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) ?
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,
Thank you all for your help. I've tried some of your suggestions and here are my results:
1. What seems to have the greatest influence is the LM317's output bypass capacitors. First I changed all three of them to 10uF so the output capacitance is 30uF. This drastically reduced the noise - see photo below:
2. Next I increased the frequency of the ADM660, this further improved the LM317's noise levels - see photo
3. Next I tried to change the ADM660 capacitors to lower value ceramic ones. However, this only made things worse (maybe the ESR was high - I didn't have the data for the cearmaic caps).
4. Adding an output load resistor to the LM317 and the ADM660 seemed to have no effect at best and sometimes even increased the noise levels.
5. Also, adding a capacitor in parallel to the LM317 feedback resistor had no noticeable effect.
All in all, with the original 10uF tantalum caps and high frequency the +5V rail looks pretty good.
However, the ADM660's -5V output is acting up (it is possible that I've missed this before but I'm pretty sure that the -5V line was stable prior to all the above changes):
As you can see the output has sharp 50mV - 25ns pulses (which are heavily coupled to the op amp's output).
1) Do you have any suggestions for reducing the output noise?
2) Am I wrong to expect a stable output from an unregulated switch capacitor converter?