Re: MMA3201 accelerometer experience?

dave.harper wrote: 
David Schultz wrote:


I haven't used that part but a quick look at the data sheet shows that
this noise level is out of specification for that part. The stated noise
is 2.8mV RMS maximum.

Did you include the recommended RC output filter? The noise
specification assumes a 1K resistor and 0.01uF output filter. (See
footnote 1 in the data sheet.)



Dave, thanks for the reply.  Yes, I forgot to mention the filter in my
first post (sorry), but I've got a low pass filter with a 1k resistor
and .25uF capacitor.


OK, that should be fine. Except that I just noticed another note in the
data sheet that specifies a maximum of 100 pF on the output to maintain
amplifier stability. This is inconsistent with the recommendation of
10,000 pF for filtering. This might require a call to an application
engineer to resolve. You might try dropping down to a 10nF part to see what happens.


Or perhaps the noise is coming from somewhere else. Check your grounding
scheme carefully.



I imagine somewhere in the above statement is the solution to my
problem.  Can you think of any grounding issues that might cause the
high noise?  (also, my power supply is 5V regulated with a big
smoothing cap)


Grounding is too big a subject to even attempt to cover here.
Start by looking at an Analog devices data sheet for a quick start. They
usually have some information on grounding.

What sort of power supply is it? If it is linear then you are likely OK. Note that some LDO linear regulators are very particular about the output capacitor.



As for the 8-LCC package not being user friendly, I didn't have any
trouble hand soldering one.



Well, I should say "user-friendly" to most people.  I've hand-soldered
their ADXL202 before, but it took about half and hour and I was
constantly worried about overheating the part.  The SOIC packages are a
little easier.

Dave


It has been a while since I soldered an ADXL210 but I know it didn't take half an hour. I also know that I am not particularly skilled at soldering. I am not bad but I am not good either.

As the data sheet provides no guidance on the difference between GND and
Vss or Vdd and AVdd, nor how to hook them up, treat them as equivalent
pins. Yes you should hook both GND and Vss to your power supply return.

Amazingly, in the two circuits shown, one shows AVdd tied to Vdd and the other doesn't show AVdd at all.

You sometimes see separate power supply pins on microcontrollers with
built in ADC's. These frequently have separate power supply connections
which isolates the digital power supply noise (at least a little bit)
from the sensitive analog circuits.

If this part had a built in ADC with an I2C interface or some such I
could see a reason for separate power supply connections. But as it is I
see none and the data sheet provides none. I really like the Analog
Devices data sheets as they provides lots of very useful information.
This data sheet on the other  hand doesn't provide much.


--
David W. Schultz
http://home.earthlink.net/~david.schultz/

"Ahh Fuji! Why is it always monkeys? Why couldn't I be attacked by
crazed super models?" - Ron Stoppable

.

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