ADC selection. How to calculate the overall error and get the final accuracy of an ADC when selecting it.

Hello, 

There is quite a bit going on here, so I am going to share pointed resources that i believe will be helpful in your design process

This great 3 part blog about accuracy of an ADC, in particular part2: Total Unadjusted Error Explained ~click here~ 

TI also has a collection of circuits created that covers multiple topologies ~click here~ in particularly one for: Temperature Sensing with NTC Circuit

There is also one for ADCs ~click here~

I suggest for your first step to be to finalize your sensor circuit, and from there defining what are your requirements for your system ie sampling rate, and then with this a proper ADC can be selected. 

Regards

Cynthia

Hello Cyntia, thank you for all the info, it is very helpful, above all the Total Unadjusted Error Explained ~click here~. I would also like to read the part 1 of this article https://e2e.ti.com/blogs_/b/precisionhub/archive/2014/10/07/adc-accuracy-part-1-is-accuracy-different-from-resolution.aspx but for some reason the link doesn't work to me.

Regarding this article: https://www.ti.com/lit/an/sboa323a/sboa323a.pdf?ts=1652384359900&ref_url=https%253A%252F%252Fwww.ti.com%252Ftool%252FCIRCUIT060002&_ticdt=MTY1MjkwNTY2NHxsaXRhOTY4YzNkMGE5NjhjM2QwYTk2OGMzZDA2MGFiYjgzZjEyNzYwY2Q2fEdBMS4yLjgzMjU1MjU1MS4xNjIxOTYxODY3 is the current design that we are using, but I find a bit difficult to calculate the error. For an ADC I know that I have to take into account Voffset, gain error and INL, but for the operational amplifier I get a bit more lost.

Regarding giving more details about the sensor circuit, here I go:

We have an NTC, which bias current has to be around 50uA (to avoid selfheating), and also it is enabled just during 100ms every second to do a reading. It is enough with getting just one trustable reading every second (so I suppose that this would answer the question about sampling rate)

My intention is to create a Wheatstone bridge with that, so that Vdd is REF7012QFKHT, with 1.25V output, 0.025% accuracy and 2ppm/ºC. R1 and R3, 20K, 0.01%, 10ppm/ºC, RX would be the NTC and R4 would be 1K 0.01% resistor.

I plan to use Vdd also as the reference voltage for the ADC

For the temperature accuracy that we need, we should detect, in the worst case, 0.9Ohm changes in the NTC (where NTC resistance is about 1107Ohm), which with a bias current of 50uA would be around a 45uV change, so that should be the final accuracy of the system.

I know that because of Vdd and R1 accuracy and tolerance, I also have a deviation, this is why I was targeting a final accuracy of 20uV instead of 45uV ini the ADC selected..

Because the total difference in the Wheatstone bridge would be 20mV (NTC value would change between 65mV and 85mV), I would need to amplify it, if possible in a factor of Vref/wheatstone range=0.9*1.024V/20mV=46 (0.9 for not using the full range of the ADC and using the most linear part of it).

For the gain, I would like to get an ADC with gain included instead of using an instrumentation amplifier.

Calibration could be an option, although it would be preferable not to do it. In case to do it, do you have a reference document for that? Do you think that in this case it would be possible to avoid the calibration, having a final accuracy so strict?

Thank you very much for all the assistance,

Best regards,

Esteban

Esteban, 

That is odd, not sure why the link isn't working for Part1, pleas try the following link 

https://e2e.ti.com/blogs_/archives/b/precisionhub/posts/adc-accuracy-part-1-is-accuracy-different-from-resolution

If I understand correctly, you are not using an op amp following the bridge circuit. But you would like a gain for the output measurement to occur in the ADC? is this correct?

there is a great document A Basic Guide to Bridge Measurements , it is a bit lengthy, but it is covers essentially every topic you are asking about here, gain and calibration. It will also help to decide if an op amp is necessary for your application, and help determine the ADC (with integrated PGA or not)