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Can't we use an Instrumentation Amplifier in front of ADC

Question asked by jebaspaul on Nov 14, 2013
Latest reply on Nov 15, 2013 by KarenNE



I am designing a Multi channel 16 bit DAS. I am using AD7663 in my application.


The signal path has an instrumentation amplifier AD8421. AD8421 is configured to give unity gain. I have calculated the Voltage noise introduced by AD8421. It comes around 150uV . Major contribution is,Output noise given by AD8421- 60nV.sqrt(Hz).

Since my voltage range is 20V, 1LSB comes around 300uV. The noise contribution of AD8421 itself gives 0.5 LSB error.

The bandwidth used for calculation is 6.2Mhz. This value is taken from the datasheet of AD7663. In its application circuit they have used 15 ohm and 2.7nF capacitance in front of ADC to provide RF filtering. I have used noise equivalent bandwidth i.e actual bandwidth multiplied by 1.57 factor.


Actual conversion rate of ADC is 30 Khz in my application i.e 24 samples per 800us.



Is the way I calculated the noise at the input of ADC correct. Please give your comments.