Just some results.  I deadbugged an LTC part I had, and the problem went aw=
ay...ok, on the right track.  This morning I had access to the scope, so I =
looked at the part..output, feedback path, etc and NO real oscillation foun=
d, at least not what I might expect so perhaps Russell was on the right tra=
ck there.  =


I replaced it with a LM324, and as expected, the problem went away.  The ci=
rcuit was designed by the client..supposed he used this before but I suspec=
t not and the reason behind it was the ability to drive out 100mA of curren=
t.  I'm thinking that using the LM324 will solve the gain problem, and usin=
g some sort of current boost on the output..any suggestions on a simple tra=
nsistor circuit?  The LM324 can source up to 40mA but they want to do at le=
ast 100mA.  Thinking a NPN/PNP pair (2N3904/2n3906) inside the feedback loo=
p might be the easiest way?  They are rated for 200mA continuous, so that w=
ould provide the necessary drive.

Thoughts? =


--- On Sat, 4/24/10, Russell McMahon <apptechnz@gmail.com> wrote:

> From: Russell McMahon <apptechnz@gmail.com>
> Subject: Re: [EE] DAC with 2x gain - isnt working as expected
> To: "Microcontroller discussion list - Public." <piclist@mit.edu>
> Date: Saturday, April 24, 2010, 5:36 PM
> > This is a single supply, no
> negative since I'm only on the positive side of
> things...figured I didnt have to use a neg supply.
> =

> Provided that a IS55004 =3D =3D =3D ISl55004
> =

> =A0 =A0 http://www.intersil.com/data/fn/fn6219.pdf
> =

> That would be your problem then.
> At least :-) !
> =

> ie that makes the circuit completely outside the IC's
> spec.
> =

> The spec sheet says:
> =

> 1. The amplifier has an input common-mode voltage range
> from 1.5V
> above the negative supply (VS- pin) to 1.5V below the
> positive supply
> (VS+ pin). If the input signal is outside the above
> specified range,
> it will cause the output signal to be distorted.
> =

> and
> =

> 2. The outputs of the ISL55004 can swing from -12.75V to
> +13.4V for VS
> =3D =B115V. As the load resistance becomes lower, the output
> swing is
> lower.
> =

> 1.8V out for 0V in is definitely distorted - so it's
> meeting the 1st
> spec OK :-).
> =

> Point 2 above says it gets down to 1.25V above its negative
> supply at
> 30V supply and is expected to not get as close at lower
> supply
> voltages so 1.8V seems not too surprising either.
> =

> LM324's are cheaper-) - depends on what level of
> performance you want.
> =

> =

> =A0 =A0 =A0 =A0=A0=A0Russell
> =

> -- =

> http://www.piclist.com PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
> =



      =


-- =

http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist