ADuC706x: main ADC chan.: min. absolute input voltage for VIN− @gain=1

For the main ADC channel, the datasheet of the ADuc7060/7061 lists an absolute input voltage range for both VIN− and VIN+ of 0.1V  to VDD−0.7V (page 7).

Additionally, in table 43 (ADC0CON MMR Bit Designations) in the primary channel ADC gain select section, for a gain of 1 the following note is given: Buffer of negative input is bypassed.

 

If the buffer of the negative input is bypassed for gain=1, does the assumption hold true, that the minimum absolute input voltage for the primary channel ADC, VIN−, in this case is AGND, not 0.1V?  (analogous to the secondary ADC with buffer disabled)

  • Hello,

    A very interesting question.

    From the specification, the ADuC706x can only guarantee the performance showed in the specification. For gain =1 and input voltage less than 0.1V, you can use the Auxiliary channel.

    While from the technical view, it's worth a try for your case.

    Best Regards,

    Barry 

  • Hello Barry, after searching once more I've found an answer by  to a related question, that might count as an answer to this question too: The negative buffer bypass in case of gain=1 appears to be implemented deliberately, to make it possible to tie the negative input directly to AGND, avoiding the 0.1V offset. From my view this could be considered a 'yes' to my question.

    (A remark found in AN-608 p. 3, "Grounded Inputs", seem to be related, in that it highlights a very similar situation.)

    Perhaps this could be expressed in the datasheet (?): 

             Absolute Input Voltage Range:

                      Applies to both VIN− and VIN+:           Min = 0.1V

                      except for VIN−, if Gain = 1:                 Min = AGND

     

    As you mentioned the auxiliary channel, where both buffers can be disabled manually:

    I suppose in the data sheet, Table 44 / ADC1CON, the note "Single-ended channels are selected with respect to ADC5. Bias ADC5 to a minimum level of 0.1 V." can be understood that way too -- if one disables the auxiliary ADC's negative input buffer, no minimum level of 0.1V is needed on ADC5, which may be directly tied to AGND then.

    Thanks,

    Michael

  • Hello Michael,

    Thank you very much for your further clarification.

    I think what you described is reasonable. I will do test to double check it.

    Best Regards,

    Barry