At 21:35 01/24/99 +0000, Dan Larson wrote:
>I am using a MAX680ACP to generate +/- 10V supplies for
>an LF383 op-amp, the output of which I am feeding to
>an LM567 tone decoder.  It works fairly well, but the
>ripple on the +10V supply (at about 12Khz) gets amplified
>and is impressed on the output whether the photo transistor
>is covered or not.  The strange thing is that there is no
>measurable ripple on the -10V supply.  Things go down hill
>fast when I try to boost my gain by using the second op-amp
>as a 1:100 gain amp.  Needless to say there is a nice square
>on the output with, surprise!, the same period as the
>charge pump ripple.  I have bypassed the +10V supply to
>the op-amp with a ferrite bead and 4.7uf cap.  It helps
>a little, but the output is still a wild waveform of
>the amplified supply ripple.  The op-amp circuit is nothing
>special. I have both stages set up with 1:100 gain using
>470K for feedback and 4k7 on inputs.  The photo diode is biased
>with a 100K resistor to V+ with the other end of the diode
>tied to V-.  The junction of the diode and the 100K is coupled
>to the non-inverting input of the first stage with a .01uf
>cap.

doesn't it look like you get the ripple from the input bias? you can check
this and use the photodiode as a current source into the inv. input (diode
between gnd and inv.inp., plus a feedback resistor from inv.inp. to the
output and noninv.inp. to gnd) without any bias from the supply. (usually
you use the bc-diode of a transistor and leave e open in this configuration
if you have a phototransistor.)

ge