A New High-Performance MEMS Relative Humidity and Temperature Sensor from IDT

IDT’s new high-performance MEMS relative humidity and temperature sensor requires no user calibration and comes in four different accuracy levels.

IDT recently announced their new HS300x family of MEMS high-performance relative humidity (RH) and temperature sensors. While all four devices in this family—the HS3001, HS3002, HS3003, and HS3004—share the same datasheet and use the same small package (a 3.0 × 2.41 × 0.8 mm DFN-style 6-pin LGA), the four ICs differ slightly in terms of the accuracy of their relative humidity and temperature measurements.

 

Figure 1. IDT’s HS300x relative humidity and temperature sensor. Image taken from the datasheet.

Reading I2C Data and Calculating the Measurements

Because the HS300x family of ICs has calibration and compensation logic integrated into the devices, no user calibration of the output data is necessary. Furthermore, because these ICs output their fully corrected data using standard I2C protocols, getting the measured data from the sensors is rather straightforward; the entire output consists of only four bytes of data. And after the data is acquired, calculating the corresponding relative humidity (in percent) and temperature (in degrees Celsius) is accomplished by means of the following two equations:

Humidity[%RH]=(Humidity[13:0]214−1))∗100

Temperature[∘C]=(Temperature[13:0]214−1))∗165−40

Although the HS300x sensors operate as slave devices on the I2C bus (supporting clock frequencies from 100 kHz to 400 kHz), only one HS300x IC can be connected directly to a single I2C bus because all the parts have the same fixed I2C address (0x44). So if you want to connect multiple sensors to a single I2C bus, you'll have to connect them indirectly by using an I2C multiplexer/switch, such as this one. It would have been handy if IDT had dedicated the unused pin (see figure below) as an optional I2C address input bit, which would allow two HS300x devices to be connected to a single I2C bus.

Figure 2. IDT’s HS300x application circuit. If the NC pin could be used for I2C address selection, two sensors could be directly connected to the same I2C bus. Image from the datasheet.

Accuracy Specs

The figure below shows the relative humidity accuracy variations between the HS3001, HS3002, HS3003, and HS3004, both at 25°C and as a function of temperature.
By the way, if you're interested in verifying the relative humidity accuracy of this family of ICs—or other relative humidity ICs, for that matter—then consider using the approach described in this article.

Figure 3. Relative humidity (RH) accuracy (at 25°C and as a function of temperature) of the four devices in the HS300x family, from the datasheet.

IDT has also provided temperature-accuracy information for the HS300x devices. Take note, however, that the HS3001 and HS3002 share the same accuracy data.

 

Figure 4. Temperature accuracy of the HS300x family, from the datasheet.

Some Guidance on Soldering and Storage

As stated in Section 10 (Soldering Information) of the datasheet, IDT tells us that when a relative humidity sensor is exposed to the high heat associated with a soldering process, the sensing element has a tendency to dry out, and this may introduce an offset in the relative humidity readings that will "slowly disappear as the sensor is exposed to ambient conditions." If a troubleshooting engineer didn't know about this "drying out" issue, then they could end up tracking a bug that would eventually pull a Houdini on them! To prevent such erroneous relative humidity readings, IDT has provided the following two methods for rehydrating the sensor:

1. After the soldering process, place the sensor in an area that's at room temperature with a relative humidity of 75% for at least 12 hours.
2. Or, place the sensor in an area that's at room temperature with a relative humidity ranging from 40% to 50% for 3 to 5 days.

Umm, "3 to 5 days"? I assume that this rather large soak time range is related to soldering-process variations (e.g., temperature, one vs. two reflow passes). In any event, if you want to verify the sensor’s accuracy immediately after performing the rehydration procedure, then it might be best to simply rehydrate the sensor for the full five days.
Regarding the storage of the HS300x sensors, IDT recommends, in Section 11 (Storage and Handling) of the datasheet, that once the ICs have been removed from their original packaging material, the sensors should be stored in metal-in antistatic bags, and that using polyethylene antistatic bags should be avoided as they may influence the accuracy of the sensors. Good to know!

Evaluation Kits

If you're interested in testing these ICs prior to incorporating them into a design, consider using either the SDAH01 or SDAH02 evaluation kit. Although both eval kits utilize the HS3001 sensor, the SDAH01 kit outputs the measured data to a PC while the SDAH02 displays the data on an LCD screen (see image below).

Figure 5. The SDAH02 evaluation kit includes an LCD that displays the measured data as numbers or as a plot. Images courtesy of IDT.

Have you used one of the ICs from the HS300x family or one of the evaluation kits? If so, leave a comment and tell us about your experiences.