[PSUBS-MAILIST] Electronic help request

[Jon Wallace <jonw@psubs.org>]

Hello folks,

I'm looking for some electronic debugging help.  I'm using an Arduino 
UNO board to interface some sensors with an LCD display.  I currently 
have two LM34 temperature sensors and a 0-1000psia sensor attached to 
the board.  The LM34 sensors are working fine but I'm having a problem 
with what I call "drift" with the ADC readings from the pressure 
sensor.  I've created a video diary to help explain the problem and also 
show the hardware I'm using at http://www.youtube.com/watch?v=uDpafXob2ko

Non-electronic types might be interested in viewing the video as well 
just to see the project.

Arduino UNO
16 digital outputs
10bit ADC (1024 steps)
6 analog inputs
5vdc output
3.3vdc output
ground

PSIA Pressure Sensor
8-30vdc input voltage
1-5vdc output voltage
1v output at 0psi
Data-Sheet at the following web address:
http://www.ssitechnologies.com/PDF/Application%20Notes/PS_AN2_MediaSensor_P51_Product_Overview.pdf

The UNO board runs at 5vdc and I have powered the LM34 sensors directly 
from the board.  However the pressure sensor requires 8-30vdc so I have 
to power that with a separate supply and have been using a laptop AC/DC 
adapter that is putting out (steadily) 18.72vdc.  Note that the 
following description applies to the pressure sensor sitting on a 
workbench with steady atmospheric pressure.

I connected the output from the pressure sensor to an analog input of 
the board and observed a pretty large drift of about 10 steps.  I 
measured the output of the pressure sensor with a volt-meter and it was 
rock-steady at 1.050 volts the entire time.  I did some internet 
searching and came across this document 
http://www.maxim-ic.com/app-notes/index.mvp/id/3775 which in Figure 2 
seems to suggest that connecting ground from the sensor to the ADC.  So 
I ran a wire from the sensor ground to the ground terminal on the UNO 
board which smoothed out the ADC drift quite a bit dropping it to about 
3 steps.  The ADC will occasionally still produce a reading that is 8-10 
steps high but that is the exception now.

Given that the pressure sensor is putting out a steady and unchanging 
1.05vdc I cannot account for the ADC drift I'm seeing.  I also do not 
think the problem is with the ADC on the board because the LM34 sensors 
are being read steadily with no step drift at all.

Is the drift caused by noise of some kind?  Is it the external power 
supply?  Something else?

Any help appreciated.

Jon




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