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Help with basic design


Robin Cull

Jan 1, 1970
Hi all,

I'm pretty new to this electronics game having come from a software
background. I am trying to design a basic interface board between the
Analog Devices ADXL202E accelerometer and the LabJack U12 USB DAQ.

I have a basic design sketched out on paper but I would like some help
with some of the details.

Firstly, the ADXL202E is an 8-pin LCC surface mount component
(5mm*5mm). I have soldered surface mount before but it's a pain and
as I only have two parts (and they're expensive to replace) I don't
want to solder them directly unless I have to. Also using SMD makes
the use of other through-hole components more difficult (two-layer
board). I have been searching, so far in vain, for a socket type
adaptor that will clamp onto the LCC package and have through-hole
pins. I've seen these things for larger (more pins) parts but not one
this small. Any tips?

Second problem is capacitor choice. The ADXL202E analogue output is
filtered by a capacitor. I've figured out that I need to use 0.47uF
filtering capacitors (this will yield a bandwidth of 12.5Hz so I can
sample on the DAQ at 25Hz). I am guessing that as these are being
used for timing purposes they need to be fairly precise so normal
electrolytics of +/-20% won't do. What kind of capacitors should I be
using for this purpose?

Thirdly, working on the assumption that I may not be able to get an
adaptor for the LCC package I've created a part in Eagle so I can make
a schematic. I'm having a little trouble with the layout. Initially
I created a symbol as a rectangle with 8 pin-outs on the right hand
side but doing it this way makes it really hard to layout the
schematic without making a real birds nest out of it. Any
recommendations on how I should tackle this?

Finally, and this question is more specific to the ADXL202E and the
Labjack, I plan to use the +5V and common 0V lines from the Labjack to
power the board, the part maxes out at 5.25V and I've placed a 100Ohm
resistor in series with Vdd to smooth the power. I assume this is ok,
looks fine from the data sheet. The signals for the analogue sampling
are coming in series after the filter capacitors. I think I've got to
go to ground after this but I am not sure how. Also the datasheet for
the ADXL202E mentions that it maybe necessary to op-amp buffer these
signals to get sufficient power out of them to read, I am not sure
whether I need to do this or not. Could anybody clarify these points
for me?

Thanks a bunch for putting up with these questions, I know they are a
little varied.