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Original question:

ADS1256: ADC Accuracy comes from INL

ADS1256: accuracy and noise free resolution in system level

ilhan taygurt
Prodigy 235 points
Part Number: ADS1256

Dear Chris,

could you pls explain the difference between accuracy and system resolution much deeper?

In your reference design you only made calculation according to system accuracy. but what is the noise-free resolution of the system including external driving circuitries, etc.

for example ADS1256 has 24-bit resolution but it has 18.6 noise-free bits. I think this is due to ADC inherent noise by shorting inputs. What about the Noise free resolution calculation in a simple ratiometric RTD circuit or thermocouple circuit with 2MR pullup&pulldown for biasing? 

Best regards

  • 0
    TI__Mastermind 44610 points

    Hi Ilhan,

    "Resolution" is a measurement's repeatability. More repeatable results generally make for better measurements; however, just because a measure is repeatable doesn't necessarily mean the result is accurate.

    "Accuracy" is what you'd expect, the error between a measured voltage and it's actual voltage.

    I think the most important takeaways are:

    • If resolution is very bad (i.e you cannot achieve repeatable results), then the resolution will hurt your system's accuracy; You cannot achieve accuracy that is better than the measurement repeatability.

    • If your system's resolution is very good, then it becomes possible to measure or "calibrate" the accuracy of the system to compensate for the measured error (assuming the error is fairly constant).

    Are you referring to the TIPD188 reference design? This design showed the noise performance with an external 10 mV signal applied (the maximum input amplitude for a Wheatstone bridge with 2 mV/V sensitivity, excited by 5 V).

    Noise performance of the ADC with external circuitry will differ depending on the design of this circuitry, so it doesn't always make for the best universal example. Typically, showing the noise performance of the ADC by itself is helpful to all applications, as it provides you with a baseline to know what kind of performance ADC is capable of achieving. With high-resolution ADC's, such as the ADS1256, external circuitry will usually only degrade performance, so it's up to the circuit designer to design this circuit to provide comparable performance which won't degrade significantly from the ADC's capabilities, including trying to find an accurate and low noise sensor, which can be difficult as sensor datasheets don't always show noise performance.