Using EVAL-CN0409-ARDZ with LED2 and PD1 mounted off board, I get 'unstable' readings, while LED1 and PD2 mounted on board (their factory fitted positions) provide stable readings.

I am using EVAL-CN0409-ARDZ to prototype a turbidity sensor to measure water turbidity accurately over the range 1NTU - 40NTU remotely with the sensor mounted on site in the site PVC plumbing.. To test the concept, I removed LED2 and PD1 from the EVAL board, and mounted these components into a piece of PVC  pipe stub (~30mm dia) and connected the components to the EVAL board using twisted pair wire (~75mm/3" long for each of LED2 and PD1). I have fitted small plastic lenses in front of the LED and PD to provide a barrier between the LED/PD components and the water in the pipe. I have glued the LED and PD into place in the pipe fitting behind the lens barriers, taking care to ensure that the glue is kept away from the face of the LED and PD.. When I perform measurements on this arrangement, with no water yet in the pipe, I am unable to get stable readings for LED2/PD1 combination, while LED1/PD2 combination, still mounted in their original positions on the board provide stable readings. By unstable reading, I mean there is a steady drift over several hours. For example, when I first started testing today, initial reading from ADC was 9929. Over the space of a few minutes this increased steadily to 10022, and then started decreasing so that after 1hr the reading was 9623 and still decreasing. I am maintaining benign measurement conditions during this period (testing is on my desk), yet something is affecting readings. Readings are very sensitive to movement of the connecting wires but I am not allowing any movement during my testing. Is it possible that the glue is interfering in some way with the PD charge transfer? Any suggestions as to what is causing my problems would be appreciated.

Top Replies

    •  Analog Employees 
    Jul 27, 2020 +1 suggested

    Hi ,

    Thank you for your interest in EVAL-CN0409-ARDZ (CN0409).

    Based on your description above, you have removed the LED and PD from the circuit and attached it to the PVC plumbing. The CN0409…