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ADS131E08: ADS131E08IPAG/TQFP

Ankush
Intellectual 565 points
Part Number: ADS131E08


Hi ,

I am using ADS131E08 for 8 channel ADC sensing differential mode .

my IC is connected to 3.3V (AVDD )w.r.t. AGND , INXP(+ve input pins) connected to Source meter and INXN(-Ve input pins)are connected to 1.5V fix voltage source and my VREFP is internal 2.4 .

Now my desired range should be in 0.3V To 2.7V but it is not working ???

Below 0.5V constantly giving me 39000(aprox.) ADC counts 

and above 2.5V constantly the ADC count is 26000 (aprox.) .

so if i am considering 1.5v as input reference i am only getting +/- 1V as range of my ADC channels 

please let me know is any thing need to be check . 

  • 0
    TI__Guru 66906 points

    Hello Bhagyesh,

    Thank you for your post.

    We will more information to understand how the ADS131E08 is configured in your test. Can you please provide a list of the register settings if they are different than default? 

    Can you also please share a connection diagram/schematic showing your inputs to the device? Remember to share the ground connection (AVSS) with your voltage source so that 1.5 V is referenced to the same ground.

    Regards,

    Ryan

  • 0 in reply to Ryan Andrews
    Intellectual 565 points

    Analog Section.pdf

    Here is my connection diagram ,Please check .

    most of configuration are set as default except CONFIG_3  = 0xc0 

  • 0 in reply to Ankush
    TI__Guru 66906 points

    Thank you, Bhagyesh! I will review this and provide feedback sometime tomorrow. 

    Regards,

    Ryan

  • 0 in reply to Ryan Andrews
    TI__Guru 66906 points

    Hello Bhagyesh,

    One thing I noticed in your schematic is that IN8N is floating. Are you, by chance, using Channel 8 to conduct these measurements? Are all channels showing roughly the same output code values?

    The differential input range is from -2.4 V to +2.4 V when VREF = 2.4 V. This is the difference between INxP and INxN. Since INxN is fixed to 1.5 V, the INxP pins can range from 0 V to 3.3 V (AVSS to AVDD) without exceeding the ADC full-scale range -> the range for valid conversions. However, each channel has a PGA, which is implemented in an instrumentation amp configuration. The input common-mode voltage requirement in section 9.3.4.1 must be satisfied in order for each PGAxP and PGAxN amplifier to drive its output to the correct voltage. Can you try increasing INxN to 1.65 V (i.e. mid-supply) and then sweep INxP? It may help us to understand the problem better if we can see a plot of INxP input voltage vs. ADC output code to understand when the ADC output is clipping.

    Even before the PGA outputs saturate, I would expect the output codes to be closer to full-scale. It may also help to share a logic analyzer capture or scope capture of the SPI communication with one complete frame (CSn, SCLK, DOUT, DRDYn) to check the conversion codes.

    Best regards,

    Ryan

  • 0 in reply to Ryan Andrews
    Intellectual 565 points

    Thank you Ryan and sorry for late reply.

    I am using all 8 channel with same configuration and i have checked every channel ,and also changed my device hope that may pcb board is faulty .

    but still not getting full range of count 0 to 65k . i had done some setting changes but not getting desired solution .

    1. I have also tried what you suggest INxN to 1.65 V (mid range ) . at 0mV count starting from 11104 and saturate at 3300mV (3.3V) with count 54372 .when i adjust my channel PGA 1(in Channel setting = 0x10 ) .

    2. I had connected JP7 and removed R54 & R58 so at INxN = 2.5V it gives 60 adc count at 0mV and 43325 adc count at 3300mV (3.3V)after that it is also get saturated

    image is showing adc count w.r.t. mV at INxP at PGA gain in 1 .

    my sensor range is 0V to 2.4V and i am expecting 0 to 65535 (approx) what will be my setting in schematic as well as in programming configuration .

  • 0 in reply to Ankush
    TI__Guru 66906 points

    Hello Bhagyesh,

    Can you please confirm which device you are using? Are you using ADS130E08 (16-bit resolution) or ADS131E08 (24-bit resolution)?

    I don't believe you are converting the ADC output codes correctly. These devices use binary two's complement format to output the data. When INxP < INxN, the differential voltage to the PGA and to the ADC will be negative. Therefore, the MSB of the raw binary output will be set to 1b. 0x800000 represents minus full-scale, and 0xFFFFFF represents -1 code.

    Regards,

    Ryan