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 13, 2013 1:02 PM over 7 years ago

    Whilst my area is really inductor based power supplies, I thought you may be interested in the article here: http://www.tnt-audio.com/clinica/regulators2_impedance1_e.html  This looks at the impedance of the LM317 across frequency and the anecdote at the end may be a similar problem to the one you are having - the LM317 may be oscillating due to the low esr input cap on the ADM660.

    There are a few things you could try to help

    - Add a capacitor into the LM317 feedback loop to improve the low frequency performance

    - Ensure there is a continuous load (the output impedance of this type of regulator tends to improve under load)

    - Add a ferrite bead between the LM317 and the input cap of the ADM660 - it may help reduce high frequency noise

    You may also want to try changing the ADM660 switching frequency - I believe you can either set 25kHz or 120kHz.  One frequency may give better performance for you

Reply
  • 0
    •  Analog Employees 
    on May 13, 2013 1:02 PM over 7 years ago

    Whilst my area is really inductor based power supplies, I thought you may be interested in the article here: http://www.tnt-audio.com/clinica/regulators2_impedance1_e.html  This looks at the impedance of the LM317 across frequency and the anecdote at the end may be a similar problem to the one you are having - the LM317 may be oscillating due to the low esr input cap on the ADM660.

    There are a few things you could try to help

    - Add a capacitor into the LM317 feedback loop to improve the low frequency performance

    - Ensure there is a continuous load (the output impedance of this type of regulator tends to improve under load)

    - Add a ferrite bead between the LM317 and the input cap of the ADM660 - it may help reduce high frequency noise

    You may also want to try changing the ADM660 switching frequency - I believe you can either set 25kHz or 120kHz.  One frequency may give better performance for you

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