Electronic – Unexpected Sensor Output Voltage Rise


I'm using a Shinyei PPD-60PV particulate sensor in a product and I've noticed something very odd in testing, and I don't know how to explain it. It's connected to a WildFire board by way of cabling to an interface adapter board. The WildFire is powered by 5V through its USB port. The PPD-60PV has two 5V/GND connections made through the interface adapter board, and an analog output that is wired to the A7 ADC input of the WildFire by way of the interface adapter board.

My product supports two fundamental modes of operation: (1) Wi-Fi connected, and (2) Offline. What I've discovered is that in Wi-Fi mode, the analog output of the PPD-60PV sensor appears to rise by about a volt. What I discovered (and painstakingly narrowed the symptom down to) was that this voltage rise happens somewhat gradually (over several seconds), only after the ESP8266 connects to a Wi-Fi network. It also recovers to a normal baseline value gradually (over a similar period of time) upon resetting the ESP8266 (and hence disconnecting it from the Wi-Fi network).

Further diagnostic experiments reveal that this voltage rise on the analog output of the sensor occurs even if I disconnect the analog output from the WildFire altogether leaving the 5V / GND connections in place and probe it with an oscilloscope.

Also if I have two assemblies plugged into the same power source, with one of them in Wi-Fi mode, and one of them in offline mode, the offline mode unit exhibits the voltage rise phenomenon. The rise is certainly there, and also noteworthy that it's to a lesser degree than when the unit itself is in Wi-Fi mode, e.g. 600mV – 700mV.

An offline unit connected to an isolated power source (e.g. a battery pack) does not experience the voltage rise, e.g. despite physical proximity to a Wi-Fi connected unit.

I wondered if maybe it was a ground path resistance issue, but everything is pretty short lengths here, and I measured the resistance from both sensor ground connections back to the WildFire ground at 0.2 ohms each, and I measured the total current of the system at about 300mA (displayed on a the LCD of a conventional benchtop 5V power supply). That certainly doesn't account for a 1V rise as far as my reasoning goes.

My understanding was that the PPD-60PV analog output is a low-impedance buffered output, but that's not entirely clear from the datasheet. I'm kind of stuck / perplexed at the moment, and I'm not sure what to do next.

So, on to my forlorn question. What could be the root cause of what I'm observing here? What advice do you have as to what I might do next to drive this issue to ground, as it were?

Best Answer

If your system uses a photodiode for detection, it is attached to a relatively high gain amplifier/integrator, and strong electromagnetic fields (wifi) can result in induced AC voltages that are rectified by the diode junction and appear on the output. If this is your problem, you can solve it by increasing the distance from your wifi transmitter or additional shielding around the photodiode. I'm betting your sensor has some shielding around the photodiode already.

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