High-Accuracy Temperature Sensing: New Digital Temperature Sensors from Sensirion

Sensirion asserts that their new series of digital temperature sensors are more accurate, have increased intelligence, and are more reliable than their predecessors.

Sensirion recently introduced their new series of highly accurate digital temperature sensors. This new series, the STS3x, uses Sensirion's "industry-proven" CMOSens® technology to achieve increased intelligence, better reliability, and improved accuracy. There are three flavors in this new series, all of which use the same datasheet:

• STS30: ±0.2°C accuracy at temperature range of 0°C to 65°C
• STS31: ±0.2°C accuracy at temperature range of 0°C to 90°C
• STS35: ±0.1°C accuracy at temperature range of 20°C to 60°C

Figure 1. STS3x series of high-accuracy temperature sensors. Image taken from the datasheet (PDF).

A Wide Operating Temperature Range, but Not Always with High-Accuracy

Although each of the three ICs within this series can function in the wide operating temperature range of -40°C to 125°C, which, by the way, is quite impressive as this range is considered the automotive-grade, the touted high-accuracy measurements do not extend to the entirety of this range. Rather, the accuracies (as can be seen in the images below) begin to change noticeably outside a narrower temperature range.

Figure 2. Accuracy vs. temperature of the STS30 and STS31. Image courtesy of Sensirion.

 

Figure 3. Accuracy vs. temperature of the STS35. Image courtesy of Sensirion

Designed for Mass Production

The datasheet states (see image below) that this IC—specifically the CMOSens® technology that it uses—is "designed for mass production." Umm, shouldn't this go without saying? I have seen datasheets state "not recommended for new designs," but I don't ever recall seeing one that specifies that the IC, or its underlying technology, is designed for mass production. This benefit makes me question if Sensirion has other ICs that are in fact not designed for mass production. It's all a bit puzzling. Have you seen other IC datasheets call this out? If so, please let us know.

 

Figure 4. Somewhat puzzling that the IC's core technology is called out as being "designed for mass production" (from the datasheet).

Comes in a Small Package and Has Alert and Reset Pins

This IC is available only in an 8-pin dual-flat no-leads (DFN) package, measuring a scant 2.5mm × 2.5mm with a tiny thickness of only 0.9mm. There is also, in addition to the eight pins, a thermal pad that is connected to ground (see image below).
Although this device uses an I2C interface with communication speeds up to 1 MHz, two of the eight pins are dedicated to Alert (pin3) and nReset (pin 6). The Alert pin is intended to be connected, if desired, to an interrupt pin on a microcontroller. According to section 3.5 (ALERT Pin) of the datasheet, "The output of the pin depends on the value of the temperature reading relative to programmable limits," and its function is "explained in a separate application note." However, and this is very perplexing, there is no additional information related to the "separate application note." My suspicion is that Sensirion intended to include a link to this app note but then simply forgot to include it; perhaps additional information will be provided in the datasheet's next revision.
The nReset pin may be used to generate a system reset of the IC. And while a reset may also be externally generated by issuing a command (referred to as a soft reset), to achieve a full reset it is recommended to use the nReset pin (or of course you can also cycle power). On the other hand, if this pin is not to be used then it is recommended that it is either left floating or tied to VDD via a series resistor of value ≥2 kΩ. The datasheet goes on to say that "the nRESET pin is internally connected to VDD with a pull up resistor of 50 kΩ." So... why are there two options for how to configure the nReset pin when it's not being used? Why not just recommend that it should be left floating (since it's already pulled high internally)? And, if there are indeed technical reasons for when the pin should be externally pulled up to VDD (for better noise immunity, as an example) then let us know what those technical reasons are.

Figure 5. Pinout and pin descriptions of the STS3x series. Image courtesy of datasheet (PDF).

Packaging and Land Pattern Details

Sensirion provides detailed information on both the package outline and the land pattern. In fact, it is recommended, because the IC's pad pitch is just 0.5 mm, that only one solder mask opening should be used for all four pads on one side. It is also suggested, when using solder paste printing, that a laser-cut stainless steel stencil with trapezoidal walls and with a stencil thickness of 0.1 or 0.125 mm should be used. See the images below. This information should be quite helpful for a PCB layout design team.

Figure 6. Package information, from the datasheet (PDF).

 

Figure 7. Land pattern information, from the datasheet (PDF).

Have you had a chance to use any of the temperature sensors in this new series? If so, leave a comment and tell us about your experiences.