• State Resolved
  • Locked Locked
  • Replies 2 replies
  • Subscribers 45 subscribers
  • Views 1058 views
  • Users 0 members are here
Support feedback
Options
Options
Related

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.

ADS1234 issue at gains 64 and 128

Sergey Goncharov2
Prodigy 20 points
Other Parts Discussed in Thread: ADS1234

Dear Support! I'm newbie on this forum, and hope you will help me. My problem is in following:

I've wired a scheme shown on thee picture 

where all power pins wired to common power supply 4.8V and I'm measuring divider voltage. I got readings (at gain = 128) something like 101847, that seems strange to me, because I expect value = (0.0108V/2.4V)*128*8388607 =  4831837. If I try gain = 64, I'll get value 51035. But if I try gain = 1, I'll get reading smth like 37426, which seems reasonable to me. And the gain = 2 gives reading 75733. 

My first guess was that I'm doing wrong data conversion, but I have risen voltage on AINP1 to about 0.634V (at gain = 128) and got maximum value 8388607 and further rising voltage led to reading still remain 8388607, so it's look like conversion is right.

So, can expect the values I mentioned above for gains 64 and 128? What am I misunderstanding or doing wrong?

Thanks,

Sergey

  • 0
    TI__Guru** 113765 points

    Hi Sergey,

    Welcome to the forum!  The problem you are seeing is you are outside the common mode input range of the ADS1234 when using gains of 64 or 128.  In other words you can only measure a signal referred to ground at a gain of 1 or 2, but not at gains of 64 or 128.  The common mode input range is shown on page 3 of the datasheet in the Electrical Characteristics table.

    When using the higher gains, the voltage at the AIN inputs must be greater than 1.5V above AGND and 1.5V below AVDD.  To investigate this and to get a proper conversion result using the higher gains, your measurement must be within the common mode range.  You could change the voltage divider so that AVDD is connected to 100k, then 470, then to another 100k connected to AGND.  You can connect the AIN inputs across the 470 ohm resistor and you should see the proper results.

    Best regards,

    Bob B