Electronics Forum / Basics / May 2008

You can use this page to design the analog filter:

http://www.analog.com/Analog_Root/static/techSupport/designTools/interactiveTool
s/filter/filter.html

It allows you to set the gain to be whatever makes sense, and computes the
right component values.

However, if you are interfacing with a microcontroller, then you might want
to use a digital filter instead. You can easily average the output over
time, and save on the external components. You still need an RC filter to
eliminate high frequency noise, since a digital filter will alias
frequencies higher than 1/2 the sampling frequency into the baseband if you
don't filter them out. You can control the gain with a digital filter as
well. There are websites that will design any kind of digital filter for you
(i.e., a java app). They usually use floating point, though. For this
application, I've used a simple RC filter, along with an averaging loop like
this:

   forever {

       while (!sample_available()) {
           do_something_else();
       }

       average = (average << 3) - average + get_sample();
       average >>= 3;
       // do something with the average here
   }

This gives each sample an exponential decay.

If that simple filter doesn't work, there are lots of others to choose from.
The choice of filter will depend on the application.

Regards,
Bob Monsen

Hi, Dave.  Your noise problem will certainly be reduced (and probably
eliminated) with good layout practices and bypassing.  You could do
worse than starting out with a .01uF to .1uF ceramic cap in parallel
with a 22uF electrolytic right across the power pins of the sensor.

The 0-2.5V analog output means you only lose one bit of resolution
with your ADC.  Might not be worth setting up a filter with a gain of
two to bump it up.  If you still have residual noise after bypassing,
try a simple R-C filter to cut some of the higher frequency stuff:

(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

Chris

You might check the archives of comp.robotics.misc. I recall several
discussions about the amount of current that some (all?) of the
Sharp IR rangers consumed.

I do know from personal experience that an IR remote control unit
such as the Siemens SFH506 (admittedly a slightly different device)
can go from "flaky" to "fantastic" when a 100Ohm/100uF filter is
added to keep the device's voltage up.

Hope this helps...

Frank McKenney
--
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   endeavors to live at the expense of everybody else.
                      -- Frederic Bastiat, French Economist
--
Frank McKenney, McKenney Associates
Richmond, Virginia / (804) 320-4887
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