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+5V Ratiometric reference for ADS1256

Damien Dusha
Prodigy 190 points
Other Parts Discussed in Thread: OPA2350, ADS1256

Dear all,

I am using the ADS1256 for a single-ended 5V sensor whose analog output is ratiometic to supply.   We are not using the on-chip buffers on the ADS1256 - they are externally buffered using OPA2350 devices.

The 5v rail is supplied by a Linear LT1763IDE low-noise LDO.   The 2.5V reference is created with a voltage divider made from precision resistors (Vishay ACASA2002E2002P100), and buffered by the recommended circuit in Figure 26 of the ADS1256 datasheet (half of an OPA2350).

My niggling concern is that, according to Figure 5, VREF is multiplied by 2, and therefore, if the 2.5 is (for example) actually 2.51, then x2 reference will clip on the supply voltage.

Is this an issue?  If so, what would be the recommended voltage reference setup to measure an analogue voltage that is ratiometric to a 5V supply?

  -- Damien

EDIT: I've also noticed on Figure 25 that there are 49.9R resistors on both VREFP and VREFN, with one going to 2.5V and the other to GND. Is this the recommended practice, even when using a buffer circuit such as Figure 26? What is the theory behind the selection of these resistors?



  • 0
    TI__Mastermind 43915 points

    Hi Damien,

    This is all going to be dependent on how much of the code range you are attempting to use. Generally speaking, ratiometric applications at 24 bit resolution do not need to utilize the entire 5V range. The gain options and resolution are so fine such that you can stay away from the input max and minimum keeping the input signal away from the supplies. This allows you to use something like a 2V reference (gained by 2 = 4V reference) and have your sensor output voltage sweep around 1-3V. It all comes down to the flexibility of your system.

      Regards,

    Tony Calabria

  • 0 in reply to Tony Calabria
    Prodigy 190 points

    Hi Tony,

    Unfortunately, the sensor has its own output amp that swings between 0-5V. It has an allowable voltage range of 4.75 to 5.25, so we'll generate a 2.4V for reference and x2 to 4.8 to drive the sensors. This should keep it ratiometric between the range we're interest in.

    Thanks,

    Damien.