Tailoring the AD7173-8 ADC input range

I would like to take full advantage of the ADC's input range while using the internal buffers.

I have configured the ADC to run from a single +3.3V supply.

If the internal buffers are turned on, the absolute analog input range become 0 to 2.2V.

If I use a 1.1V external reference, the ADC range will match the absolute analog input range.

I don't think I want to use a resistor divider off of the ADR441 to create a 1.1V reference.

Should I buffer this signal with an amplifier for the reference?

- James

  • 0
    •  Analog Employees 
    on Aug 14, 2015 5:19 AM

    Hi James,

          The AD7173-8 has a buffer on it, I would recommend to use these rail-to-rail buffers. ADI has a ADR512 1.2V reference that I would recommend that is closest to your requirement of 1.1V. Or you check on the link below to check for other references.

    http://www.analog.com/en/products/linear-products/voltage-references.html

    Regards,

    Jonathan

  • Hi Jonathan,

    I don't think the ADR512 is a suitable replacement.

    It has 4x the output noise; 20x the temperature coefficient.

    I noticed that the ADR441 is used on the evaluation board and that there appears to be a resistor divider between the output of the part and the input of reference inputs. 

    I'm thinking about inserting an amplifier in a non-inverting configuration with a gain of 0.44 to create the 1.1V reference from the 2.5V reference output of the ADR441.  Is this a suitable approach to the problem and if so what amp would you reccommend?

    I'm thinking a low input bias current amp like the AD8615 would do the job.

    What are your thoughts?

    - James

  • 0
    •  Analog Employees 
    on Aug 19, 2015 12:07 AM

    Hi James,

        The ADR441 has a voltage noise of 1.2uVp-p and a 48nv/rtHz of voltage noise density. The AD7173-8 has an available internal buffer for the internal reference. Using a resistor divider to have a 1.1V to ADC Ref pin maybe suitable, since the output impedance of the ADR441 is zero or almost zero ( please refer to fig. 31 of the ADR441 datasheet) at lower frequencies and considering the effect of resistor thermal noise could be very minimal ( you may find the formula in the web) and enabling the buffer in the AD7173-8 at the same time to have a High input impedance on the buffer, this will have 1.1V on the Reference of the ADC.

          There is a 0.44 attenuation amplifier available with ADI like AD8475 have a  2.5uVp-p output voltage noise.

          I'll try to check on the people here from our amplifiers group if they can recommend. Do you have some specifications that need to be consider other than that was mentioned before?

    Regards,

    Jonathan

  • Hi Jonathan,

    I gave it a little more thought and decided using an amp in the non-inverting configuration would not work well because it would not take advantage of the ADR441 and it would create a situation where the amp would be sinking current to maintain the 1.1V reference.

    Instead I decided to divide the signal from the ADR441 down to 1.1V and buffer this with a low offset amp like the ADA4528-1.  This will ensure that the reference input of the AD7173-8 see a low source impedance.

    If this is how I plan to drive the reference input, would be it advantageous to disable the reference input buffer?

    I was unable to find any tangible metrics in the datasheet that described how the reference buffers directly impact the performance of the converter.

    Again, thanks for your feeback and your support.

    Best Regards,

    James

  • 0
    •  Analog Employees 
    on Sep 1, 2015 10:24 PM

    Hi James,

        

         I would recommend to use the internal buffer. This buffer is design and proven to work best with the AD7193. And using the internal buffer, it will save space on the board.

    Regards,

    Jonathan