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ADS1255 ADC giving wrong data

Mayank Kumar
Intellectual 260 points

Other Parts Discussed in Thread: ADS1255

Hi!

We are using ADS1255 24 bit ADC from TI for our design. We have interfaced it using SPI with C5515 EZDSP for data acquisition at 31.7 KHz sampling rate. The ADC used to give good data for long time in the past. But, sometime back it started behaving strangely.

It collects good relevant data for sometime and then start giving junk values. By Junk I mean signals which span the whole range from -2^23 to 2^23 and change very fastly i.e. it changes from 2^23 to -2^23 in span of 3-4 samples. Though our test sine wave signal is very low in amplitude (10^5) and frequency (30 Hz). This clearly indicate that sometime I am able to collect good data and at other times, I am getting bad junk data. Analog signal till the ADC pin is coming correct. PLEASE SEE THE ATTACHED FIGURE.

We are unable to find the problem that suddenly started appearing. It will be really helpful if somebody could direct us for some possible causes of such irregularities which started appearing lately.

Some other changes which might have some effect is that we tried powering the whole setup using battery supply for an elongated time.

  • TI__Mastermind 43915 points

    Hi Maynak,

    Are you feeding a differential signal to the device? Is the input buffer enabled?

    Reason I ask is that when the input buffer is enabled, the input voltage is limited to AVDD -2.0V. This spec is the individual input pins relative to the ground of the device. Therefore if AIN- is at 2V and AIN+ is at 4V (2V differential), the device is run out of spec. When the input buffer is driven beyond its limits, it will try to reset in which the conversion result may appear as you have shown in your attachment. May sure that your input signal common mode is set correctly. We have seen a lot of configurations where a function generator is simply tied to the ADC inputs without making sure that the ground from the signal source is at the same potential as the chip.

    Regards,

    Tony Calabria

  • in reply to Tony Calabria
    Intellectual 260 points
    Hi Tony!
    Though we are testing it using a fully differential signal, the input buffer is disabled. And the signal is properly referenced to common mode.

    Regards,

    Mayank

  • in reply to Mayank Kumar
    TI__Mastermind 43915 points

    Hi Mayank,

    Sorry for the delay. Were you ever able to solve your issue? How many codes / volts is your peak to peak change when reading back the bad results? It is hard to tell from the picture your provided.

    Regards,

    Tony