Charge pumps and OP-AMPS are NOT a very good combination...

Although PSRR may be stated at 60dB or better, the induced noise on the
input, escpecial for FET input OP-AMPS can be very high. The best
overall solution is to use a shield around the charge pump as stated by
Matthew and to ensure that single point ground connections are deployed
throughout the PCB layout. Ground connection from the charge pump should
return to the supply input, the same goes for the GND reference for the
input and output signals. This will ensure no ground loops, a big
headache for audio/analog cct's using charge pump supplies. The art of
Electronics (Horowiz & Hill) presents a very good discussion on this
particular topic.

Matthew Ballinger wrote:
>
> Hi, I designed  an electronic crossover for my  car stereo a while back. I too
> used a charge pump to produce the (-)V for the OPAmps. I had a huge problem
> with the switching noise. I tried low ESR caps of various sizes. Even after
> boosting the switching FREQ above audible, it was still there. Don't no why.
> Finally I added shielding (aluminum foil) around the charge pump IC and
> components and it eliminated the problem completely. I'm not sure you have the
> same problem though. It may be worth experimenting. Good luck. Matt.

Nino.
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* Antonio (Nino) Benci                               *
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