Humidity sensor HIH-5030 false readings on PCB

Question

I've created a circuit (schematic below) to measure humidity with a HIH-5030 according to its datasheet and an ATtiny85 and tested it on a breadboard, the readings are very accurate.

Soldered on a small PCB (below), the readings drop within about 5 minutes after connecting it from correct 50% relative humidity to about 30% and I am trying to figure out why.

• I've measured the output voltage of the sensor to verify that AD conversion is correct, which it is
• The board warms up slightly but if I warm it up even more while disconnected and then connect it, the behaviour is the same
• The sensor is installed close to the controller but even in power down sleep mode the behaviour is the same so I suppose interference by the controller can't be the issue
• Below the sensor there is a ground plane and some tracks, can this cause issues because this is a capacitive sensor?
• When the sensor is connected to the board with wires about 3 cm long, the readings are correct

I suppose I've done some real beginners mistake in my PCB design? For the "minimum load" resistor R7 I have chosen 80k instead of 65k because the sensor is operated with 5V instead of 3.3V.

Answer 1

Oddly the datasheet is doesn’t mention a maximum supply voltage (or current), but suggests a 3 or 3.3V supply so I suspect it’s out of spec on 5V. Adding a 3.3 supply would likely help, otherwise, depending on the current it draws, you might be able to power the device from an IO pin if you can spare one, and power it up only when you need to make a measurement. I don’t know if that would help but may be worth a try.

Answer 2

Eventually I desoldered all parts besides the controller from the PCB and got correct measurements from the humidity sensor. So probably the sensor does not like to be so close to the controller. What puzzles me however is that putting the controller in power down sleep mode did not make any (noticeable) difference.

The issue can be reproduced in the test circuit on the breadboard. If the sensor is near the controller, the humidity reading slowly drops at least 5% to more than 10% below the actual value. Even the temperature measurement seems to be affected; it is 1-2 °C too high.

With the humidity sensor at least 3 cm away from the controller the readings are super accurate and stable.