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ADS127L01: ADS127L01 START pin with a resistor

MIKHAIL SALOV
Expert 5755 points
Part Number: ADS127L01

Good day Colleagues,

According to the ADS172L01 data sheet all hardware mode pins shall be pulled to DVDD or DGND via 100kOhm resistors, but START pin.

Also in the EVAL-board the START pin is pulled to DVDD via a 100kOhm resistor.

Is there a reason for that? And how to utilize START pin correctly?

Thank you, Mikhail

  • 0
    TI__Guru 60976 points

    Hello Mikhail,

    Thank you for your post.

    The 100 kOhm pull-up resistors are meant to define a known I/O state at power-up while minimizing power consumption.

    The START pin is used to control whether or not the delta-sigma modulator is converting by pulling the pin to a logic high or low level. Alternatively, the user may send the START and STOP op-code commands via SPI to accomplish the same thing. However, the START pin must be held low if the SPI commands are used to control conversions. 

    Note that the ADS127L01 is a delta-sigma ADC, which requires a conversion history to provide accurate, settled conversion data. Therefore, the intention is to leave the modulator converting continuously and not to start/stop conversions frequently.

    Best regards,

    Ryan

  • 0 in reply to Ryan Andrews
    TI__Expert 5755 points

    Hi Ryan,

    Unfortunately it’s not clear to the customer at the moment.

     Should the START pin be pulled to DVDD with a 100kOhm although the datasheet does not say so?

    Customer assumes yes, as it is so in the EVAL-board of the ADSL127L01.

     Is customer's assumption correct that the 10µA leakage current of the digital inputs will drop the voltage at the pin by 1V with a 100kOhm resistor?

    Thank you, Mikhail

  • 0 in reply to MIKHAIL SALOV
    TI__Guru 60976 points

    Hi Mikhail,

    100-kOhm pull-up resistors were added to START and RESETn pins on the EVM to define a start-up condition in a manner consistent with the Hardware Mode pins. Including a pull-up resistor instead of directly connecting to the supply also provides a means of debug if needed. It is possible to connect START directly to the supply if the customer does not need to control it dynamically. 

    The leakage current is only specified as a min/max of +/- 10uA. The voltage drop across a 100-kOhm pull-up resistor will usually be much less than 1 V.

    Regards,

    Ryan