AVR-Chat

At 12:48 PM 7/9/2006 -0500, Thomas J Keller wrote:
>Robert Adsett wrote:
> >
> > > At 12:33 AM 7/9/06 -0500, Thomas J Keller wrote:
> > >> Best bet is to diode protect (for polarity) the input to an
> > >> opto-isoltor on each GPIO. This will protect you against
> > >> static discharge, excessive positive or negative inout, etc, etc., at
> > >> very little cost. Cost of a single diode, and a $0.25 (US)
> > opto-isolator.
> > >
> > > A question, where do you get a good linear opto for $0.25?
> > Especially if
> > > you are not dealing with a low impedance signal source.
> >
>I don't know.  Since I referr34ed to GPIOs, I don;t see the relevance of
>the question.

Well the subject line is analog inputs.

>    As for linear opto-isolators, those might cost you $1 or $2 (US)
>each.  Most of them are
>pretty linear, and plenty fast enough for any analog signal you're
>likely to be digitizing with
>the onboard ADC of an AVR.

Fast enough but kind of pricey.  They also usually require a low impedance 
source, and/or  an isolated power supply (to drive the feedback chain to 
linearize the opt curve).

>    Nor, given that we're discussing the onboard peripherals of an AVR,
>are we looking at ultra-precision
>instrumentation.  Let's face it, with a 10 bit resolution, any
>non-linearities in an inexpensive linear opto-
>isolator are not likley to be very noticeable in the final results.

Well 10bits is < 0.1%, but a lot of signals aren't that good to begin 
with.  I do agree there is not much sense striving for greater accuracy 
than your sensors or input can provide.

Likewise, it doesn't make sense to spend multiple $ to protect an input 
with an opto if you can do it passively for considerably less.  Of course 
for one or two you could well save enough in design to make it worth using 
optos.  Also in at least some test equipment I'd use a fully isolated 
linear and conditioning module, expensive but I don't have to worry about 
the voltage levels and grounding of the DUT.

Robert









>And the conversion speeds of the onboard ADCs
>inVRs are sufficiently slow that even the least expensive of linear
>opto-isolators should have MORE than
>enough bandwidth to do the job.
>
>    There is no sense in paying for precision and accuracy which we canot
>use in our circuitry.  Sure, you *CAN*
>purchase linear opto-issolators that will accurately (wuithin, say, 1%
>linearity) handle analog signals of up to
>many, many Mhz (posssibly hundreds?).  Once you have so accurately and
>quickly so coupled the signals,
>to pass them into an AVR is a joke.  Scale your expectations to the
>capabilities of the processor you are
>working with.
>
>   I am an absolute fan of the AVR family (indeed, my "nick" on eFnet IRC
>is avrFreak), but I think I have a
>realistic view of what the AVR can and cannot do.  I am not likely to
>attempt to use an AVR to build a
>spectrum analyzer, or a video data stream encryption block.  On the
>other hand, I am not likely to use a much
>more expensive and more difficul to program DSP to control my AC
>induction motor speed, either.  Granted,
>I might achieve somewhat better precision of control, but in very few
>instances would that additional precision
>prove of any usefulness whatsoever.  So why pay the costs associated
>with achieving it? (monetary costs,
>as well as costs in complexity and time-to-market, etc.)
>
></rant mode>
>
>tom
>aka avrFreak on eFnet IRC
>Check out #avrFreaks chat channel on eFnet ISC
>
>
>
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>

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