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)
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.
Hello Barry, after searching once more I've found an answer by MMA 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.
Thank you very much for your further clarification.
I think what you described is reasonable. I will do test to double check it.