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ADS1278: Max # of chs in case of Daisy-Chain with Discrete Data Output Mode

Shota Mago
Expert 6810 points
Part Number: ADS1278
Other Parts Discussed in Thread: ADS1220, ADS1232, ADS1261

Hi team,

My customer considers to use ADS1278 and provided some questions.

  1. Is it possible to output 12ch data using Daisy-Chain with Discrete Data Output Mode?
  2. if you have the data, Could you provide ENOB @ 32ksps/Vref=2.5V at each mode?

Best regards,

Shota Mago

  • 0
    TI__Mastermind 37570 points

    Hello Shota Mago,

    Daisy chain only works with one data output line.  Discrete Data output uses a DOUT line for each channel.  For a 12 channel configuration, you would have 12 DOUT lines.  However, you can daisy-chain multiple ADS1278, but the total number of channels is limited by the maximum Fsclk=Fclk.  Please refer to Table 15; in order to support 12 channels on a single daisy-chained DOUT line, you can operate in either High-Resolution Mode or Low-Power, CLKDIV=1 Mode.

    I used the ADS1278EVM to take the below data with the inputs shorted.  The input CLK was adjusted for each mode to maintain an output data rate of 32ksps.  Please note I am providing Effective Resolution, which is based on the ratio of the Full scale voltage range to the input RMS noise. Also note that Low-Speed mode only supports a maximum output data rate of 10.5ksps.

    I am also providing an estimate of ENOB based on the shorted input noise data.  The actual ENOB calculation assumes a sinewave input, where the maximum input range for a sine wave is equal to the full scale range divided by 2√2, and it also takes into account any non-linearity response that results in distortion.

    If you have questions on how Effective Resolution and ENOB were calculated based on the input RMS noise, please refer to the ADC section of the Analog Engineer's Pocket Reference Guide.

    Regards,
    Keith Nicholas
    Precision ADC Applications

  • 0 in reply to Keith Nicholas
    TI__Expert 6810 points

    Hi Keith-san

    Thanks for your reply and taking measurement.

    I understood that Daisy Chain only works at single DOUT line.

    Let me check clarify definition of ENOB/Effective Resolution.

    Datasheet of ADS1232 and ADS1220 describe that ENOB = ln (FSR/RMS noise)/ln(2).

    Meanwhile, ADS1261 calls this equation as Effective Resolution

    ADS1261:Effective Resolution or Noise-Free Resolution = ln (FSR / en ) / ln (2)

    According to your explanation, although these equations are same, actually "ENOB" and "Effective Resolution" are technically different term.

    Equation ln (FSR/RMS noise)/ln(2) should be called as "Effective Resolution" and it's slightly different from "ENOB" which assumes sinusoidal input and its distortion.

    Is this understanding correct?

    Best regards,

    Shota Mago

  • 0 in reply to Shota Mago
    TI__Mastermind 37570 points

    Hello Shota-san,

    Yes, your understanding is correct.  The ADS1232 and ADS1220 datasheet should define the equation ln (FSR/RMS noise)/ln(2) as effective resolution.

    ENOB is an IEEE specification based on the SINAD (SNR+THD) measurement of the device, which is based upon a sine wave input.  The maximum signal level for a sine wave that an ADC can accept is FSR/2√2.  This factor, along with the distortion components that show up using a sine wave input reduce the ENOB by about 2b vs the Effective Resolution calculation.  

    The equation used for ENOB = (SINAD-1.76)/6.02.

    Regards,
    Keith

  • 0 in reply to Keith Nicholas
    TI__Expert 6810 points

    Hi Keith-san

    Thanks for your kind explanation!

    Best regards,

    Shota Mago