At 09:22 AM 8/10/2004 -0400, you wrote:
>Hi all. Few questions about opamps. If I see a circuit running off a
>split power supply, but the negative voltage is only used for the
>opamps, can I redesign it using single power supply opamps such as the
>TLC272?

"Single supply" generally means that input common mode range and the
output range extend to the negative supply rail and close to the negative
rail respectively. It may be possible to *redesign* the circuit
so that it uses single supply (or rail-to-rail) op-amps, but it will
usually be more than a simple substitution of parts. If you have to
ask, the answer is probably "no".

>If I must use a split power supply, would something like the
>http://focus.ti.com/docs/prod/folders/print/pt5062.html from TI
>(+5Vin, -15+15Vout) be a bad idea due to switching noise? This is for
>audio purposes.

You should not have a problem with *audible* switching noise, since
that part operates at 650kHz, however if you use a very wideband
op-amp as used for "professional" audio, you might end up with some
high frequency noise finding its way through the circuit if you are
not careful with layout, bypassing and filtering. Sometimes a relatively
crummy part like a TLC272 (unity gain bandwidth only 2.2MHz typ.) will
be more stable and cause less problems.

That power supply part should work fine if you don't mind the $16
cost. Note that +/-15V will definitely fry a TLC272, though. +/-8 maximum
(16V total). I suggest at least a 0805 ferrite bead series then a 1uF ceramic
cap to ground on each output, with careful attention paid to layout to
keep the 650kHz noise under control.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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