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ADS131M08: Powers supply

Part Number: ADS131M08
  1. To reduce consumption of a board powered from battery, I plan to keep off the LDO for AVDD. It powers also input buffers with maximal 30mA consumption. DVDD is powered by other LDO that powers the microcontroller that reads data from SPI. Are there side effects that AVDD is zero and DVDD is 3.3V?

According to paragraph “10.2 Power-Supply Sequencing” should be OK.

  1. As AVDD and DVDD are same value, it will be better to have same power supply for both than powering DVDD from the same power supply with the microcontroller?
  2. Considering that DVDD current is rally small it is acceptable to insert a RC filter between LDO (AVDD) and DVDD?

 

Best regards,

Radu

  • Hello Radu,

    1. ...Are there side effects that AVDD is zero and DVDD is 3.3V? 
    2. As AVDD and DVDD are same value, it will be better to have same power supply for both than powering DVDD from the same power supply with the microcontroller?

    Unfortunately, I'm not really sure I understand the question. It sounds like you don't want to have separate LDOs for each supply voltage. In general, this is possible.

    AVDD powers the PGA, so having it at "zero" or GND would be improper use of the device. However, I think you want to have DVDD, AVDD, and the MCU all powered off the same voltage rail. We have this option on the EVM so you can absolutely do that. I would only recommend looking at the quiescent or supply current specs for the ADC and the MCU and make sure the LDO won't be overburdened.

    1. Considering that DVDD current is really small, it is acceptable to insert a RC filter between LDO (AVDD) and DVDD?

    Any particular reason? With a cut-off of ~160kHz, I would expect that to be some sort of EMI filter where a ferrite bead would be a bit better. Otherwise, increasing decoupling capacitance should be sufficient if you have too much voltage ripple on the voltage line between the LDO to DVDD.

    Best,

    -Cole

  • Hello Cole,

    Thank you for the answers.

    Clarifications. Initial plan was to have two LDOs: first for all analog parts (AVDD + buffers etc.) and second for all digital parts (DVDD + MCU). LDO for digital part will remain always on. As analog part is consuming more, it is possible to power the LDO for analog (via Enable input) only before measurements and to turn it off till next measurements? No side effects?

    Thanks,

    Radu

  • Hello Radu,

    From the datasheet, you can keep the analog supply if you'd like as they can be sequenced in any order. The issues is keeping the inputs voltages below the absolute maximum. I will also note, you have to wait for t_POR after turning on the analog supply.

    As shown below, if AVDD is low, then the reference or inputs cannot exceed 0.3V or it might risk damaging the device. You might consider extra protection diodes, external to the device, if you cannot guarantee following the spec. The tradeoff is that leakage from the diode results in error and more higher power consumption during regular operation.

    Edit: I forgot to mention that only turning off AVDD is an unknown state of the device. I'm not sure if register contents are lost as the digital circuitry will remain on but you will need to check

    Best,

    -Cole