What do you mean by 2r 2 times r?

Andrew

On Fri, 17 Mar 2000 13:44:44 -0600 Don Hyde <DonH@AXONN.COM> writes:
> An R-2R ladder can be used to turn a group of PIC output pins into a
> DAC for
> a few cents.  Below is a 3-bit DAC.  Using 3 PIC output pins (beware
> of the
> odball open drain pins, they're no good for this), this circuit will
> provide
> 0 - Vcc*(7/8) volts out, with a 2R source impedence.
>
> It can be extended to more bits by replicating the MSB R-2R end of
> the
> ladder.
>
> Up to 4 bits can be achieved with 5% resistors.
>
> Up to 6 bits can be achieved with 1% resistors.  I know this works
> because
> I've used it.  R=10K works fine.
>
> Up to 10 bits might be achieved with .1% resistors.  Beware, this
> circuit
> uses Vcc as its reference, so you would have to use a precision
> reference
> for your Vcc to achieve this accuracy, and I would still figure it
> was
> doubtful due to FET on-resistance, digital noise, etc.  Besides, .1%
> resistors are pricey enough that you still might be ahead to buy a
> DAC in a
> chip.
>
> In many applications, it would be advisable to buffer the output
> with an
> op-amp.
>
> A circuit like this can achieve waaay higher output frequencies than
> PWM,
> and can be easier to filter, at the expense of using up a bunch of
> PIC pins.
>
> OUT o--+-/\/\/\/\----o MSB
>        |    2R
>        \
>        /
>        \ R
>        /
>        |
>        +-/\/\/\/\----o Middle bit
>        |    2R
>        \
>        / R
>        \
>        /
>        +-/\/\/\/\----o LSB
>        |    2R
>        \
>        /
>        \ 2R
>        /
>        |
>       ---
>       ///
>
> > -----Original Message-----
> > From: Andrew Kelley [mailto:kelleyat@JUNO.COM]
> > Sent: Friday, March 17, 2000 12:35 PM
> > To: PICLIST@MITVMA.MIT.EDU
> > Subject: Re: D/A with an f84
> >
> >
> > It is not critical at all.  I just want to be able to control
> > the speed
> > of a motor like slow medium or fast.
> >
> >
> > Andrew
> >
> > On Fri, 17 Mar 2000 12:06:17 -0600 Scott Dattalo
> <scott@DATTALO.COM>
> > writes:
> > > On Fri, 17 Mar 2000, Andrew Kelley wrote:
> > >
> > > > Does anyone know how to make an analog voltage/signal with an
> f84?
> > >
> > >
> > > This will make a DC signal on the lsb of porta:
> > >
> > >   bsf  porta,0
> > >
> > > as will this:
> > >
> > >   bcf  porta,0
> > >
> > > A "strategic" combination of these can be concatenated to
> produce a
> > > waveform
> > > whose average value can be used to produce an analog voltage.
> > >
> > > Or as we prefer to say on the list every few days: PWM. Here's a
> > > software pwm
> > > that works on the f84:
> > >
> > > http://www.dattalo.com/technical/software/pic/pwm256.txt
> > >
> > >
> > > But it's probably overkill for what you want to do. But, OTOH, I
> > > have no idea
> > > what you exactly want...
> >
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