The HDC3120 is Texas Instruments’ first humidity sensor with an analog ratiometric output. The HDC3120 provides continuous voltage signals corresponding to temperature and relative humidity, making this well-designed for low-noise, analog front-end systems.

Unlike digital sensors which require communication protocols, the HDC3120 allows direct access to sensor outputs without writing I2C commands or configuring registers. This simplifies integration in systems where an analog-to-digital converter (ADC) is already available.

To interpret the output signals, users can apply the appropriate conversion equations (provided below) to translate voltage levels into temperature (°C) and relative humidity (%RH).

An important characteristic of the HDC3120 is the ratiometric behavior. The output voltages for temperature and humidity scale linearly with the device’s supply voltage (VDD), which also serves as the internal reference. This design provides immunity from noise and drift in the power supply, enabling reliable measurements. This relationship is illustrated in the following graphs for both temperature and humidity output curves.

Understanding the ratiometric nature of the HDC3120 output is critical – especially when interfacing with an ADC since the sensor’s output voltage directly scales with the supply voltage (VDD). If the chosen ADC also uses its supply as a reference, measurement consistency is preserved in the event noise or drift appears within the voltage supply. This alignment eliminates gain mismatch issues and helps maintain consistent sensor readings.

How to Choose an ADC for the HDC3120:

Table 3-1 provides some ADCs for the HDC3120:

When selecting an ADC for the HDC3120, the following process must be considered.

1. Determine the ADC’s least significant bit (LSB)
2. Calculate the HDC3120 temperature LSB
3. Calculate the HDC3120’s humidity LSB
4. Compare LSB Values

Choose an ADC LSB that is smaller than the HDC3120’s temperature LSB. The same consideration can be used for humidity LSB, however, since the HDC3120's temperature sensor has a higher accuracy, it is used to determine the minimum LSB required for an ADC.

Additional information on pairing the HDC3120 with an ADC can be found in section 8.2.2 in the HDC3120 datasheet.

Example Scenario:

For this example, the BOOSTXL-ADS7142-Q1 EVM was paired with the HDC3120EVM. Both devices were powered using the same 3.3V supply. From here the LSB’s of both devices can be compared using the steps above:

1. Determine the ADC’s least significant bit (LSB) using the following equation:
   Equation 5. $\frac{FSR}{Resolution}=LS{B}_{ADC}$
   1. Since the operating voltage is 3.3V and the ADC features a 12-bit resolution, the LSB is:
   Equation 6. $\frac{3.3V}{2^{12}}=0.805mV$
2. Calculate the HDC3120's Temperature LSB using the following equation:
   Equation 7. $V_{TEMP}=V_{DD}\times \left[T\left(°C\right)\times 4.571\frac{mV}{°C}\right]$
   Equation 8. $Tem{p}_{LSB}=3.3V\times \left[\right(0.1)\times 4.571\frac{mV}{℃}] = 1.508 mV/°C$
   Important:

   The value for temperature (T(°C)) is 0.1 to reflect the HDC3120's typical temperature sensing accuracy.

3. Calculate the HDC3120's Humidity LSB using Equation 9:
   Equation 9. $V_{RH}=V_{DD}\times \left[\left(\%RH\right)\times 8\frac{mV}{\%RH}\right]$
   Equation 10. $R{H}_{LSB}=3.3V\times \left[\left(1.0\right)\times 8\frac{mV}{\%RH}\right]=26.4 mV/\%RH$
   Important:

   The value for humidity (%RH) is 1.0 to reflect the HDC3120's typical humidity sensing accuracy.

4. Compare LSB Values
   1. Since the Temperature LSB size of the HDC3120 is 1.508mV at 3.3V, and the ADS7142 has an LSB of 0.805mV; this means users retains measurement precision of at least 1°C. If a higher degree of precision is desired, such as 0.5°C, a higher resolution ADC such as the ADS7066 should be used.

In this example setup, the HDC3120EVM was connected to the BOOSTXL-ADS7142-Q1 EVM. Both devices shared the same VDD rail. The setup procedure is as follows:

1. Connect the TEMP output from the HDC3120EVM to the AIN0 input on the BOOSTXL-ADS7142-Q1 EVM BoosterPack? using a jumper wire as illustrated in Figure 3-4.
   Figure 3-4 HDC3120EVM Outputs
   Figure 3-5 BOOSTXL-ADS7142-Q1 EVM BoosterPack? Pin Connection Location
2. Connect RH output from the HDC3120EVM to AIN1 on the BOOSTXL-ADS7142-Q1 EVM BoosterPack?.
3. Launch the ADS7142EVM GUI and navigate to the homepage.
4. Click the gear icon to access the configuration menu, then under Conversion Modes, select I2C Command Mode.
5. Select Auto SEQ Mode from the Operating Mode drop-down menu and click the red SET button.
6. Once configured, click Start Sequence to begin capturing measurements in 12-bit decimal format.

To interpret the results, users can apply the HDC3120’s conversion equations. Equation 11 and Equation 12 illustrate the reinterpretation of the ratios from the HDC3120's conversion equations. Equation 13 provides additional definitions for the variables.

Figure 3-6 plots temperature and humidity measurements of the HDC3120. Channel 0 (top graph) represents the temperature reading, while Channel 1 (bottom graph) represents the humidity reading. The latest data is provided next to "Latest Data" for each channel. The curves in the graph represent approximately 3-second intervals where HDC3120 heater was enabled for three seconds before being disabled via the heater enable switch on the HDC3120EVM. Therefore, a peak is observed for temperature each time the heater was enabled, while a trough (opposite peak) was observed for humidity as the device dries from heating (this is normal behavior within humidity sensors with integrated heating elements).

The calculations below utilize the most recently collected data point for temperature and humidity.

Temperature:

ADC Temperature Output (Channel 0): 1725

HDC3120 to ADS7142 Temperature Conversion Equations:

Using Equation 14, the converted temperature result is: 25.2°C

Humidity:

ADC Humidity Output (Channel 1): 1833

HDC3120 to ADS7142 Humidity Conversion Equation:

Using Equation 16 the converted humidity result is: 43.4 %RH