Magic 15V on output pin B AD8599 and AD4075

Hi Guys,

This one has me totally confused. I have a circuit designed in house by AD for a project I'm working on as an input stage for my ADC-DSP section.

The problem i have is that the circuits not functioning and even more frustrating is a 15V dc voltage that appears form no where.

I have tested the board and the pad has no DC voltage at all yet as soon as the op amp is soldered in place BAM 15v even removing all caps and resistors on section B I'm still seeing 15v

HELP I'm begginign to loose patience and i really need to figure out whats going on.

I have attached a screen grab of the tracking, the original AD supplied schematic and my version used to gee rate the board. The correct chip is the 8599 but in desperation i have swapped in a 4075 and i still have the same problem

Many thanks

Graeme

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  • gr4eme,

      The left side of R84 should go to OUTA.  The right side of R84 should go to INB-.

    Break the connection from OUTA to R84.

    As a side note, the abs max for the AD8599 is +/-18V.  If you have a small spike on the +18V,

    all bets are off and we will not accept returns.  You may affect the long term reliability or

    cause a catastrophic failure.

    It looks like you are building the classic four resistor difference amp.  All four resistors have

    to be tight tolerance to get decent DC CMRR.  The caps have to be matched also (expensive!)

    to get decent AC CMRR.  See:

    http://www.analog.com/library/analogdialogue/archives/48-02/diff_amp.html

    You may want to consider the AD8271 or AD8274.

    Harry

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