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ADS127L11: Daisy chain design, pull up on the SDO/SDI line

Part Number: ADS127L11

I'm working on a PCB design where I have 2 ADS127L11's daisy chained.

Figure 8-36 of the data sheet shows an example:

The diagram shows each chained SDO->SDI line with a pull up resistor to IOVDD. However, I can't find any recommended values elsewhere in the data sheet, nor any other descriptions regarding that pull up. The evaluation board design also does not appear to have a pull up.

The only reference in the datasheet for a potential need for pull up is where it mentions the SDO/DRDY line is high-z when CS is not active. However, I don't quite understand why it matters what the state of SDO is anyway at that point, since the bus isn't engaged. Also, is there a chance the SDI pin, similar to a microprocessor input pin, has a weak pull up (100k?) that would set the state of that line anyway?

For context, I'm using the ADS in the mode where the SDO/DRDY pin is single function SDO.

Thank you for any context you can provide on the need for a pull up.

  • Hello Adam,

    The purpose of the pull-up resistor is to avoid 'long' periods of time with ADS127L11 SDI input floating, which could cause shoot-through current on the IOVDD supply.  This is not a big concern during power-up since the time for an IO pin to be configured as an output driver is usually very short.  However, when in daisy-chain mode, it is possible that the SDO pin could be in a high-z state for extended periods of time.  If the 'Data In' pin on the MCU has an internal weak pull-up/down, then the SDO output connected to this pin does not need an additional pull-up resistor.

    Regarding the value of the pull-up, if you were using the SDO pin as a dual-function SDO/DRDY, then I would recommend using a 10kOhm pull-up for fast response times between SPI frames.  (Important if using edge triggered interrupt.)  However, if only using the pin as an SDO, then a much weaker pull-up can be used, in which case a 100kOhm value will work well.

    Unfortunately, there is no internal weak pull-up for the SDI pin, which is why we recommend including the external resistor when using daisy-chain.

    Keith Nicholas
    Precision ADC Applications