• State TI Thinks Resolved
  • Locked Locked
  • Replies 1 reply
  • Answers 1 answer
  • Subscribers 46 subscribers
  • Views 655 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

ADS1256: ENOB is higher than Datasheet

Kailyn Chen
Genius 9560 points
Part Number: ADS1256

Hi，

About the ENOB calculation, most of the equation of the datasheet is the same with AD1256 datasheet:

ENOB=ln(FSR/RMS Noise)/ln2

But I found the book of the data converter which describes the ENOB equation is as this

So could you please help to analyze why the common equations of ENOB has not minus 1.79?

In addition, when PGA>8 during using ADS1256, the ENOB value is higher than the datasheet listed. What about the possible reasons?

Best regards

kailyn

  • 0
    TI__Guru 52496 points

    Hi Kailyn,

    I am not able to find any documentation that shows your specific RMS noise equations. Can you provide a link to the source?

    The term "LSB/√12", commonly-written as "q/√12", is the RMS value of ideal quantization noise. That is noise related specifically to the quantization process.

    ADCs also have thermal noise, sometimes referred to as "input noise". This is not calculated the same way as quantization noise, and is instead determined from the standard deviation of a shorted input test. It seems like the equations you are referring to are confusing these two ideas (quantization and thermal noise), as I have never seen them expressed in this manner. Typically effective resolution (sometimes called "ENOB", which is incorrect) is a DC performance metric that is calculated using thermal noise.

    For your measurements: the noise measurements given in the ADS1256 datasheet are with the ADC inputs shorted together (VIN = 0V) over a certain number of samples. Unfortunately the datasheet does not say how many samples were used to derive each noise value. However, if you use more (or sometimes fewer) samples, you might end up with better noise performance than the datasheet suggests. Since the noise is derived from a statistical distribution, it is possible that this distribution is slightly different for different devices and/or the number of samples used to determine the standard deviation.

    In other words, this is common behavior for a higher resolution ADC.

    You can learn more about ADC noise via our Precision Labs training content - see modules 6.1, 6.2, etc.: https://training.ti.com/ti-precision-labs-adcs

    -Bryan