AVR-Chat

Why? This isn't a divider circuit. I think I see what you're getting at, but 
I'm not sure I'm buying it. 

-Micah 

On Monday 09 January 2006 1:57 pm, Jim Wagner wrote:
>  Sorry, but wrong about the tolerance!
>
>  Each successive leg has half the tolerance contribution of
>  the leg just above it in the ladder. With 1% resistors, the
>  over-all tolerance will be in the 3-4% range.
>
>  What may be more critical is monotonicity, especially the
>  step going from 0x7f to 0x80 (in an 8-bit) DAC.
>
>  An R-2R DAC also relies on the on-resistance of the FETs in
>  the logic output circuit.
>
>  Jim
>
>
>  On Mon, 09 Jan 2006 15:59:30 -0600
>
>  Thomas Keller <tkeller1@neb.rr.com> wrote:
>  > Anurag Chugh wrote:
>  > > why only PWM?.. why not use an R2R ladder instead with
>  >
>  > precision
>  >
>  > > resistors...
>  > > i know they would hog a whole PORT but arent they
>  >
>  > easier to implement ??
>  >
>  > > and they can do DC output values too... a simple opam
>  >
>  > in front
>  >
>  > > (voltage follower) will boost current output too...
>  > > i am sure most people dont use R2R... but please tell
>  >
>  > me why?? i know
>  >
>  > > there must be a reason...
>  >
>  > Anarag:
>  >
>  >   I can speak only for myself.   Consider an R-2R ladder
>  > for 8 bit DAC: 
>  > it contains 16 resistance elements.  Now, let us assume
>  > that each of these resistance elements is an indiivdual
>  > 1% tolerance resistor.  Total overall tolerance of the
>  > R2R ladder is now 16% (not very good, is it?)   1%
>  > resistors cost between $0.15 and $0.25, depending on
>  > where you purchase them and how many you purchase 9unless
>  > you're puschasing thousands, of course).  Somewhere
>  > between $2.40 and $4.00 for A +/- 16% dac?  *HORRIBLE*
>  > Assuming you can obtain (at any price) +/- .1% resistors
>  > (at any price), you still have only +/- 1.6% tolerance,
>  > at a trmendously higher price.
>  >
>  >   Now, consider 8 bit PWM as implemented using an AVR.
>  >  Tolerance is
>  > going to be +/- 1/255 of Vcc.  Assuming a +/- 5%
>  > tolerance on your Vcc (which you *REALLY* ought to have,
>  > for correct AVR operation), this yields an "absolute"
>  > tolerance of  +/- 1.9%, at ZERO additional hardware cost
>  > (unless you need post filtering, in which case, a .25W,
>  > 5% resistor and a small cap cost what, maybe $0.25 in
>  > combination?).  Of course, there is the question of
>  > software devlopment costs, but 1) those are one time
>  > engineering costs, not repeating BOM (bill of materials)
>  > costs, and 2) let's face it, PWMing ADC is pretty
>  > straight forward, and takes very little time and effort
>  > to program, thanks to the foresight and engineering
>  > skills of the engineers at Atmel.
>  >
>  >   Granted, 8 bit PWWM isn;t going to yield great results
>  > for, say audio
>  > purposes, but then, neither is an 8 bit R2R ladder (in
>  > fact, pretty much exactly the same results, in terms of
>  > signal fidelity/distortion).
>  >
>  >   If I have gone on too long, my apologies.
>  >
>  > Tom
>
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