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How to change Vref of ADS8568 in hardware and software mode?

Bhavin Lapasia
Prodigy 80 points
Other Parts Discussed in Thread: ADS8568

Hi,

I am using ADS8568 ADC. Currently I'm using it in hardware mode by connecting HW_bar/SW pin to DGND. I'm using it in serial mode with internal reference enabled. Pin no. 34, RANGE is connected to DGND. So I assume that my Vref will be 2.5V and my range will be ±4Vref giving me a voltage range of ±10V. 

When I check voltage on REFIO pin, I am seeing a voltage of 3.18V. Also the range which I'm getting is ±2Vref i.e. I get output of 7FFF for a voltage of 6.3V. Anything above 6.3V I get output as 8000 and hence a negative number. Kindly pls explain what wrong must be going? I am getting Vref = 3.18V and range as ±6.3V.

Also I was trying to run the ADC in software mode to set it in ±4Vref mode. Changed the HW_bar/SW pin to DVDD. As explained in datasheet, I am following this sequence:

  1. ADC reset pulse (active high)
  2. Wait for BUSY signal pulse (However, I'm not receiving any change in BUSY signal so bypassed this)
  3. Write data on SPI as 800083FFh

I checked the Vref value after this, it stills showed 3.18V whereas I was expecting 2.5V. Datasheet says that changes are active with 32nd SCLK falling edge of the access where the register content is updated in serial mode, so ideally after writing 800083FFh, it should get updated. Kindly check whether my Config reg value is correct or not. How can I check whether the register is updated or not?

Request your pointwise answers to the above mentioned queries. Thanks

  • 0
    TI__Mastermind 45455 points
    Hi Bhavin,
    Could you please upload your schematic here? For serial and hardware mode, I guess you have connected Pin 8 to high voltage, Pin 20 to ground and Pin 11 to high voltage. There are different hardware connections for software mode (not only changing HW/SW pin to DVDD) and the configurations are done by register bits. Please let me know your connections for different modes when you upload the schematic.
    For software mode, you should not have to monitor the BUSY signal before programming the CONFIG register. After a RESET, the register is loaded with default value (0x000003FF), you can re-configure this register if you want the configurations different from the default state. After configuring the register, you can read back the CONFIG register to verify by setting Bit 30 in the configuration register after you programming the register.
    Thanks.

    Best regards
    Dale Li
  • 0 in reply to Dale Li
    Prodigy 80 points

    Hi,

    Let us first discuss the problem while in I'm running in hardware mode. Yes I've connected the ADC as you have mentioned. I've also connected pin 34 to ground. Ideally I should get a Vref voltage of 2.5V and range of ±4Vref i.e. ±10V. However, I'm getting Vref = 3.18V with range as ±2Vref i.e. 6.3V. The hex value corresponding to 6.3V is 7FFF which means ADC is getting saturated at 2Vref.

    What can be the possible reason for this behavior? 

  • 0 in reply to Bhavin Lapasia
    TI__Mastermind 45455 points
    Hi Bhavin,
    Because you have connected Pin11 (REFEN) to high in hardware mode as you mentioned, so the international reference should be enabled, the 2.5V reference voltage on the REFIO pin should be gotten because the default value for VREF bit is zero which indicates default internal reference 2.5V and REFDAC setting bits in CONFIG register is 3FFh(2.5V typ). The potential reason which is not getting 2.5V reference voltage may be due to an improper decoupling or grounding. That's the reason we want to check the schematic. Thanks.

    Regards
    Dale Li
  • 0 in reply to Bhavin Lapasia
    TI__Mastermind 45455 points

    Hi Bhavin,

    It's wrong to connect REFN (Pin55) to a capacitor (C131) because it's a negative reference pin and should be connected to analog ground directly. This should be the reason you can not get correct internal reference voltage. Please check the reference voltage again after correct the connection for Pin55, make HW/SW(Pin41) to low for hardware mode and make Asleep(Pin36) to low.

    The capacitor value ADS8568 requires between REF_xP and REF_xN is mimnum 4.7uF and recommend 10uF, but you are using all 1uF for all of them. You are using separated analog and digital ground, please make sure they have a good and proper connection close to ADC.

    Thanks.

    Regards

    Dale Li

  • 0 in reply to Dale Li
    Prodigy 80 points

    Hi Dale,

    Sorry for a very late reply. I by-passed that capacitor and connected REFN to gnd and got Vref = 2.5V. Thanks for the help.

    Regards,

    Bhavin