# 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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#### AlbertHall

Joined Jun 4, 2014
11,598
The easiest way is to remove the two diodes. Why do you have those two diodes in the circuit?

#### AlbertHall

Joined Jun 4, 2014
11,598
OIC, the diodes are so you can measure negative temperatures, in which case you need a circuit like this.

#### 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:

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!

#### joeyd999

Joined Jun 6, 2011
4,477
I'm not a big fan of silicon temperature sensors. I'd scrap all the complexity and use a thermistor.

#### 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.

#### AlbertHall

Joined Jun 4, 2014
11,598
If you want a negative output voltage for negative temperatures then the opamp will need positive and negative supplies.
OOPS, and swap the inputs.
Yep. R2 should connect LM35 Gnd and R4 should connect to LM35 output.

#### 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.

#### AlbertHall

Joined Jun 4, 2014
11,598
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.
That's what I said.