• State TI Thinks Resolved
  • Locked Locked
  • Replies 1 reply
  • Answers 1 answer
  • Subscribers 46 subscribers
  • Views 455 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

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.

ADS131A04: Fully differential analog input behavoiur

Michele Gazzarri
Prodigy 10 points
Part Number: ADS131A04

Hi guys,

I'm designing a data acquisition board that will have the ADS131A04 to acquire data from an amplified load cell. 
The signal will be single ended with a maximum amplitude spanning between +10V and -10V. In order to correctly acquire it I'll enable both internal reference (at 2.442V) and the charge pump.
Assuming that I will scale this amplitude with some precision resistors, I can't figure out what is the maximum (either positive or negative) value allowed at the inputs of the adc and what reference I had to consider to calculate the output word.

From page 38 of the datasheet I read that a positive full scale signal (> VREF/GAIN) produce an output of 0x7FFFFF and a negative full scale signal (< -VREF/GAIN) produce 0x800000.
So from this I can assume VREF = 2.442V and provide at input a +/- 2.442V signal? But if so, it will go in conflict with absolute maximum ratings where I read that the minimum input voltage can be AVSS (=0) -1.65V.

If it's not possible to provide a fully differential signal I can't understand why have you putted inside it a charge pump with this kind of limitations.

  • 0
    TI__Guru* 95185 points

    Michele,


    The ADC of the ADS131A04 is fully differential. If the reference is set to ±2.442V then the full scale of what it can measure is +2.442V to -2.442V. However, the input is measured by the difference in the input voltages for AINP and AINN. The ADC measures AINP-AINN. If the AINP is set to 1V and AINN is set to 3V, then the ADC measures -2V because AINN is higher than AINP.

    For the ADS131A04, we give an Absolute Maximum Rating on page 7 of the datasheet. The analog input voltage (either AINP or AINN) cannot be larger than AVDD+0.3V or lower than AVSS-0.3V (AVSS-1.65V with the charge pump enabled). Exceeding these voltages may cause damage to the device. Additionally we give a Recommended Operation Condition table on page 8 of the datasheet. This lists the absolute input range to be AVSS to AVDD (AVSS-1.5V to AVDD with the charge pump enabled).

    For your application, your signal is really pseudo differential. The AINN negative input is set to a ground and then the AINP positive input changes around that. For a differential signal, the input common-mode voltage is constant and both AINP and AINN swing around that.

    You could just use a simple voltage divider, but as you have noted, the input would be limited because you have the AINN grounded, and the negative range of the input doesn't go as far as -2.442V. In this case, you would need to level shift the input to be in the full range of the ADS131A04 ADC. You could either use a fully differential amplifier (FDA) to shift the input to the range of the ADC, and set the output common-mode of the FDA to be mid-supply of the ADC. You could also use some sort of instrumentation amplifier set negative input to a different level.


    Joseph Wu