An example application of a ADAS sensor monitoring system using the HDC3120 is shown below. The purpose of the humidity sensor to sense the humidity withing the cabin of the vehicle. In this application, placing the humidity sensor on the same board as the main system controller is not possible. Wires must be used to connect the sensor board to the microcontroller board.

Figure 8-1 HDC3120 Connection to ADC

In Figure 8-2, the cable capacitance and resistance are drawn to highlight that the HDC3120 is capable of driving capacitive loads up to 47nF. When connecting HDC3120 to an ADC, using an ADC with an external reference voltage is recommended, which must be tied to the same voltage supplying the HDC3120. The HDC3120 RH and Temperature outputs are ratiometric to the VDD of the device, so if there is a spike or noise on the VDD line, that noise can be seen on the outputs as well. By having the ADC reference voltage match the HDC3120 VDD, noise events occurs for both ICs, which allows the ADC to match and eliminate the noise. On the analog outputs, implementing an RC filter near the ADC is recommended. The capacitor simultaneously helps to filter noise in the RC filter. The capacitor also serves as a charge repository for the ADC during sampling to prevent sampling glitches.

The RC filter values featured in the above circuit example are suggestions, and can be modified to meet the desired cutoff frequency. The output DACs for RH and Temp are strong outputs, capable of driving across long cables without the need for an external buffer amplifier. Take care to verify that the total load capacitance remains less than 3µF for both cabling and filter capacitance. The R-C values selected also need to take into account what the chosen ADC can support and the desired sampling frequency.

Figure 8-2 HDC3120 Connection to Comparator

In the above figure, the HDC3120 outputs are connected to a dual channel comparator to build a local temperature and humidity control system. The analog outputs of the HDC3120 are compared against a resistor divider reference. In this example, the resistor divider input for the RH output is set to a 90%RH threshold, and a 50°C threshold for the temperature output. For a 3.3V V_DD, the RH comparator voltage is set to 2.706V, and the Temp comparator voltage is set to 1.763V. So the comparators trips high if the RH exceeds 90%, or the temperature exceeds 50°C.

This circuit can be used to control fans in a system, or other system logic to protect against high temperature or high humidity conditions. The 51kΩ positive feedback resistors are needed to create comparator hysteresis. This is necessary to prevent the comparator "bouncing" when the HDC3120 output is close to the resistor divider control voltage.

The hysteresis levels can be set by changing the feedback resistors on each amplifier. In this example, the %RH output channel has a hysteresis of 5%. The hysteresis levels of a comparator can be set according to the instructions in Comparator With and Without Hysteresis Circuit. For a 5%RH hysteresis, the comparator activates at 90%RH, and does not clear the output low until the RH of the HDC3120 drops below 85%RH. This is illustrated with the following figure:

Figure 8-3 Comparator Circuit RH Hysteresis