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Ground plane under AD8003 and what to do with unused amplifier?

Question asked by josh.vear on Mar 26, 2015
Latest reply on Mar 31, 2015 by JGunao

Hi all,

 

The datasheet for the AD8003 triple 1.5GHz Op Amp recommends the use of ground and power planes, however, it also says planes should not be used under any of the pins of the AD8003 in order to minimise stray capacitance at the inputs and outputs.  The AD8003 has an exposed paddle and the datasheet suggests incorporating thermal vias into it's mounting pad.  BUT, if there is no ground plane under the device, then these vias have nothing to connect to and as far as I can tell will have minimal impact on thermal transfer.  The evaluation board (see UG-022) has this arrangement (ie vias from the mounting pad that do not connect to anything on the other side of the board).  Is this the recommended method for connecting the exposed paddle mounting pad?

 

I am only using two of the AD8003's amplifiers.  The third is turned off using the Power Down pin.  How should I connect the inputs and outputs of this unused amplifier?  I currently have +IN connected to ground, a 200ohm resistor from the feedback pin to -IN and a 50ohm resistor from Out to ground.

 

In case it makes a difference I will describe my application.  We have a pulsed laser with pulse width of ~10ps and repetition rate of 100MHz.  I need to produce a 100MHz clock signal that is in phase with the laser.  I also need an output to monitor the pulse shape on an oscilloscope.  I am using a reverse-biased high-speed (70ps rise-time) PIN photodiode with 50 ohm load to detect the laser pulses.  This signal is buffered by two of the AD8003's amplifiers.  One output feeds a connector, the other is low pass filtered to stretch the pulse and feeds a comparator.  The comparator then feeds a zero delay PLL buffer which produces a 50% duty cycle CMOS clock signal, which is made available on a connector.

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