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Question regarding datasheet of AD7718

Category: Datasheet/Specs
Product Number: AD7718
Hello,
we have a question regarding the NEGBUF bit on AD7718.
The default mode of the ADC is to have NEGBUF bit cleared, which means single-ended operation.  I've found ambiguous information in the datasheet about the exact operation in this mode.  If AINCOM is really tied internally to AGND when NEGBUF = 0 [as indicated by page 37], then we will likely create an internal short.  The specification tables, however, seem to indicate that such operation is okay.
 

page 27:

This bit controls the operation of the input buffer on the AINCOM input when a channel is configured for pseudo-differential mode of operation. If cleared [default behaviour], the analog negative input (AINCOM) is unbuffered allowing it to be tied to AGND in single-ended input configuration. If this bit is set the analog negative input (AINCOM) is buffered, placing a restriction on its common-mode input range


page: 37

Single-Ended Operation
The NEGBUF bit in the mode register is used to control the operation of the input buffer on the AINCOM pin when configured for pseudo-differential mode of operation. If cleared, the analog negative input (AINCOM) is unbuffered. It should be noted that the unbuffered input path on the AINCOM provides a dynamic load to the driving source. Therefore, resistor/capacitor combinations on this input pin can cause dc gain errors depending on the output impedance of the source that is driving the AINCOM input. AINCOM is tied to AGND for single-ended operation. This enables all pseudo-differential inputs to act as single-ended analog inputs.

 

 


 
Can you confirm there is no internal short between AINCOM and AGND after reset, when NEGBUF=0 (default) and it is safe to apply AINCOM voltage limits before setting NEGBUF to 1?
Thanks & regards,
Michael
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  • Hi Michael,

    I am sorry for the confusion with the datasheet. Yes, you are right, there is no internal short between the AINCOM and AGND after reset and it is safe to apply AINCOM voltage limits before setting the NEGBUF to 1. You can apply between AGND-30mV and AVdd+30mV for your voltage limits of your AINCOM when NEGBUF=0. For single-ended operation, you can externally tie your AINCOM to the AGND. Thank you.

    Best Regards,

    Rod

Reply
  • Hi Michael,

    I am sorry for the confusion with the datasheet. Yes, you are right, there is no internal short between the AINCOM and AGND after reset and it is safe to apply AINCOM voltage limits before setting the NEGBUF to 1. You can apply between AGND-30mV and AVdd+30mV for your voltage limits of your AINCOM when NEGBUF=0. For single-ended operation, you can externally tie your AINCOM to the AGND. Thank you.

    Best Regards,

    Rod

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