At 01:31 AM 1/05/98 -0400, you wrote:
>  I'm interested in using an Analog Devices ADXL05 accelerometer
>in a project with a PIC 16F84.  The accelerometer outputs a
>voltage in the 0-5v range.  At its default and most sensitive
>setting, the accelerometer outputs 200mV per g.  (It measures
>-5 to 5 g's.)  Although I don't know for sure (yet), the A/D converter
>may have to be able to detect voltage changes as small as 2-4mV.
>(Is this a problem?  How sensitive are most A/D converters?)
> Of course, on top of all this, I'm also looking for something relatively
>inexpensive.  :-)
>
>   I'm very new to microcontrollers and electronics in general, so
>I don't know much about A/D converters.  I know the theory behind
>what they do and how they work, but that's it.  I am trying to find
>an A/D converter to use for this project, with no success.  I was
>hoping that someone here might have used the ADXL05 with an A/D
>converter that they might recommend.
>
>  Or maybe someone could tell me what to look for in an A/D converter?
>I understand a couple of things regarding them; resolution in bits (I
>assume 8 bits would give me 0-255 (or 1-256?) as an output, which would
>work well with the F84) and to be sure they are microprocessor compatible,
>but there are things like the following taken from page 784 of the Analog
>Devices catalog which I have no idea what they mean:
>
>  Throughput Rate kSPS Max
>  Linearity Eror (LSB @ TA = +25 degrees C)
>  SHA BW (kHZ Typ.)
>  Reference Voltage Int/Ext
>
>   I've looked through past archives of this list, done many WWW
>searches, asked people more knowledgeable than I, but I haven't found
>what I'm looking for yet.  If anyone could give me some help, or point
>me to someplace where I could get this information, I would greatly
>appreciate it.
>
>  Thanks very much in advance!
>
>                                              -Jon-
>
>  .--- stei0302@cs.fredonia.edu -- http://www.cs.fredonia.edu/~stei0302/ ---.
>  | DoD# 1038, EAA# 518210, NMA# 117376, USUA# A46209, KotWitDoDFAQ, RP-SEL |
>  |     '96 Dodge Dakota v8 SLT Club Cab, '96 Kolb FireFly 447 (#FF019)     |
>  `-------------------------------------------------------------------------'
>   I do not speak for the SUNY College at Fredonia; any opinions are my own.
>
>

This project sounds like fun!.

I have gine this as a basic guide, note that there are many more factors to
consider, but this should get you going.
If out need to measure 2 to 4mV (0.01 G), then you will need a converter
that can (I am just assuming a simple 5V project here), give you the dynamic
range required of 5000/2 or 2500 counts -> 12bit ADC (5000/4096 -> each bit
is equal to around 1mV (You may need to know more about this, am I going too
fast?)

Through put, this basicaly is the rate or number of conversions per second
(Can also be the serial data rate (Thats if not // converter)! So you will
need to find out how often you need to measure the input.

Linearity, this is how close that the converter follows the input voltage
over supply voltage and temperature. LIn your case, if the converter is 12
bits, then + -1bit or less will do.

Reference votage. This is important, it will define the accuracy and RANGE
(Input voltage that can be measured for a given output count) of the converter.

As this is your first project, I can recommend either Linear Tech, or Maxim
converters. Both of these manufactures have good application circuits and
notes on how to use the devices (Including pit falls etc.) I do suggest that
you look for a device that is serial driven, has an internal reference and
has an inout range of 0 to 5V. Now I don't think that any of these types of
converters are what you could call cheep! If this is required then you will
have to look at a dual slope converter.

I hope that this will help

Dennis
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Dennis Plunkett: Embedded Hardware, Software design
NEC Australia DRMASS
Line Interface cards
TRX software
ISDN interface card
ph 03 9264-3867

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