Why do we need a load resistor connected between the output pin and ground? reference to a detailed description of the sensor
From the link:
Since the output of the sensor is analog voltage it can be interfaced directly to any micro-controller which has internal ADC. It can also be interfaced directly to Arduino.
It acts as a pull-down resistor that drives the input to micro-controller to a known logic
0, in case your sensor's output floats for some reasons.
I have no experience with the HIH 4000 (or other humidity sensors,) but I thought I'd have a look and see what I could find.
From what I have found, the datasheet isn't telling you that you need to put an 80k resistor from the output to ground.
What the datasheet trying to say is that the load impedance connected to its output must be higher than 80k ohms.
This thread on the Velleman forums refers to an ADC module with a 20k ohm input imopedance. An HIH 4000 connected to that module (the Velleman K8061) would consistently show a wrong output (as measured with a voltmeter) when connected to the K8061, but would show a proper voltage when disconnected from the K8061. The recommended solution was to remove the 20k resistor in parallel with the input on the K8061. Alternatively, an op amp could be used to buffer the HIH4000 output.
Something similar happened with an ADC for a Raspberry Pi. Again, the solution was to increase the input impedance of the ADC module.
Summary: The 80k is the minimum input impedance of the ADC or buffer amplifier you connect to the sensor.
Given that you have a high impedance output coupled to a high impedance input, you have a very good chance of picking up stay noise over the wires from the sensor to the ADC. The solution to that is to use the minimum load allowed in all cases (80k in parallel with the ADC input) or to install a buffer right at the sensor output so that you have a lower impedance line going to the ADC.