I plan to use the AD1791 for a weight scale design using AC excitation.
Is it allowed to switch between REFIN(+)=AVDD & REFIN(-)=AGND to REFIN(+)=AGND & REFIN(-)=AVDD?
Yes, AD7191 and AD7195 belongs to the same AD719x family and must have the same in some specifications. However, the AD7195 has an additional AC excitation feature which AD7191 doesn't have. This extra feature makes the AD7195 capable of accepting an inverted reference and analog input as shown in Figure 50, because when ACX is enabled the AD7195 assumes that the input and reference terminals are reversed on alternate sampling cycles and the part synchronizes the ac excitation with the conversions and performs the averaging internally. But with ACX disabled, the part must accept a reference input voltage of (1V - AVDD) as AD7191 specified. If you wish to use AD7191 in an AC bridge excitation, this will require a lot of switching requirements on your frontend reference and analog input connections to keep the voltage (as seen by the ADC) positive and within the specification. Thus, I would suggest to use the AD7195 instead of AD7191 for your AC bridge excitation application.
Sorry, I mean the AD7191
REFIN(+) is a Positive Reference Input that can lie anywhere between AVDD and AGND + 1 V. While REFIN(-) is a Negative Reference Input which can lie anywhere between AGND and AVDD - 1 V. The nominal reference voltage, (REFIN(+) - REFIN(-)), is AVDD, but the part functions with a reference from 1 V to AVDD. So the answer to your question is no since the minimum reference voltage limit is 1V.
Sorry for appearing ungrateful but are you 100% sure about that?
When comparing the datasheet of the AD7191 and AD7195 both are specified exactly the same. They even have the same comment on the comment column. See page 4 of both datasheets. (1V - AVDD and abs. max GND-0.05 - AVDD +0.05).
For the AD7195 reversing the polarity is allowed though, see figure 50 of the datasheet.
Maybe the spec should be |REFIN| instead of REFIN.
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