From: dplatt@radagast.org (Dave Platt)
Newsgroups: sci.electronics.components
Subject: Re: Need replacement for LM357 single op-amp (National discontinued 
	it)
Date: 11 Mar 1999 18:25:41 GMT

>> I have an amplifier design that requires an LM357 op-amp (8-pin single op-amp)
>> National has discontinued this component and was not able to recommend
>> a replacement. They do make the LM358 which is a dual version, but I need
>> pins 1 and 8 for offset adjustment, making the dual version unpractical.

>It sounds like you are building something new rather than replace an old
>device.
>
>You can offset null an op-amp even if it doesn't have terminals
>specifically set aside for doing this.  Just sum a DC voltage into the
>inverting input.

Another option is to eliminate the nulling entirely.  Process and
design improvements in op amps, over the past decade or two, have
produced quite a few "precision" op amps which have extremely low
input offset voltages, and which exceed the other specs of the LM357
by a comfortable margin.  In many cases, you can drop one of these op
amps into a circuit which uses an older op amp, by simply
disconnecting the nulling circuitry, and end up with performance as
good as you'd have gotten from a manually-nulled less-precise op amp.

[A nulling hint from Bob Pease of National Semiconductor: don't try to
 use an op amp's nulling pins to trim out a voltage offset which is
 due to the op amp's input currents looking back into different
 impedances (e.g. a bipolar-input op amp with the positive input
 grounded and the negative input tied to the feedback network).  This
 way lies madness, says Bob - the varying temperature coefficients of
 the impedances involved will cause the output to drift when the
 circuit temperature changes, and you'll never get it to be stable.
 Balance the input impedances first... and _then_ null, if you still
 need to!]

--
Dave Platt                                           dplatt@radagast.org
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