transfer function of a temperature sensor

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Anonymous
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OK I am new at temperature sensors, I have a STLM20 sensor with an attached below transfer function.

   

according to my calculations, T should equal the following:

   

T=85.543*(1.8641-V0)

   

is this correct? because it is measuring -15.75 degrees C.

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15 Replies
Anonymous
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 Ok so I solved the negative temperature problem.

   

the problem is now it is reading too high (27.5 degrees) in a 20 degrees environment.

   

any advice?

   

 

   

int main()

   

{

   

float T;

   

int32 voltcount;

   

float volt;

   

char str[16];

   

LCD_Start();

   

ADC_Start();

   

ADC_StartConvert();

   

    for(;;)

   

    {

   

ADC_IsEndConversion(ADC_WAIT_FOR_RESULT);

   

        voltcount=ADC_GetResult16();

   

        volt=ADC_CountsTo_Volts(voltcount);

   

T=85.543*(1.8641-volt);

   

sprintf(str, "%+1.1f", T);

   

        LCD_Position(0u,0u);

   

        LCD_PrintString(str);

   

CyDelay(1000);

   

    }

   

}

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Anonymous
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Is this reading by the PSoC5 or by a multimeter?

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Anonymous
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 by POSC5.

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Anonymous
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Is the ADC reference set to a Vdd derived reference or the internal

   

bandgap 1.024 reference ? If former Vdd is +/- 5% typically out of

   

a regulator.

   

 

   

You need to perform a complete signal path error analysis to know

   

what accuracy, precision you will get.

   

 

   

Regards, Dana.

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Bob_Marlowe
Level 10
Level 10
First like given 50 questions asked 10 questions asked

The equation given in the PDF is mixing units mV and V. You should take that into account when you rearrange the equation,

   

And where do you get the Item 85.543 from?

   

 

   

I calculate

   

Vo - 1.8663V = T[°C] * 11,8639mV/°C = T * 11.8639mV

   

T = (Vo - 1.8663V) / 0.011.8639 V

   

As you can see a calibration at the midpoint of your temperature scale could get rid of some errors.

   

 

   

Bob

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Anonymous
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Anonymous
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dividing 1/0.0118639 yeilds 85.543.

   

I have used (0.0 to 2.038 single ended 0 to Vref*2) input range of the ADC with a 100K conversion rate and it gave me more reliable results. but still there is a big difference between the real value and the measured value.

   

I am switching to the long equation. any idea on how to get the sqaure root of a float number?

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Anonymous
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The sensor tolerance is +/- 1.5 C, what are you using to determine T to

   

compare to PSOC result ? Ice bath one of the easy methods to cal a T

   

sensor at one point.

   

 

   

GCC and sqrt -               http://forums.debian.net/viewtopic.php?f=8&t=109385

   

 

   

Regards, Dana.

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Anonymous
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 I am using an old digital thermometer that is installed in a digital watch. it is currenttly reading 26 degrees while my sensor is reading 24.5 degrees.

   

1.5 degrees is tolerable error. thanks everyone 

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Anonymous
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 The first thing you should do is to use the equation for V0 that is relevant for your temperature range (given in Table 2 of the attached document). Secondly use a more accurate temperature sensor than using a digital thermometer that is installed in a watch. This will give you true temperature reading to compare with PSOC result.

   

 

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ETRO_SSN583
Level 9
Level 9
250 likes received 100 sign-ins 5 likes given

@deniz, just a thought, do you think customer still watching a 6 month old thread ?

   

 

   

Regards, Dana.

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Anonymous
Not applicable

 Well Dana, most people won't do that. But I'm a special case, I even check threads that are more than a year old 

   

weird? or simply eccentric? 

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HeLi_263931
Level 8
Level 8
100 solutions authored 50 solutions authored 25 solutions authored

At least regular visitors here can look for forum with new stuff since their last visit (you just need to remember when it was :). And inside there the newest threads are on top making it easy to find new stuff even after a week or so.

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ETRO_SSN583
Level 9
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Realtime examination of all your thread history history pretty awesome.

   

 

   

But is this a geometric progression of time needed, and soon you will have no time

   

to eat, sleep 😞

   

 

   

Reagrds, Dana.

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Anonymous
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 Hahahahaha you're absolutely right. I need to stop doing that 

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