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ADS1262: Noise free bits and resolution related clarification

Praveenlal Edappala
Prodigy 30 points
Part Number: ADS1262

Tool/software:

We are using the for one of the critical low amplitude measurements. We have optimized the configuration according to requirements.

The table 8.1 of datasheet indicates that the noise level is 27nv p-p for 2.5SPS, Sinc4 and 32 gain.

The table 8.2 of datasheet indicates that the noise-free bits is 23.3 for 2.5SPS, Sinc4 and 32 gain.

These two parameters are mentioned for VADD = 5V and Vref = 5 V at 25de C as mentioned in the heading of these tables 

Now, in our configuration, we are using AVDD 2.5V, AVSS -2.5V, and external reference 1.25V, under these conditions, can you please suggest the deviations in the values of the above-mentioned parameters. i.e. due to a change in the AVDD and reference voltage, what will be the change in the noise free bits and notice level in p-p.

your response will be a great help in the verification and validation of our design for benchmarking.

  • 0
    TI__Mastermind 43140 points

    Hello Praveenial,

    The input referred noise in terms of voltage will be approximately constant over supply voltage and reference voltage.

    In your case, using AVDD=+2.5V, AVSS=-2.5V, and Vref=+1.25V, your can expect the same 27nVpp of noise when using the SINC4 filter, Gain=32, and 2.5SPS.  However, the noise free bits depends on the full scale input range, which will be effected by the lower reference voltage.

    We can calculate this using equation 8:

    FSR=2*Vref/Gain=2*1.25/32=0.078125V

    Vnoise=27nVpp

    Noise Free Bits = ln(0.078125/(27*10^-9))/ln(2)

    Noise Free Bits = 21.5b

    Regards,
    Keith Nicholas
    Precision ADC Applications

  • 0 in reply to Keith Nicholas
    Prodigy 30 points

    Thank you Keith. I could clear my confusion.

    Can you pls help me to know the FSR for the same setting? Confused because of the table 9-2 shows that the FSR is +/-0.078V for 32 gain and ref was 2.5V

    SO will it be bipolar even though the ref is unipolar

    in our case we are applying Vref+ is 1.25 V and Vref- is 0V

    kindly help

  • 0 in reply to Praveenlal Edappala
    TI__Mastermind 43140 points

    Hello Praveenial,

    The full scale differential input range with Vref=1.25V, Gain=32, will be +/-39.0625mV.  The voltages on the inputs with respect to ground can be positive or negative.  AINP=+10mV and AINN=-10mV would be converted as a positive differential voltage of (AINP-AINN)=+20mV.  If AINP=+20mV and AINN=0V, the input would still be a positive differential voltage of +20mV.

    Yes, the inputs will still measure differential signals, and the input voltages can be positive or negative with respect to ground in your configuration.

    Take a look at the below FAQ, which may provide some additional insight.

    https://e2e.ti.com/support/data-converters-group/data-converters/f/data-converters-forum/1048636/faq-can-my-adc-measure-negative-voltages

    Regards,
    Keith

  • 0 in reply to Keith Nicholas
    Prodigy 30 points

    Dear Keith,
    Thank you again for the awesome technical support. Now probably the last confusion.

    So, with our settings, of G=32, External VRef = 1.25V, Supply +/-2.5V, I can conclude, that Input FSR is +/-39.0625 mV, Noise is 27nV P-P and ENOB is 21.5 bits, So the resolution will be 26.34nV. Can you pls confirm the resolution?

    Also, i need to confirm that as the noise level itself is 27nV the minimum detectable/measurable input voltage should be greater than 1 LSB which is greater than 27nV. Thank you in advance

  • 0 in reply to Praveenlal Edappala
    TI__Mastermind 43140 points

    Hello Praveenlal,

    Effective Resolution (ENOB) is based on RMS noise, which is 7nVrms and will be 23.4bits.  Noise free resolution will be 21.5bits.  

    Yes, in the case of using SINC4 filter at 2.5sps, the practical minimum measurable input will be 27nV, which is equal to the peak-to-peak noise.  Each code change on the ADC, or 1LSB, will be smaller than this value, or 2*39.0625mV/2^24=4.7nV.  In other words, the measured output codes will change by 27nV/4.7nV/code = 6 codes.

    Regards,
    Keith

  • 0 in reply to Keith Nicholas
    Prodigy 30 points

    Hi Keith,

    Thank you once again for nice explanation. I am very clear about the values now.

    So, in my design, the configuration is

    AVDD 2.5V, AVSS -2.5V, External Ref 1.25V, Sync 4, 32 Gain

    • ENOB = 23.4
    • Noise Free Bits = 21.5
    • Resolution = 4.7nV
    • The minimum detectable/measurable voltage = 27nV P-P (7nV RMS)

    Now I would like to clear two more confusion related to this

    1) If noise-free bits are 21.5, then what is the significance of specifying ENOB as it is of no use?

    I mean can we improve NFBs to reach to ENOB by doing any kind of further offline processing like filtering or coherent averaging?

    2) Table 8.2 of the datasheet of ADS1262, whether or not it is with Chop mode on?

    If Table 8.2 is with chope mode off, and if we enable it, will the noise be further reduced by a factor of 1.4, as mentioned in section 8.8 Noise Performance in the datasheet?

    Pls help to resolve the above two queries as well...

    Thank you Keith in advance for your great support.