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ADS8912B: 3-wire SPI interface

Taketo Sato
Genius 10335 points
Part Number: ADS8912B


Hello team,

My customer wants to omit SDI line to reduce interface line of ADS8912B. Can we omit SDI line?

In this case, ADS8912B will work normally with SPI-00-S, correct?

Also, could you let me know if there is anything that we should be careful of with the 3-wire(CONVST+/CS, SDO, SCLK) SPI?

Best regards,

  • 0
    TI__Mastermind 47070 points

    Hello Sato-san,

    Yes, you can connect SDI line to GND, and SPI communications will be fixed at SPI-00-S mode.  Please note that in this mode, you cannot change any register values.  If using a reference voltage other than 5V, the customer will not be able to adjust the OFST_CAL register, and there will be additional offset error.  However,  this can be corrected when doing a system level calibration.

    Yes, you can further reduce the number of IO lines to 3 by connecting the CONVST and /CS lines together.  In this configuration, you will be limited to Zone 1 transfer, which will require a higher SCLK frequency, but you will still be able to operating at the maximum 500ksps rate.  Please refer to section 7.5.3 in the datasheet for more details.

    Minimum SCLK needed with CONVST,/CS connected together, 3-wire mode, for ADS8912B.

    t(read-Z1) <= 2000ns-1200nS- 30ns <= 770nS (Equation 5)

    f(SCLK) >= 18/t(read-Z1) >= 18/770nS >= 23.4MHz (Equation 6)

    Regards,
    Keith Nicholas
    Precision ADC Applications