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ADS1278: Toggling between two levels

Lars Berre
Prodigy 40 points
Part Number: ADS1278
Other Parts Discussed in Thread: THS4521

I am using ADS1278 with the following setup: f_clk = 4.096MHz, VREFP = 2.5V, Mode = Low-speed -> data output rate = 8kHz

I do observe toggling between two levels, delta V approx. 20µV

Toggling period decreases with increasing temperature. Time between toggles can be several hours.

This is what it looks like:

 

The figure is recorded during a temperature ramp from +85degC down to +25degC. Each data-point in the figure represents the average of 15 seconds of data (120,000 samples)  

 

The phenomenon has also been observed at steady-state temperature.

 

The phenomenon has also been seen on different ADC input configurations.

 

Have anyone seen such a phenomenon, could it be a Delta-Sigma issue?


 

 

 

 

  • 0
    Prodigy 40 points

    Seems like the figure did not came out the way it was intended. I am trying one more time:

  • 0 in reply to Lars Berre
    TI__Guru 57346 points
    Hello Lars,

    Thanks for your post.

    Let me check with the designer and see if he can offer an explanation. I do know there is a typical "warm up" drift response where the offset voltage will take some time to stabilize after power up. Once it settles, I don't believe you should see much variation without some external stimulus (i.e. temperature drift or different MODE setting). Can you give some indication as to how quickly the temperature is changing in your plot above?

    Best Regards,
  • 0 in reply to Ryan Andrews
    Prodigy 40 points
    Thank you Ryan

    The temperature is changing -0.06degC/minute.

    Best regards
    Lars
  • 0 in reply to Lars Berre
    TI__Guru 57346 points
    Hi Lars,

    We've discussed this internally and so far, the conclusion is that this does not appear as typical ADC behavior over temperature. Excluding other disturbances to the clock, voltage reference, MODE, etc, this kind of fluctuation in offset may be the result of popcorn noise from the amplifier(s) in the input signal path.

    Can you tell us what kind of amplifiers you are using in the input signal path? Are they BJT? To confirm whether they are introducing popcorn noise, you could try bypassing the amplifiers and shorting the ADC inputs together directly at the channel input pins and repeating the temperature sweep.

    Best Regards,
  • 0 in reply to Ryan Andrews
    Prodigy 40 points

    Thank you very much Ryan. I have tried with many different ADC input solutions both buffered and unbuffered. By unbuffered I am thinking of different sensors, with unknown output stage, driving the ADC directly.The buffered solution is acording the recomodations in the datasheet by use of THS4521.

    Yes popcorn noise seems like a plausible explanation. For sure I will try shorting the ADC inputs. I will post the result.

    Best regards
    Lars Berre