Management was griping over having to pay $0.28/piece @ 2500qty. for the
-55C rating in a quad op-amp; to use a single $2.00 op amp would probably
take quite a bit of fight.

The LM124 was chosen because it is a "jelly-bean" op amp.
(1) it will probably be available in some form the next 30 years (hard
requirement) - the bar is set *really* low for a pin compatible replacement
to be put in.
(2) it is cheap
(3) it is rated for -55C operation
(4) it is "HV" rated. We are using a 15V supply that may be subject to
transients.

The issue is that I can't push too hard against the limitations of the
LM124, I don't want batch/manufacturer specific behavior. So I will have to
figure out a way around the slew rate. Probably by reducing the PWM
triangle wave to 1V peak-to-peak.

Out of curiosity what makes the LM324 family "bad"? Voltage swing? Slew
rate/bandwidth? Noise/Offset? All the above ;) ?


On Tue, Jul 10, 2018 at 11:08 AM, Sean Breheny <shb7@cornell.edu> wrote:

> Hi Jason,
>
> Is there a reason for using such an ancient op-amp? This is part of the
> LM324 family which is only really ever used now in either ultra cost
> sensitive devices or legacy designs. Except for cost, there are many
> op-amps which are better in every regard.
>
> I think you are right to be concerned. Slew rate depends on internal
> capacitances, resistances, and current magnitudes. All of those are
> temperature dependent AND manufacturing dependent. I would not be
> comfortable assuming this part can do 0.2V/us unless either the datasheet
> specified a minimum slew rate of more than that over all temperatures and
> manufacturing variation OR if the typical value were at least 10x the
> necessary value (2V/us).
>
> Sean
>
>
> On Tue, Jul 10, 2018 at 10:29 AM, Jason White <
> whitewaterssoftwareinfo@gmail.com> wrote:
>
> > In my current design the operating temperature range is rather large:
> -55C
> > to 125C.
> >
> > I am planning on using the LM124DT from ST in an analog PWM circuit. My
> > design uses a 0.5 to 4.5v triangle wave at 25kHz (required slew rate
> > 0.2V/us).
> >
> > The LM124DT has a "typical" slew rate of 0.4V/us at 25C. A minimum slew
> > rate is not specified and slew is not specified at other temperatures.
> > Other manufacturers such as TI don't seem to specify this either.
> >
> > I would like to get an intuitive sense of how (or if) slew rate might
> > change over temperature. Additionally I'd like to get some opinions as =
to
> > whether 0.2V/us could be too fast for the LM124.
> >
> > Thanks,
> > Jason White
> > --
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