ADS1119: Ads1119 Good procurement practices

Hi kawann Silva,

kawann Silva said:

I have a circuit in which I am using 3-channel reading, I am using an external reference of 1.25 V to read 2 channels and in 1 channel I am using an internal reference of 2.048V.
I would like to know what would be the best practices for making an acquisition, as I realize that the value at the ADC input is varying (oscillation) in the order of 10uV, but in the acquisition the value ends up varying by 30uV.

For the ADS1119, the noise table 1 on page 14 of the datasheet shows that when using the internal reference the ADC can resolve to the LSB value (or 1 code) which is 62.5uV.  So that is the best resolution you can get with the 2.048V internal reference.  Using the 1.25V external reference you are making a couple of assumptions.  One assumption is that the reference is very low noise.  The second assumption is that the ADS1119 can resolve to 1 LSB (38.14uV).  If you short the inputs to the ADC when using the external reference, you will then know what the best capable resolution of the ADC is when using the external reference.  The noise could actually be higher when an input voltage greater than '0' is applied.

kawann Silva said:

* What would be the best time to perform the channel change, if I take a reading and change the channel could I have a problem related to delta sigma acquisition?

The easiest method for cycling through the mux is to use the single-shot conversion mode.

1. Set to single-shot mode
2. Set to desired input configuration
3. Issue the START/SYNC command
4. Monitor DRDY for high to low transition or read the value of the DRDY bit for a logic high in register 1 indicating end of conversion
5. Read the conversion result and go back to 2.

kawann Silva said:

*When is the best time to calibrate the ADC Offset?

You can do this after power-up.  If the temperature of the ADC varies you may want to issue periodic offset calibration checks.  The frequency as to how often is really system dependent and relative to the required accuracy of the measurement.

kawann Silva said:

* Could you give me some good practices, because I intend to work very close to the minimum resolution of the ADC (1.25V/32768 =38.14uV)

The measurement can pick up noise from external sources, including power line-cycle noise and EMI/RFI.  A good analog input filter for antialiasing can be beneficial.  Good PCB layout practices are also important.  A good ground layout is very important.  For example use of a ground plane as opposed to just point to point ground connections using traces alone.

Best regards,

Bob B