SNAS758A February   2025  – June 2025 HDC3120

PRODUCTION DATA  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Device Comparison
  6. 5 Pin Configuration and Functions
  7. 6 Specifications
    1. 6.1 Absolute Maximum Ratings
    2. 6.2 ESD Ratings
    3. 6.3 Recommended Operating Conditions
    4. 6.4 Thermal Information
    5. 6.5 Electrical Characteristics
    6. 6.6 Typical Characteristics
  8. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1 Device Power-Up
      2. 7.3.2 Device Disable and Enable
      3. 7.3.3 Conversion of the Signal Output
        1. 7.3.3.1 Relative Humidity (RH%) Measurement
        2. 7.3.3.2 Temperature Measurement
      4. 7.3.4 NIST Traceability and Unique ID
      5. 7.3.5 Output Short Circuit Protection
    4. 7.4 Device Functional Modes
      1. 7.4.1 On-Chip Heater
        1. 7.4.1.1 Operating Principle
          1. 7.4.1.1.1 Heater Configuration Example
        2. 7.4.1.2 Heater Electrical Behavior
        3. 7.4.1.3 Heater Temperature Increase
        4. 7.4.1.4 Heater Usage Guidelines
  9. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Application
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Procedure
    3. 8.3 Power Supply Recommendations
    4. 8.4 Rehydration Recommendations
    5. 8.5 Layout
      1. 8.5.1 Layout Guidelines
      2. 8.5.2 Layout Example
    6. 8.6 Storage and PCB Assembly
      1. 8.6.1 Storage and Handling
      2. 8.6.2 Product Storage
      3. 8.6.3 PCB Assembly Flow
      4. 8.6.4 Rework Consideration
      5. 8.6.5 Sensitivity to Chemicals and Vapors
      6. 8.6.6 Exposure to High Temperature and High Humidity Conditions
      7. 8.6.7 Recovering Sensor Performance: Bake and Rehydration Procedure
  10. 9 Device and Documentation Support
    1. 9.1 Documentation Support
      1. 9.1.1 Related Documentation
    2. 9.2 Receiving Notification of Documentation Updates
    3. 9.3 Support Resources
    4. 9.4 Trademarks
    5. 9.5 Electrostatic Discharge Caution
    6. 9.6 Glossary
  11. 10Revision History
  12. 11Mechanical, Packaging, and Orderable Information

8.2.2 Detailed Design Procedure

HDC3120 HDC3120 Connecting RH Analog Output to ADC InputFigure 8-4 HDC3120 Connecting RH Analog Output to ADC Input

To best use the HDC3120, keep in mind that the RH and Temperature are analog outputs from a pair of buffered DACs. Hence, RH and Temperature have an LSB. Since the output of the HDC3120 is ratiometric to the VDD supplied, LSB size varies with different VDD levels. LSB size is calculated by the following formula:

Equation 8. 1 LSB = FSR2n-1 = 0.8×VDD4095

The output DACs are 12 bits each, and the full-scale range (FSR) is 80% of VDD (10% of VDD is minimum output, 90% of VDD is maximum output). For a nominal VDD voltage of 3.3V, 1 LSB is 644.7μV. If a smaller LSB is required, VDD needs to be lowered since as VDD decreases, LSB size decreases proportionally.

To get started with the HDC3120, first identify the desired sensing conditions and the supply voltage and how the user wants to receive the analog output. For example, connecting to an ADC so that the analog outputs can be received by a microcontroller is a common use-case. In this example situation, the user needs to sense RH from 5% to 95%, in a temperature range of 10°C to 50°C, with a VDD of 5V. A 5V voltage supply creates a DAC LSB of 977μV for both temperature and RH outputs.

Next, once the LSB is identified, the user must choose the ADC. In this case, the ADC must have a full scale range (FSR) that can satisfy between 10% and 90% of VDD, so in this case, the FSR must satisfy a range of 500mV to 4.5V at a minimum. The HDC3120 has an automatic conversion rate of 4 measurements per second after start up, so the ADC must sample at least 4 times per second, but very fast sampling is not necessary. When choosing the ADC FSR, verify that the LSB size of the ADC is less than the LSB of the HDC3120. In this case, the LSBADC must be less than 977μV. Using the HDC3120 VDD for the ADC reference voltage is recommended, so that noise on the sensor is matched on the ADC. This can impact the chosen ADC as well. If the desired VDD is 5V, then the ADC needs to be able to accept a +5V reference voltage. So the ADC for this scenario needs a maximum LSB of 977μV and able to handle inputs up to 4.5V with a reference voltage of 5V.

To build on this, the ADS1115 can be used to interface with HDC3120. The ADS1115 is a 16-bit ADC with customizable FSR. To satisfy the necessary voltage output range of the HDC3120, the largest FSR range needs to be chosen, ±6.144V. This option has an ADC LSB of 187.5μV. This LSB is still much less than the 977μV needed for 5V power supply, so this ADC choice satisfies the design needs. A 16-bit ADC is typically a good choice to pair with HDC3120 since the ADC LSBs is much smaller than the HDC3120 LSB.

After the ADC is chosen, the user must decide how to connect the HDC3120 to the ADC. The HDC3120 DAC output has an internal buffer, so there is no need for an external buffer amplifier to drive long cable lengths or lower the output impedance. Adding a capacitor to the input of an ADC to act as a charge bucket to prevent noise from ADC sampling and a resistor for stability and filtering is typically unnecessary. The HDC3120 can drive up to 3µF capacitive load, and because the HDC3120 has a 4Hz conversion rate, an RC circuit does not cause a signal slowdown issue, even with a large capacitor value.