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ADS7138: ADS7138 Averaging

MANIKANTA SRINIVAS
Prodigy 140 points
Part Number: ADS7138
Other Parts Discussed in Thread: ADS7066

Tool/software:

Hi TI Team,

There are a few questions related to the 12-bit ADC converter (ADS7138):

  1. How can we ensure that the ADC is set to either 12-bit or 16-bit mode? The documentation mentions that oversampling converts the 12-bit output to a 16-bit effective result, but is the core resolution still 12-bit only?

  2. Though the ADS7138 is a 12-bit ADC, it seems to output 16-bit values after averaging. Can you explain how it achieves 16-bit results? This isn’t very clear from the datasheet.

  3. The high threshold for alerts is specified as 0xFFF in the datasheet. If the averaged output is 16-bit, should the threshold be set to 0xFFFF? How does this work given the 12-bit core resolution?

  4. What is the purpose of the OSR[2:0] bits in the OSR_CFG register?

  5. What is the maximum current that the ADS7138 can measure?

Thanks,

Manikanta

  • 0
    TI__Expert 6790 points

    Hi Manikanta,

    Apologies for the delayed response, as I was on business travel this past week. 

    1/2. The core resolution will always be 12-bit. The output value will be 16-bits long, but does not necessarily have the resolution of a 16-bit device. Theoretically, every 4 samples averaged together yield a 1-bit dynamic range increase. At a maximum of 128 sample OSR, you are getting a 3.5-bit increase (log_4(128)) over the 12-bit base resolution, or about 15.5 bits of resolution. Keep in mind these are rough numbers, and it does not improve static ADC characteristics (such as INL). 

    This function and theory is briefly described in the document linked below for a similar device, the ADS7066.

    https://www.ti.com/lit/ab/sbaa441/sbaa441.pdf

    3. Yes, thresholds while getting a 16-bit conversion should also be specified as 16-bit long thresholds. These are just mathematical comparisons between register values, not necessarily related to the core ADC resolution.

    4. The OSR[2:0] field sets the amount of 12-bit conversions that are averaged together when obtaining the 16-bit long conversion. The more samples averaged together, the more effective dynamic range, but also lower achievable sampling rates.

    5. The ADS7138 inputs don't measure current, per se, but voltage. However, the input pins do have a maximum input current that they can tolerate without being damaged. This is 10mA in or out per the absolute maximum ratings of the datasheet.

    Regards,
    Joel