Overview

The SHT40 is Sensirion's 4th-generation industry-leading digital relative humidity and temperature sensor. Housed in a microscopic $1.5 \times 1.5\text{ mm}$ 4-pin DFN package, it delivers class-leading $\pm 1.8\%\ \text{RH}$ humidity accuracy and $\pm 0.2^\circ\text{C}$ temperature accuracy.

Operating across an extended supply range of $1.08\text{V}$ to $3.6\text{V}$ DC with a idle current of just $80\text{ nA}$, the SHT40 features an integrated variable-power internal heater ($200\text{ mW}$ max) for self-decontamination and condensation removal, 16-bit ADC output, and an $I^2C$ interface (0x44 default).

Quick reference

Pinout

Breakout module header & STEMMA QT / Qwiic connectors:

Specifications

$I^2C$ Commands & Conversion Formulas

• Measure High Precision: 0xFD (6.9 ms conversion).
• Measure Medium Precision: 0xF6 (4.5 ms conversion).
• Measure Low Precision: 0xE0 (1.7 ms conversion).
• Activate Heater ($200\text{ mW}$, 1 sec): 0x39

Temperature Calculation ($^\circ\text{C}$)

$\text{Temperature } (^\circ\text{C}) = -45.0 + 175.0 \times \left( \frac{\text{Raw 16-bit Temp Register}}{65535} \right)$

Relative Humidity Calculation ($\%\text{ RH}$)

$\text{Relative Humidity } (\%\text{ RH}) = -6.0 + 125.0 \times \left( \frac{\text{Raw 16-bit RH Register}}{65535} \right)$

(Clamp RH output to $0\%\dots 100\%\text{ RH}$).

Wiring

Example (Adafruit_SHT4x Library)

#include <Wire.h>
#include "Adafruit_SHT4x.h"

Adafruit_SHT4x sht4 = Adafruit_SHT4x();

void setup() {
  Serial.begin(115200);
  Serial.println("Adafruit SHT40 Humidity & Temp Test");

  if (!sht4.begin()) {
    Serial.println("Couldn't find SHT40 sensor! Check I2C wiring.");
    while (1);
  }

  sht4.setPrecision(SHT4X_HIGH_PRECISION);
  sht4.setHeater(SHT4X_NO_HEATER);
}

void loop() {
  sensors_event_t humidity, temp;
  sht4.getEvent(&humidity, &temp);

  Serial.print("Temperature: "); Serial.print(temp.temperature); Serial.print(" °C | ");
  Serial.print("Humidity: "); Serial.print(humidity.relative_humidity); Serial.println(" % RH");

  delay(1000);
}

Common mistakes

• Leaving internal heater running continuously: Running the $200\text{ mW}$ heater continuously causes heavy sensor self-heating ($+15^\circ\text{C}\dots +30^\circ\text{C}$ temperature bias). Use heater pulses for short 1-second intervals only.
• Confusing conversion formulas with SHT31/SHT21: The SHT40 uses updated temperature formula offsets ($-45.0 + 175.0 \times \dots$) compared to SHT31 ($-45.0 + 175.0$) and SHT21 ($-46.85 + 175.72$).

Notes

• Sensirion SHT40 Family: SHT40 ($\pm 1.8\%\ \text{RH}$ standard grade), SHT41 ($\pm 1.5\%\ \text{RH}$ high accuracy), SHT45 ($\pm 1.0\%\ \text{RH}$ ultra-high precision).