 15, 2017
3
Hi,
I'm new in electronics.
I have a problem with Voltage difference. I want to take voltage difference between two points, but respect to the ground.
Here is the schema: LM35 is temperature sensor and Vout is analog voltage proportional to the temperature :


I wаnt Vout to supply to one input chanel of analog do digital converter. I can supply + to one channel and - to another input channel of ADC and calculate the difference, but i wаnt to use only one channel and supply to this channel difference voltage Vout betwen + and - . What is the most accurate and easy way to generate this voltage respekt to the ground ?
 

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EM Fields

Joined Jun 8, 2016
583
Hi,
I'm new in electronics.
I have a problem with Voltage difference. I want to take voltage difference between two points, but respect to the ground.
Here is the schema: LM35 is temperature sensor and Vout is analog voltage proportional to the temperature :


I wаnt Vout to supply to one input chanel of analog do digital converter. I can supply + to one channel and - to another input channel of ADC and calculate the difference, but i wаnt to use only one channel and supply to this channel difference voltage Vout betwen + and - . What is the most accurate and easy way to generate this voltage respekt to the ground ?
Use a single-supply difference amplifier; for example: http://www.ti.com/lit/ds/symlink/ina152.pdf or a single-supply instrumentation amplifer
 

Dodgydave

Joined Jun 22, 2012
10,088
Use a TMP36 instead, gives out same 10mV/c except it goes down to -50C,

so its zero mV at -50C
and 500mV at zero C,
700mV at 20C, etc
 

Thread Starter

rashev

Joined Feb 15, 2017
3
Use a TMP36 instead, gives out same 10mV/c except it goes down to -50C,

so its zero mV at -50C
and 500mV at zero C,
700mV at 20C, etc

The sensors have different accuracy, according to their datasheets. The TMP36 has ±2°C typical accuracy, while the the LM35 has 0.5°C typical accuracy (over 25°C). This is the reason
 

Thread Starter

rashev

Joined Feb 15, 2017
3
Thank you for the answers and suggestions!
I use operational amplifier LM741 and adapted the scheme suggestion by AlbertHall:newSchema.JPG

Pin 1 and pin 5 from LM741 - "offset null" not use them. Pin number 8 is N/A
Pin 7 "Positive supply voltage" i connect with common +5v voltage supply and pin 4 "Negative supply voltage" to the ground.
Is this true?
Is it all right in my shema?

Thanks again for help boys!
 

hp1729

Joined Nov 23, 2015
2,304
Thank you for the answers and suggestions!
I use operational amplifier LM741 and adapted the scheme suggestion by AlbertHall:View attachment 120595

Pin 1 and pin 5 from LM741 - "offset null" not use them. Pin number 8 is N/A
Pin 7 "Positive supply voltage" i connect with common +5v voltage supply and pin 4 "Negative supply voltage" to the ground.
Is this true?
Is it all right in my shema?

Thanks again for help boys!
I would not suggest the LM741 for that. An LM358 or newer Rail-to-Rail input and output op amp would be better. The LM741 does not work too well with an input close the a power rail voltage. Your circuit would be okay if you were powering it from + and - 9 V, or so.
OOPS, and swap the inputs.
 

hp1729

Joined Nov 23, 2015
2,304
If you want a negative output voltage for negative temperatures then the opamp will need positive and negative supplies.

Yep. R2 should connect LM35 Gnd and R4 should connect to LM35 output.
I don't think so. The resistors are connected right on the op amp. Feedback goes to the inverting input. The connections to the LM35 need to be reversed.
 
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