ADS7853: External Reference Voltage Not Working as Configured

Part Number: ADS7853
Other Parts Discussed in Thread: REF5025

Tool/software:

I wish to configure the ADS7853 to take external reference voltage for data conversion. We have REF5025AIDRG4 as a reference voltage generator to give 2.5V to the pins REFIO_A & REFIO_B. The decoupling caps have also been populated as per the reference design. I set B.9 to 1, B.6 = 0, and B.7 = 0 in the CFR to use 2 x VREF for the ADC conversion. Hence, it effectively gives the ADS7853 5V for ADC conversion reference voltage. 

The issue is that irrespective of the configuration setting, 2 x VREF doesn't appear as the reference voltage for conversion. The REFDAC_A and REFDAC_B were also set to 0x9000 and 0xA000 to ensure they were not in use.

Also, I depopulated REF5025AIDRG4 and gave varying voltages to REFIO_A & REFIO_B from 0 to 2.5 volts to check whether 2 x VREF happens at any other voltage, but the result remains the same VREF with no change at all.

Am I missing some configuration here, or could there be a different problem?

A quick reply would be appreciated

Thank you

  • Hello K Pruthvi Raaj, 

    Thank you for your question.  

    Just to make sure I understand your set up,

    • the device is currently in SE mode (B7 = 0), which means your AINx_M should be shorted to GND
    • The internal reference is disabled (B6 = 0), and you are using REF5025 to set the 2.5V external reference
    • The FSR of the device is 2xVREF (B9 = 1)

     

    This should be similar to your set up:

      

    What I would like some clarification with is what you are inputting and what you are reading out? Do you always read back but in reference to just VREF not 2x VREF?

     

    Is there a way you can confirm the CFG register is actually being written to? It seems like B6 & 7 are set similar to what you have by default but B9 isn’t. So, if by any chance the device isn’t successfully being written to then it would remain in 1xVref mode. (also, as a remainder, if please confirm your read back is aligned with the data format set, B4)

     

    Another thing to note is the ADC reference input has a dynamic load to account for, so it is recommended to have a reference buffer for each channel.

     

    Best regards,

    Yolanda

     

  • Hello,

    Is there a way you can confirm the CFG register is actually being written to?

    I read the configuration set in the ADS7853 by writing it 0x3000, and I read back the configuration. The configuration read matches the configuration so written. 


    What I would like some clarification with is what you are inputting and what you are reading out? Do you always read back but in reference to just VREF not 2x VREF?

    I did not understand this. To clear it up, I wish to set it to 2xVREF, but the data comes out only in VREF reference.


    Also, can you let me know all the settings that need to be done across all the registers to set 2xVREF ?

    Thank you

  • Hello K Pruthvi Raaj,

    Thank you for confirming the readback.

    Setting wise, you have the correct bits in the CFR register to configure for the 2xVREF, Single-Ended, with external VREF mode of the ADCs. This should be enough to read back data with the other default/reset setting of the device.  

    External settings would be the next thing to confirm, if possible could you please share more on the type of set-up you have for this device? Like supplies, filters, drivers, etc. or if you have a schematic that would be very helpful. 

    One very important external setting for the set-up mentioned, is the AVDD requirements for 2xVREF functionality. AVDD must be greater that or equal to 2xVREF, so if your external VREF is 2.5V, then AVDD must be at least 5V.

    To clear it up, I wish to set it to 2xVREF, but the data comes out only in VREF refere

    This clears it up, thank you

    Thank you for your patience, I look forward to hearing back from you. 

    Best regards, 

    Yolanda

  • Hello,

    AVDD in our design is 5V. 

    External settings would be the next thing to confirm, if possible could you please share more on the type of set-up you have for this device? Like supplies, filters, drivers, etc. or if you have a schematic that would be very helpful. 

                                                                     

    This is the schematic. The voltage measured at the voltage dividers at the input of ADA4528 and the output of ADS4528 matches the theoretical value calculated from the voltage divider ratio (The gain of ADS4528 is '1'). 

    The 2.5V Vref input to the ADS7853's REFIO pin has been derived from the reference voltage generator IC. Can you please confirm whether the connections at REFIO pins are correct? The datasheet suggests decoupling caps.

    Can you also confirm whether the REFDAC registers have to be written something to make sure they are not interfering in any case?

    Thank you

  • Hello K Pruthvi Raaj,

    Thank you for sharing your schematic. From looking at your schematic you do have some basic requirements for the ADS7853 to function, but I do have to suggest, for optimum ADC performance, to look into Section 9 (Application and Implementation) of the datasheet. It is recommended to use at least an antialiasing filter at the inputs of the ADC, since this device is a SAR ADC it is critical that the sample and hold (CHS = 40pF) is charged within the acquisition time (~520ns) of the ADC.  

    The other recommendation within that section is having a reference buffer for each ADC channel. Considering not just the recommended 10uF but also the ADC’s input reference dynamic load is important when deciding on a reference buffer or and unbuffered reference.  

     

    That being said, if it is possible, are you able to measure the ADC input, REFIO pins, and the AVDD close to the device? Especially though an acquisition and conversion sequence.  Since the required supplies and software configurations seem to be met, it should be working as expected, but since it isn’t I am concerned that some input might be overdriven and the device is not seeing what is actually expected.

     

    Another thing to try is to use the ADS7853’s internal reference to see if it works with that, this will help determine if the external reference is able to drive both ADC’s references.  

     

    Best regards,

    Yolanda

  • Hello,

    It turned out that the ground pin of the REF5025AIDRG4 was not soldered properly. Once this was fixed, we received the expected data in the correct format.

    Thanks for the help