hi,
Look at this option.
Note the zero trim pot.
E

The OPA supply can be set back to 5V

I get the same result even i simulated your attached file.
I do not know where this error exist.

 

Is trim pot essential in opamp circuit to set opamp gain ? because it would be difficult set the value of Trim pot value in mass manufacturing

Hi,
If you buy components, there is no way you could guarantee that a particular component will have exactly the same parameters, so you will need so method of trimming out errors.
I did not realise that this project was for a mass production market.??

Which part of my simulation could not duplicate.??

E

BTW: the sim in post #81 is not my circuit.

 

Hi,
If you buy components, there is no way you could guarantee that a particular component will have exactly the same parameters, so you will need so method of trimming out errors.
I did not realise that this project was for a mass production market.??

Which part of my simulation could not duplicate.??

E

BTW: the sim in post #81 is not my circuit.

You posted in post #74
The schematic was mine but your simulation results is different from mine.
In your simulation resistance in Xaxis increases from 6586 to 43834 ohm and at 43834vohn vout reaches Zero.

Could you guide me how to plot this resistance ?


Yes this project is for mass market and volume is high.

Regards

 

hi,
For some reason, the identical circuit plots from High resistance to Low on your PC and mine plots from Low to High resistance.
Also the Zero is different.
My LTSpice is up to date with the latest version.

I will keep looking

Edit:
post #69 @crutschow plots the same as mine Low to High resistance.

 

hi,
For some reason, the identical circuit plots from High resistance to Low on your PC and mine plots from Low to High resistance.
Also the Zero is different.
My LTSpice is up to date with the latest version.

I will keep looking

Thanks

pl look if you can find ?

Regards,

 

hi
Check this, place your cursor on the Xaxis label, Right Click, see a pop up , note the settings , are they same as mine.
Post an image of the pop up.
E

 

hi
Check this, place your cursor on the Xaxis label, Right Click, see a pop up , note the settings , are they same as mine.
Post an image of the pop up.
E
View attachment 182770

Thanks got correct simulation results.
Regards,

 

Thanks got correct simulation results.
Regards,

So what was the problem?

 

So what was the problem?

Incorrect : I wrote Rt list in descending order
Correct: Ascending Order



Thank you so much to you.
You helped me a lot .!!!
Keep doing good work.
Training young engineer and leaving legacy is great job. !!!

 

Hi All,

I am opening this thread again with some issue.
Hope i will get some recommendation from this forum.
Below is the resistance value form NTC vs temperature.

-5	42.327
-4.5	41.2435
-4	40.16
-3.5	39.1365
-3	38.113
-2.5	37.1495
-2	36.186
-1.5	35.2765
-1	34.367
-0.5	33.5085
0	32.65
0.5	31.84
1	31.03
1.5	30.264
2	29.498
2.5	28.7745
3	28.051
3.5	27.367
4	26.683
4.5	26.037
5	25.391
5.5	24.7805
6	24.17
6.5	23.5925
7	23.015
7.5	22.4665
8	21.918
8.5	21.401
9	20.884
9.5	20.393
10	19.902
10.5	19.436
11	18.97
11.5	18.5305
12	18.091
12.5	17.6735
13	17.256
13.5	16.8585
14	16.461
14.5	16.0855
15	15.71
15.5	15.355
16	15
16.5	14.6625
17	14.325
17.5	14.003
18	13.681
18.5	13.377
19	13.073
19.5	12.782
20	12.491
20.5	12.2155
21	11.94
21.5	11.6805
22	11.421
22.5	11.1725
23	10.924
23.5	10.686
24	10.448
24.5	10.224
25	10
25.5	9.787
26	9.574
26.5	9.3695
27	9.165
27.5	8.972
28	8.779
28.5	8.5925
29	8.406
29.5	8.2305
30	8.055
30.5	7.9135
31	7.772
31.5	7.587
32	7.402
32.5	7.249
33	7.096
33.5	6.9515
34	6.807
34.5	6.6695
35	6.532

Used Circuit :



NTC Details :

https://www.st.com/resource/en/datasheet/stm32f405rg.pdf

Source Variation for Vadc = 3.3V with 2% Accuracy.
Now my question is -

1. How much accuracy i can get using this circuit. ?
2. Can i able to get accuracy of 0.5degC or less. ?
3. How to theoretically calculate the accuracy of the NTC. ?
4. Can i get 0.1degC step measurement using this NTC ?


Thanks i advance.!!!

 

If the voltage supply to R38 is the same as Vadc but isn't stable then I don't think you can expect better than 2% error, regardless of any NTC error. However, if the voltage is stable but has a 2% tolerance then you should be able to calibrate the system at accurately known temperatures, so that the error can be allowed for.

 

If the voltage supply to R38 is the same as Vadc but isn't stable then I don't think you can expect better than 2% error, regardless of any NTC error. However, if the voltage is stable but has a 2% tolerance then you should be able to calibrate the system at accurately known temperatures, so that the error can be allowed for.

Thanks Alec_t

It was quit good information and although I am not expert but i had same thought about calibration.

Now I want to make it very clear that 3.3V is coming from LDO which has accuracy of 2%. I believe LDO output is stable but it has 2% variation thoughout the temperature. So LDO output may vary 2%.

The datasheet of NTC says that it has 0.2degC variation itself throughout the temperature range.

So as per my understanding the error at 35degC equal to ....

35×0.02= 0.7degC

Add 0.2 degC variation of NTC so total error is 0.9degC at 35degC temp and that is close to 1degC theroritical considering ADC itself error..

Now if you suggest for calibration how can it be done.

Could you suggest the calibration process as i am not much familiar about it ?

I think 1degC error is not acceptable.

Please enlight me more .

Thanks a lot

 

Could you suggest the calibration process as i am not much familiar about it ?

Calibration would involve recording the circuit output when the LDO regulator is kept at one or more accurately known temperatures and the NTC thermistor is at each of a set of accurately known test temperatures. Therein lies a problem: you would need an alternative, very accurate method of maintaining and measuring those test temperatures.
To reduce error you could perhaps make use of a more accurate reference voltage source to supply the thermistor.

 

Calibration would involve recording the circuit output when the LDO regulator is kept at one or more accurately known temperatures and the NTC thermistor is at each of a set of accurately known test temperatures. Therein lies a problem: you would need an alternative, very accurate method of maintaining and measuring those test temperatures.
To reduce error you could perhaps make use of a more accurate reference voltage source to supply the thermistor.

Hi Alec_t
Thanks for information,

As per current circuit both mcu adc supply (reference ) and NTC are connetected with same source and if there will be variation in LDo output there will be a change in both voltage.

Are you talking about fixing more stable source for NTC at R38 or microcontroller adc pin supply voltage.?

Still I did not get more perception NTC calibration ?

 

One more addition to above points that my micro-controller does not Vref Pin so it is considering Analog supply for its measurement.

Regards,

 

One more addition to above points that my micro-controller does not Vref Pin so it is considering Analog supply for its measurement.

hi 87,
Which type of MCU are you using and programming language.?
E

 

If your micro is an STM32F, according to section 3.10 of its datasheet it has an internal reference voltage source and an internal linear temperature sensor, complete with calibration values. Can't you use those features?

 

hi 87,
Which type of MCU are you using and programming language.?
E

STM32F405 series with C++ prgramming language

 

If your micro is an STM32F, according to section 3.10 of its datasheet it has an internal reference voltage source and an internal linear temperature sensor, complete with calibration values. Can't you use those features?

Hi All,

I am still struggling with same issue trying with several permutation and combination but at the end no solution found that can be easily implemented.
https://www.digikey.com/product-detail/en/stmicroelectronics/STM32F405ZGT6/497-11909-ND/2757671

Calibrating temperature sensor or internal reference is tedious task in production.

Regards,

 

Hi All,

I am still struggling with same issue trying with several permutation and combination but at the end no solution found that can be easily implemented.
https://www.digikey.com/product-detail/en/stmicroelectronics/STM32F405ZGT6/497-11909-ND/2757671

Calibrating temperature sensor or internal reference is tedious task in production.

Regards,

If you really want tight tolerances, high precision, and no calibration, you may need to fundamentally change your approach.

I'm not an expert at this, but I seem to remember that taking a high quality RTD and reading it through a wheatstone bridge is one of the more reliable ways of achieving better accuracy and precision without the need for manual calibration on a per-unit basis.

Most ways of measuring temperature will inevitable require individual calibration to achieve the accuracy you seek.