AD7787 AIN2 input voltage range

If REFIN is tied to VDD on the AD7787, is the AIN2 input voltage range from GND to VDD (when configured as a unipolar input)?  Are there any issues with that configuration?  VDD=5V in my application.

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  • +1
    •  Analog Employees 
    •  Super User 
    on Feb 17, 2021 7:08 AM

    Hi Jessica,

    The AIN2 input voltage range varies depending on the mode of buffer you select. 

    When the mode is selected to unbuffered, the AIN2 input voltage will range from 0 to 5v based on the VDD of your application. But there is some disadvantage of using the unbuffered mode. First is you will have a limitation of RC values at the inputs. The second is will contribute to some gain errors in the system. 

    When the mode is selected to buffered, the AIN2 input voltage will only have a range of 0.1v to 4.9v. Since there is a 100mV headroom voltage at the buffers. But the advantage of using buffered mode is that it allows you to place source impedances on the front end without contributing gain errors to the system. 

     

    Regards, 

    Andrei

  • Thanks Andrei - I would like to use this to measure a voltage in the range of ~1.5V to 4.5V, so it sounds like the buffered mode would work.

    And just to confirm, when unipolar input is selected, a high precision reference voltage at REFIN should not be necessary, correct?

  • 0
    •  Analog Employees 
    on Feb 18, 2021 2:03 AM in reply to JRiley529

    Hi, 

    I think the accuracy of the reference voltage depends on the target accuracy of your application. So if you want a high precision, high accuracy I would recommend to use a low noise, high accuracy external reference input. 

    Thanks,

    Jellenie

  • Thank you Jellenie.

    I am trying to find a solution which will measure a 1.5V to 4.5V signal with no more than 1mV of total error.  My supply voltage is 5V (4.75V to 5.25V).  Can you recommend a reference voltage IC which will accomplish this?  Or is there another ADC I should look at?

  • 0
    •  Analog Employees 
    on Feb 18, 2021 3:52 AM in reply to JRiley529

    Hi, 

    For the family of low noise, low drift and high accuracy references you may check this: ADR445, ADR4550.

    But I guess there's also good references with LTC products. I'd suggest ask someone from Voltage References - EngineerZone (analog.com) as they may recommend something. 

    In terms of the ADC, it is an old product but we do not have new product with single channel. You can check AD717x family I think it has a good internal reference with more number of channels, higher speed and optimum performance.

    Thanks,

    Jellenie

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