At 09:32 PM 2/5/1998 +1100, you wrote:
>Has anyone experimented with the (Honeywell) Hycal Humidity sensor eg IH
>3602 - L
>
>I have the sensor working, but have 100mv or so of noise superimposed on
>the output signal. We are building a PIC based networked temp / humidity
>sensor system.
>
>Any guidance or good alternatives appreciated.
>
>Tim Modra: tim@modra.com.au
>
>Modra Technology: http://www.modra.com.au
>Custom Sensors and Embedded Control
>
>
Hi Tim,

Just a thought...

Although you could low-pass filter the output of the sensor to reduce the
noise, you can also take another approach, if your PIC is "idling around"
waiting to take a reading every few seconds - or whenever.  Instead of
using an op-amp to implement a low pass filter before the A/D conversion,
if the noise is random, you can implement a low-pass filter in software!
Simply take a large number of A/D samples in a given time period (say, for
the sake of arugment, in 5 seconds) and average them.  If the noise is
randomly distributed (in amplitude), then this will have the same effect as
low-pass filtering.  This works when the actual signal is a slowly varying
one w.r.t time, as is the case of the humidty sensor.

Another approach along the same lines:
If you are using an 8 bit A/D converter, you can take 256 consecutive
samples, accumulating the result in a "16 bit" register (i.e. in two
single-byte registers, one for the high byte of the sum and one for the low
byte of the sum).  After you accumulate 256 samples, simply discard the
low-byte register in the 16 bit sum.  The high-byte of the 16-bit
"register" will contain the averaged (i.e. low pass filetered) 8 bit A/D
value.

The same approach can be used to obtain 65536 averaged samples, using a "24
bit register" and so on.  The upper byte of the summation will always
contain the averaged 8-bit A/D value.

This approach is very easy to implement in software, since division is not
required, just addition!

Some food for thought....

Regards,

Gary Pepper
e-mail: gpepper@capitalnet.com
Ottawa, Ontario, Canada