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ADS1119: ADS1119 power supply recommendation

Christof Reinarz
Prodigy 90 points
Part Number: ADS1119
Other Parts Discussed in Thread: TPS63802, PCA9306

Hi,

I am planing on using the ADS1119 in a system to measure temperature with NTC thermistors but I have some doubts on which power supply architecture is best. The board will be powered by a single Lithium Ion cell. Under normal circumstance ambient temperature will be around 0ºC. From the data sheet of a Panasonic NCR18650 (https://datasheet.octopart.com/NCR18650B-Panasonic-datasheet-34154111.pdf), at 3.3V (needed for the uP) there will be still about half of the energy left in the battery. I guess using a buck-boost converter like the TPS63802 solves this issue but, on the other hand, noise might be become problem so I probably need a low noise LDO for the ADC and the NTC's, perhaps a LP5907MFX-3.0. Now, given that the uP will be powered directly from the DC/DC converter and the analog subsystem (i.e. ADC and thermistors) from the LDO should I:

a) supply both the ADC's VDD and AVDD plus NTC's from the LDO or

b) supply ADC's VDD from the DC/DC converter and AVDD plus NTC's from the LDO

To make things more complicated, the uP will be in deep sleep mode mode most of the time and wake up to take a measurement every 5 to 30 minutes so I can't keep the NTC's on during sleep.  Option a) would allow me to power down the ADC and the NTC's by simply shutting down the LDO with the EN pin but possibly creating problems with other devices on the I2C bus and the ADS1119 itself. Option b) on the other hand would keep the digital side of the ADC powered at all times while shutting down only the analog side but I am not sure if this an option at all.

I would greatly appreciate some advise. Perhaps I am over engineering this and there is a simpler solution.

Best regards and many thanks in advance,

Christof

  • 0
    TI__Guru** 113765 points

    Hi Christof,

    There are a number of pros and cons to each of your potential solutions.  You mention other devices on the I2C bus.  It is not clear how the ADS1119 will react to a voltage applied to the SDA and SCL inputs when the device is unpowered.  So I believe that at least the DVDD portion should be powered.

    If DVDD is powered and AVDD is not powered, the ADS1119 will be held in a reset state.  Here again it is unclear what the DVDD current would be when held in the reset state and the AVDD is unpowered.

    My first inclination would be to power everything from the DC-DC converter.  It is good practice to follow up the DC-DC converter with an LDO to clean up the switching noise, but with a 16-bit device I'm not sure it is necessary.  I would also use the supply as the ADC reference. Using the supply for both the reference and the NTC excitation should cancel noise of the excitation source in the measurement and reduce the drift in the measurement as well.  I would then put the ADS1119 in POWERDOWN mode to reduce power consumption when not taking a measurement.  To break the break the excitation of the NTC, I would use a MOSFET as a switch.

    As to option a or b, I think this will depend on what else is connected.  Option b may be preferred if there is power on the digital input pins.  However, option a will will have the lowest overall power consumption if you can isolate the I2C from the inputs.  You may be able to use a device like the PCA9306 which is a level translator, but I think this or similar type devices could be used to isolate the LDO circuit from the DC-DC converter circuit power.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 90 points

    Hi Bob,

    thank you for your answer. I will look into powering the ADC directly from the DC/DC converter and leave the ADS1119 always on. This resolves my issue.

    Best regards,

    Christof