ADM660 input switching noise

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).

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.

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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  • 0
    •  Analog Employees 
    on May 17, 2013 10:48 PM over 7 years ago

    Hi Alex,

    Looks like you've made a lot of progress so far.  Here's a couple of comments in response to your results and questions:

    1.  Ceramic caps:  I'm surprised that ceramic capacitors don't help your ripple--especially for the output cap.  Ceramics have much lower ESR's then tantalum.  Try using the same value (10uF) the tantalum ones and see if that doesn't help. 

    2.  Vout ripple:  The short spike in the Vout ripple is due to parasistic inductances. Probably in the output capacitor or in the trace to the output capacitor.  Ceramics usually have a lower ESL than tantalums, so that may help with the parastic inductance spike.  Or you can always add a small LC filter to the output. 

    3.  Stable Output:  The output of an inverting charge pump is alway "stable."  Meaning it does not oscilallate because it has no feedback path (it is not sensing the voltage and adjusting any of the power delivery).  So it is never unstable.  However, like all switching converters, it will have output voltage ripple, which is normal. 

      - Jon

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  • 0
    •  Analog Employees 
    on May 17, 2013 10:48 PM over 7 years ago

    Hi Alex,

    Looks like you've made a lot of progress so far.  Here's a couple of comments in response to your results and questions:

    1.  Ceramic caps:  I'm surprised that ceramic capacitors don't help your ripple--especially for the output cap.  Ceramics have much lower ESR's then tantalum.  Try using the same value (10uF) the tantalum ones and see if that doesn't help. 

    2.  Vout ripple:  The short spike in the Vout ripple is due to parasistic inductances. Probably in the output capacitor or in the trace to the output capacitor.  Ceramics usually have a lower ESL than tantalums, so that may help with the parastic inductance spike.  Or you can always add a small LC filter to the output. 

    3.  Stable Output:  The output of an inverting charge pump is alway "stable."  Meaning it does not oscilallate because it has no feedback path (it is not sensing the voltage and adjusting any of the power delivery).  So it is never unstable.  However, like all switching converters, it will have output voltage ripple, which is normal. 

      - Jon

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