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ADM660 input switching noise

Question asked by AlexKu on May 13, 2013
Latest reply on May 17, 2013 by Jon.Kraft

Hello,

 

I'm trying to power an AD8052 opamp with a +/- 5V power supply. The positive +5V supply for the op amp is generated by an LM317 voltage regulator (the input is a 9V battery). While, the negative -5V supply is generated by the ADM660 whose input is the LM317's +5V output. The ADM660 negative output directly powers the negative supply of the AD8052 op amp (see schematic below).ADM660.JPG

 

Now, when I view the ADM660 output on a scope i get a pretty stable -5V voltage. However, if I probe the +5V input voltage I get quite large switching noise (order of 150mV and frequency is approximately 44khz). If I disconnect the ADM660, the noise on the +5V line disappears. The 10uF capacitors are tantalum low ESR as recommended by the datasheet. Placing a large electrolytic cap between the +V input and GND of the ADM660 actually reduces the noise a bit but it is still significant.

 

IMG-20130512-WA0000.jpg

 

1) Is this a normal phenomena with those kind of switch capacitor voltage inverters or am I doing something wrong?

2)  What can be done to reduce the noise amplitude?

3) If bypass caps are the way to go, what  values would you recommend (I already have three caps on the +5V line which are placed about 2cm from the ADM660) ?

 

P.S.

If it makes any difference, the board is a 4 layer one and the +5V input to the ADM660 is routed through a via to a +5V plane.

 

Thank you in advance,

Alex

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