By using external reference would it increase measurement accuracy ?
If so, which reference IC is recommended ?
The quick answer is yes, using an external reference will likely increase your converter accuracy assuming the reference chosen is more stable than the internal reference.
There are a few recommended reference parts on the AD7091 product page. (http://www.analog.com/en/products/analog-to-digital-converters/ad7091r.html#product-recommendations )
I am moving this into the data converter forum in case you have any follow up questions on this device.
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