From: Brent Brown <brent@eds.co.nz>
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Date: Tue, 19 Oct 2021 19:21:24 -0700
Subject: Re: [EE] Op amp buffer behaviour
Thread-Topic: [EE] Op amp buffer behaviour
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Hi Neil,

Yes, but you're putting the 980Hz square wave into the input of the Op Amp,=
 first=20
trace, then observing the output, second trace... desiring that to be a buf=
fered=20
version of the input? Sorry if I've got this wrong, feel free to correct. T=
he slew rate of=20
this Op Amp is typically around 3V/milli-second (seems very slow, but that'=
s what=20
the data sheet says). That could explain why you are seeing a very triangul=
ar=20
shaped wave at this frequency?

A single gate logic part (74HC1Gxx) might do a good job of buffering the sq=
uare=20
wave.

Brent

On 19 Oct 2021 at 20:36, Neil wrote:

> I failed to mention that the PWM frequency is 980Hz (IIRC).=A0 The 1.6Hz=
=20
> is the RC filter cutoff frequency.
>=20
> On 10/19/2021 8:05 PM, Brent Brown wrote:
> > Sadly, the datasheet might agree with your scope, see figure 2-31. Sugg=
ests the
> > part is just not fast enough to accomdate your 1kHz square wave. But pu=
tting the
> > buffer on the analog side should be fine though.
> >
> > On 19 Oct 2021 at 19:41, Neil wrote:
> >
> >> Working on a proof-of-concept, but scratching my head over this.
> >>
> >> First, I attached an RC filter (fc =3D 1.6Hz) to the output of an Ardu=
ino
> >> PWM pin.=A0 Got a decent analog voltage and it tracks the PWM duty cyc=
le
> >> decently.=A0 That will then go into a comparator later for measuring v=
oltage.
> >>
> >> Second, the circuit will really be used on a Teensy and there will be =
an
> >> isolator on the teensy output pin, so I decided to add a unity-gain
> >> buffer (MCP6042) to the (still arduino for now) PWM output pin before
> >> the RC filter.=A0 Circuit here...
> >> http://orlandorobotbuilders.com/stuff/IVM_Proto_Circuit.png
> >>
> >> Top image here is the input to the unity-gain op-amp...
> >> http://orlandorobotbuilders.com/stuff/IVM_Proto_Scopes.jpg
> >>
> >> But the behaviour is odd... I expected a clean strong square wave sign=
al
> >> at the output of the unity-gain buffer op-amp but getting a triangle
> >> wave, that's very low voltage up to about 67% duty-cycle (middle image=
),
> >> then at about 67% it flips to a higher voltage (bottom image).=A0 A
> >> circuit simulator somewhat follows, but I don't understand why.=A0 I
> >> expected the output of the buffer would be just like the output of the
> >> arduino, just strong/clean.=A0 Not sure what I'm missing here.
> >>
> >> I see that a basic active op-amp RC filter has the RC components on th=
e
> >> input side, but the intention of the buffer was to not add any
> >> noticeable load to the teensy or isolator (even though R=3D100k).
> >>
> >> Can anyone explain this to me.=A0 Thanks!
> >>
> >>
> >> --=20
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