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buffer of AD7705

Question asked by shichen717 on Aug 4, 2013
Latest reply on Sep 11, 2013 by JohnnyG

Hi.

I use AD7705 to measure differential voltage, with PGA=32, Sa=50, CLK=2.4576MHz.

From Datasheet, AD7705 has 16 bit ENOB, 0.65uV rms noise, which means 15.1bit noise free resolution, either buffer is on or off.

 

When I short Ain1+ and Ain1- to Vref=2.5V, I got 15.1bit noise free resolution when buffer was off, about 2 LSBs reading noise within 5 second, or 3-4 LSBs noise in a hour.

But when the buffer is on, noise reached 4 LSBs in a short time, or maximum 6-7 LSBs in a hour.

 

I want to realize the characteristics of the noise of the buffer, ie., the input noise density, which is not mentioned in datasheet.

I think the buffer is not noise free, the noise table in the datasheet implies it's noise is very low, but how low is it, and how the noise of the buffer affect ENOB?

 

And, I also have a question on 15.1bit, does it lead to 2 LSBs P-P reading noise? How to calculate the long time reading noise from the rms noise?

 

Thank you.

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