ADS7806: DNL Error, Accuracy,

Sarang Suryawanshi

Part Number: ADS7806

Hello,

I am referring to application report: http://www.ti.com/lit/an/sbaa004a/sbaa004a.pdf

I have following two queries on page no. 7, 8. Find the attached snapshot for easy reference.

[1] No. of codes for temperature measurement for accuracy of 0.25°C over the span of 1000°C is 4000 (1000/0.25= 4000). This is okay. 12bit ADC will solve the purpose.

However the DNL error of one-half LSB (1/2 LSB) makes the accuracy to 0.244°C. It would be helpful if somebody gives the calculations involving the DNL error effect on accuracy.

[2] I could not find "Typical Error" given in Table 2 in the respective part datasheets (I am particular about ADS7806). Where did the statement "As a first order approximation, the total error for this system would be 0.0147%." come from? ( 0.003% + 0.0025% + 0.01% + 0.01% is not equal to 0.0147% )

Any help would be really appreciated.

Regards,

Sarang

over 7 years ago

0 Michael Stout over 7 years ago

TI__Genius 16890 points

Hello Sarang,

1. Since the ADC is 12 bit there are 4096 codes possible. 1000C/4096 = 0.244C.

2. Take all the errors and rms add them together: sqrt(0.003%^2 + 0.0025%^2 + 0.01%^2 + 0.01%^2) = 0.0147%

Mike

0 Sarang Suryawanshi over 7 years ago in reply to Michael Stout

Prodigy 140 points

Hello Mike,

Thank you for the prompt reply.

1. The accuracy of 0.244C is when there is no DNL error. However, statement "The accuracy, assuming the DNL error for the converter is one-half LSB, will be 0.244°C." considers the DNL error. So, what is the effect caused by DNL error?

2. Thank you! the calculation works out perfect. But I could not find those Typical error values in the datasheet. Could you show me these in the datasheet?

Best Regards,

Sarang

0 Michael Stout over 7 years ago in reply to Sarang Suryawanshi

TI__Genius 16890 points

Hi Sarang,

1. The DNL is the deviation between an ideal step size and the actual step size for each ADC code. If DNL< 1LSB the ADC is guaranteed not to have any missing codes.

2. I am not sure where they got the typical error from. The author of the app note may have had access to the characterization data from when the part was being designed. The app note is over 20 years old, so I do not have that data.

Mike

0 Sarang Suryawanshi over 7 years ago in reply to Michael Stout

Prodigy 140 points

Hello Mike,
1. Okay. What would have been the effect on accuracy if DNL >1LSB (say DNL=2LSB)? Also, with what parameter is the INL related to?
2. Alright.
Best Regards,
Sarang

0 Michael Stout over 7 years ago in reply to Sarang Suryawanshi

TI__Genius 16890 points

Hi Sarang,

There are a couple of application reports on the TI website that explain DNL and INL along with other ADC specifications.
They are sbaa147b and slaa013. You can use the search function on ti.com to find the documents.

Mike

0 Sarang Suryawanshi over 7 years ago in reply to Michael Stout

Prodigy 140 points

Dear Mike,

I shall read sbaa147b.

Thank you for the support extended.

Best Regards,

Sarang

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ADS7806: DNL Error, Accuracy,