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CN071 Reference Design (based on AD8495), weird +2.5V input

Question asked by tfm on Feb 18, 2018
Latest reply on Feb 23, 2018 by JinoL

Hello,

We have purchased some units of the AD8495 and we are about to test its performance. So I downloaded the CN0271 reference design:

CN0271 Circuit Note | Analog Devices 

and

http://www.analog.com/CN0271-DesignSupport 

 

And I can't make much sense of this +2.5V going to IN- input of AD8476 (pin 1). It can be seen on the schematic:

 

layout:

 

And on the ref. design webpage:

 

 

BOM confirms that R5 is a short and is populated:

 

Since the AD8476 is just a differential amplifier, unity gain, its differential output should be (AD8495 output ) - 2.5V. However, the AD8495 reference is grounded, and its output equation is:

 

Considering the whole board is based on a single +5V supply, that looks a bit awkward.

It looks to me that pin 1 should be grounded. AD8476 main purpose seems to be just convert the single ended output from the AD8495 to a differential signal and drive the ADC. Most of the common-mode is already addressed by the instrumentation amplifier, anyway.

But I am probably missing something here, since I believe this is a tested circuit. Could you advise?

 

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