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Difference between AD623 and AD8421


I use both AD623 and AD8421 as instrumentation amplifiers for subtracting an electrical signal. Although both are set to the same gain, AD623 shows much better signal to noise ratio. 

So, my question is, what is the differentce between AD623 and any other instrumentation amplifier produced by Analog Devices? I checked the datasheet but didn't get what is the front end of this amplifier configuration and if there is any mean to copy same effect to other instrumentation amplifiers of higher quality.

All help is appreciated thanks