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ADA4817 eval board oscillating

Question asked by rmauro on Nov 13, 2015
Latest reply on Dec 1, 2015 by KRZ

I've been having some trouble with a prototype circuit using the ADA4817-2.  My circuit uses that dual amp as the input of a 3-amp IA - with a gain of 2.  I am trying for 500MHz BW.  The input amp peaks and is prone to oscillate around 180Mhz.  I tried everything and couldn't get the circuit stabilized. 

Finally - I got an evaluation board (see attached UG083).

I installed a single ADA4817-1 and the following components (refer to fig 2 of eval board guide):


C3, C4 33uF
C5, C6 .47uF 0603 ceramic
C2 .047uF 0603 ceramic
R6, R7 1kohm  (jumper to not power down)
R5 274ohm
R3 10ohm
R4 49.9ohm
R9 49.9ohm


R1,R2,R8,R10,C1   not installed


I am running the part at +/-5V supplies, and the thermal pad is tied to gnd.


This should be a simple unity gain follower.  I swept the frequency response (50ohm source and load) into a scope.  The orange trace (F5) is the max of the FFT.  You can see that the output is about -6dBm (with a 0dBm input) at 10MHz - and this peaks up by about 5dB at around 190MHz - then the amplitude 'jumps' and then drops down with increasing frequency.  This seems to be a near oscillation.  This is nearly identical to the response that I get on my circuit. 

(See attached file: eval_amp_freq1.png)


The data sheet warns of peaking at unity gain, but it should be within reason with 274ohm fb resistor.  The data sheet specifies a G=1 BW of 1050MHz - and I'm not even getting half that, with instability. 


I could sure use some help in getting this device to work.  I would think by using the AD eval board I would get very similar performance to that shown on the data sheet. 

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