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AD8436 current meter - Part II.

Question asked by Jest62 on Jan 3, 2013
Latest reply on Jan 10, 2013 by jstaley

Hello,

 

Finally received PCB with AD8436 connected as it is on following picture:

 

cid:image001.jpg@01CDE9C9.96C0ABC0

 

PCB layout is compact and simple:

 

cid:image002.jpg@01CDE9C9.96C0ABC0

 

But result is far away from what I expected and what I got on AD8436 evaluation board.

 

At the input, there is 50Hz sinus with 2,5V DC offset:

 

cid:image003.jpg@01CDE9C9.96C0ABC0

 

After input capacitor, DC component is removed and everything looks nice except that signal is going below 0V???

 

cid:image004.jpg@01CDE9C9.96C0ABC0

 

First nonsense comes out from input buffer. Signal should be 2x in Pk-Pk around same DC offset of 2,5V. But it is not.

 

cid:image005.jpg@01CDE9C9.96C0ABC0

 

On next point DC component should be removed for entering RMS stage. And it is not...

 

cid:image006.jpg@01CDE9C9.96C0ABC0

 

Of course output is something strange and cannot be used.

 

Any idea why this happens? OK, now I know, that design as it is cannot be used with 2x amplifier in input buffer since there is only 5V power supply. So, input stage should be follower and output stage 2x amplifier. But the answer remains: what is happening with my input buffer to act like it is present on the scope?

 

Thank you for support and best regards,

Andrej

 

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