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ADS8568: Strange behaviour with ADS8568SPM

Carmine Fiorillo
Prodigy 40 points
Part Number: ADS8568

In practise sometime at power on the internal vref is 1Volt instead of the already programmed 2.5Volt.

The 1V that I maesure is at steady state not in transitory phase and the desidered 2.5V can be reached only if I restart my board.

I use the HW mode placing a pull-down resistor on pin41 and pull-up resistor on pin11 for Vref enable. In this case I don't write anything inside the configuration register and we use its in default setting (000003FFh).

On the other hand, we use all the Power supplies raccomandations as depicted in the datasheet (Bypass and decoupling capacitors). The output voltage of the DAC is decoupled with a 470nF.

The problem that I explained is randominc and I can see it on 4 board of the 20 total of our production.

On one board we try to change the AD converter and in this way we rosolve the problem, but we don't understand the reason of this wrong behaviour.

  • 0
    TI__Guru 64236 points

    Hello Carmine,

    Thank you for your post.

    What is the state of pin 20 in your circuit? Pin 20 is the /REFBUFEN pin and should be tied low to enable all internal reference buffers when using Hardware Mode.

    Also, can you confirm which interface mode you are using: parallel or serial?

    Best regards,

  • 0 in reply to Ryan Andrews
    Prodigy 40 points

    Hi Ryan,

    your observation is good but I use the ADC in Parallel mode (with pin 8 connected to ground), for this reason pin 20 is used as DB11.

    In this case the buffue is enabled by default?

    Thanks for your support,

    Best Regards

    Carmine Fiorillo

  • 0 in reply to Carmine Fiorillo
    TI__Guru 64236 points

    Hi Carmine,

    Yes, in this case, the reference buffers should be enabled at the same time as the internal reference voltage (when pin 11 is taken high).

    Are you consistently able to restart the board and correct the issue every time this happens?

    If you switch to software mode, write 0x000003FF to the CONFIG register, and return to hardware mode, does that correct the issue? This is just for debugging purposes.

    Best regards,

  • 0 in reply to Ryan Andrews
    Prodigy 40 points

    Hello Andrews,

    thanks for your quick reply.　

    Yes, of course. During my test I verified that the REFEN/WR* (pin 11) is correctly connected to a pull-up resistor connected at DVDD. On the oscilloscope, when the Vref is 1V the voltage on the pin 11 is 3.3V. 　

    We observed that after some seconds the initial wrong value of 1V (stable since seconds when acquired) reach the correct value of 2.5V.　

    I can't switch the ADC from HW to SW mode because the pin 41 in connected to ground with a pull-down resistor.

    In our design the Vref is used for as a built in test for the device, so if this voltage is 2.5V +/- 10% the ADC works correctly. In this way, if at power-on we read a wrong maesurement, our state machine goes in its safety loop.　

    We read the Vref with a microcontroller on its analogue input pin. In fact, the internal ADC converter of the microcontroller confirm the 1V measured with the oscilloscope.　

    We also tried to change the microcontroller but the result obtained does't change. We tried to change the 470nF capacitor but the result obtained does't change.　

    As final stage we changed the ADC and the problem seems to be resolved.

    I think that it can be a production defect of the device. You agree?

    Best regards

    Carmine

  • 0 in reply to Carmine Fiorillo
    TI__Guru 64236 points

    Hi Carmine,

    Thanks for the added information and for confirming the supply and VREF voltage levels.

    Carmine Fiorillo said:

    I can't switch the ADC from HW to SW mode because the pin 41 in connected to ground with a pull-down resistor.

    Ok - I only suggested it thinking that an MCU GPIO pin could be used to control the pin as well. No problem.

    So you're seeing the VREF pin ramp to 1 V, stop for several seconds, and then eventually reach 2.5 V? I don't think we've come across this issue before. One last suggestion I can make is to take one of the devices/boards which show this behavior and disconnect the VREF pin from everything except for the 470 nF capacitor. Remove the connection to the MCU ADC pin and anything else which might be loading the reference. Then repeat the test with that same board and device.

    Best regards,