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ADS1255: Bad histogram on ADS1255. Wrong Vref circuitry?

HUGO DE MORAIS
Prodigy 10 points
Part Number: ADS1255
Other Parts Discussed in Thread: OPA350, ADS1256, REF6225, REF5025

Hi,

We´re getting the following distribuition of the readings on ADS1255. We suspecting of the Vref circuitry, where we using the ADR431 reference. The ADS1255 manual shows, on pages 28-29, two ways to use the reference. Which circuitry we had to use with ADR431? Could be another problem than Vref? 

  • 0
    TI__Mastermind 44620 points

    Hi Hugo,

    Welcome to the TI E2E forums!

    Have you tried probing the +2V5 voltage to see if it is stable?

    My guess is that the capacitive load on the ADA4004-4 (U1D, specifically) is probably making this buffer become unstable. Figure 26 in the ADS1256 data sheet shows the OPA350 driving a large capacitor load; however, the OPA350 is specifically designed to be able to handle large capacitive loads. You might need to remove the 47uF capacitor and maybe even one of the 100 nF caps.

    Note: We do have low-noise buffered reference sources, such as the REF6225 or REF5025 that would be able to drive the reference input of the ADC directly and handle the large capacitive load. The REF5xxx and REF6xxx devices were released after the ADS1255 so that's probably why the OPA350 is the recommended device in the ADS1255 datasheet.