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New ADC

A project log for Street Sense

Portable electronic device to measure air and noise pollution

Mike TeachmanMike Teachman 03/18/2019 at 16:170 Comments

A new Texas Instruments ADC was added to the design -- ADS1219

The ADS1219 replaces the ADS1015 ADC.  This new ADC offers these benefits:

I wrote and published a MicroPython driver for the ADS1219 device.  The control of the device in MicroPython was quite simple.

The V05 schematic includes the new ADC.   You will notice that the initial integration of this ADC is lacking most sensible approaches to minimize noise effects:

This approach allowed me to write the MicroPython driver and get the device integrated into the rest of the MicroPython code.

Given the lack of care to deal with power supply noise, it wasn't surprising to see poor results when reading the analog values from the ozone and NO2 sensors.  Results showed considerable variability in back-to-back measurements for gas concentration.   An oscilloscope capture shows that the Vgas outputs from the gas sensors oscillate during the time they are being sampled by the ADC. 

You can see voltage fluctuations on the 3.3V rail that are associated with the oscillations. These oscillations happen each time the ADC performs a single-shot conversion.  

Next steps:   Time to put on the analog design hat and investigate decoupling methods to provide low-noise analog power for the ADC and gas sensors.

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