What I meant by DC...is not AC (audio, etc)...but basically being used to p=
osition actuators that accept 0-10V.  I guess by saying there is no AC comp=
onent, there isn't a deliberate AC component being generated.

--- On Mon, 4/26/10, Olin Lathrop <olin_piclist@embedinc.com> wrote:

> From: Olin Lathrop <olin_piclist@embedinc.com>
> Subject: Re: [EE] DAC with 2x gain - isnt working as expected
> To: "Microcontroller discussion list - Public." <piclist@mit.edu>
> Date: Monday, April 26, 2010, 8:45 AM
> alan smith wrote:
> > The output is driving just DC voltages,
> =

> That's a silly statement.=A0 DC as apposed to what,
> AC?=A0 And what's that
> supposed to tell me?=A0 If it were really DC it couldn't
> change.=A0 Obviously
> your output must have some AC component, so this statement
> tells me nothing
> useful about the load.
> =

> > sourcing the current with the
> > loads being actuators and the like used in HVAC
> > so it never needs to
> > go negative (or should never)at all.
> =

> Then a NPN emitter follower will work fine as long as you
> have a opamp that
> can drive near ground on the output and with a common mode
> input range
> extending to ground.=A0 You will need at least the B-E
> drop headroom for the
> opamp on the high side, but probably more for the opamp
> itself.
> =

> > Running a 24V rail also gives plenty of headroom.
> =

> Yes, that is way more than enough headroom even with the
> emitter follower in
> there.=A0 Your 3904 transistor isn't going to handle
> that at 100mA though.=A0 As
> I suspected, you need something considerably more beefy,
> probably with a
> collector resistor.
> =

> =

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