This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

  • Resolved

[FAQ] ADS129x: How do I convert ADC output codes to volts?

Genius 17305 points

Replies: 1

Views: 971

I can successfully clock data out of the device, but how do I convert these output codes back to volts? 

Best regards,

Alex Smith
Applications Engineer | Precision Delta-Sigma Converters

Check out our helpful resources: BIO-FAQ: Common Questions for TI's ADS129x Family of Bio-Potential ADCs

TI Precision Data Converters | TI Precision Labs - ADCs | Analog Engineer's Calculator Data Converters Learning Center | Selection Guide

  • Great question! I'll use the ADS1298 for this example, but the theory can be applied to any device!

    To convert output codes into volts, you must first calculate the least-significant bit size or LSB. One LSB represents the voltage weight of one code. In other words, the input voltage must change by one LSB size in order to increment/decrement the ADC output. The full-scale range (FSR) of the ADC divided by the total number codes gives you the LSB size:

    LSB = FSR / (2^n - 1), where 'n' is the ADC resolution.

    In the ADS1298, the differential input voltage to each ADC can range from -VREF to +VREF, so the full-scale range = 2 x VREF. Remember that there is a PGA in front of each ADC as well, so the differential input to each channel must be limited from (- VREF / Gain) to (+VREF / Gain). The LSB size is then calculated as:

    LSB = (2 x VREF)/ Gain / (2^24 - 1)

    Next, you must know the output data format for your ADC. The ADS1298 outputs data in binary two's complement, where 0x7FFFFF represents positive full-scale and 0x800000 represents negative full-scale.

    FIGURE 1: ADS1298 Ideal Transfer Function

    The most straightforward way to convert your output codes back into input voltage is to AND the data with 0x800000 and test the MSB (most-significant bit). If the MSB equals 0, simply multiply the decimal equivalent by the LSB size. If the MSB = 1, you must first subtract 2^n from the decimal equivalent, then multiply by the LSB size. 

    For example, using VREF = 2.5 V, N = 24 bits, and Gain = 4, we can calculate the LSB size as 74.506 nV. An output code of 0x147AE1 would correspond to an input voltage of +100 mV. Meanwhile, an output code of 0xD70A3D would correspond to an input voltage of -200 mV.

    I hope this helps answer your question.

    Link to BIOFAQ homepage

    This post applies to ADS1191ADS1192ADS1291ADS1292ADS1292RADS1294ADS1294RADS1296ADS1296RADS1298ADS1298RADS1299-4ADS1299-6, and ADS1299.

    Best regards,

    Alex Smith
    Applications Engineer | Precision Delta-Sigma Converters

    Check out our helpful resources: BIO-FAQ: Common Questions for TI's ADS129x Family of Bio-Potential ADCs

    TI Precision Data Converters | TI Precision Labs - ADCs | Analog Engineer's Calculator Data Converters Learning Center | Selection Guide

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.