TE Connectivity Releases Pressure Sensor for Harsh Environments
The MS5839 is a highly integrated, water- and chlorine-resistant device that utilizes delta-sigma A/D conversion to provide high-resolution digital data.
Sensing applications typically benefit from high-resolution measurements. Fortunately, many of these applications have fairly low sample-rate requirements.
In contrast to communication systems (such as a software-defined radio) or high-quality audio processors, sensors often operate on a more “human” time scale.
We generally don’t need thousands of temperature or air-quality or light-intensity measurements every second, and sometimes just a few samples per second is more than enough.
The Delta-Sigma ADC
Low throughput requirements allow us to take advantage of an ADC architecture known as delta-sigma, also written as ΔΣ.
The basic operating principles of a delta-sigma ADC are quite different from what we observe in a successive approximation register (SAR) ADC or a pipelined ADC.
With these two architectures, the converter samples the analog input signal according to the desired update rate and generates an output word whose length reflects the specified resolution of the device. Thus, a 12-bit, 1 MHz SAR ADC will sample the input signal one million times per second and generate a 12-bit digital value for each sample.
A delta-sigma ADC, on the other hand, uses high-frequency, 1-bit A/D conversion to generate low-frequency, high-resolution output values.
This block diagram conveys the general architecture of a delta-sigma ADC.
Those who speak the engineering language, which happens to be written using both Roman and Greek letters, know that delta represents a difference (e.g., ΔT is the difference or change in temperature) and that we use sigma to indicate that a summation is taking place.
Thus, a delta-sigma ADC involves both difference and summation (or integration), as shown in the following diagram.
Diagram of a delta-sigma ADC involving both difference and summation (or integration).
The result of delta-sigma modulation is a digital signal that captures analog amplitude changes as changes in the rate of transition between logic low and logic high.
If you provide a sinusoidal input to the delta-sigma modulator, it will produce a bitstream that transitions less frequently when the sinusoid has low rates of change (i.e., near the minimum- and maximum-amplitude portions of the waveform), and that transitions more frequently when the sinusoid has high rates of change (i.e., near the zero-crossing portion of the waveform).
After delta-sigma modulation, the ADC applies digital filtering and decimation to the bitstream. The low-resolution but highly oversampled digital signal is thereby converted into higher-resolution values delivered at a sampling rate that is consistent with the requirements of the system.
For much more information on this topic, take a look at AAC’s article on delta-sigma analog-to-digital conversion.
Underwater Pressure and Temperature Sensing
TE Connectivity recently released MS5839, which utilizes MEMS technology and incorporates a signal-processing and communication IC, provides temperature and pressure measurements with 24 bits of resolution.
It has only four pins—VDD, GND, and the two pins needed for an I2C interface—and requires no external components except for a 100 nF decoupling capacitor and the typical I2C pullup resistors.
Typical application circuit of the MS5839 used in conjunction with an MCU for mobile altimeter applications. Image used courtesy of TE Connectivity
Owing to its pressure-measuring capabilities, the MS5839 can be used as an altimeter. The datasheet indicates a resolution of 13 cm, though this spec is not clearly explained, so I’m not sure how exactly to interpret it.
TE emphasizes the device’s suitability for underwater applications, for example as a sensor component in swim watches, diving computers, or underwater vehicles, and according to the datasheet it is “optimized for” environments in which chlorine and saline are present (I’m assuming that it works just as well in non-chlorinated, salt-free water).
Supply Voltage Range
The MS5839 accepts supply voltages from 1.5 V to 3.6 V. This wide range is particularly advantageous in battery-powered applications because it helps the designer to avoid power-supply circuitry that would otherwise be required to compensate for the gradual reduction in battery voltage.
However, if you’re thinking about incorporating this part into a high-precision system, keep in mind that supply voltage influences the accuracy of both the pressure measurements and the temperature measurements.
In the case of the pressure sensor, accuracy degrades at lower supply voltages:
Pressure error vs. supply voltage of MS5839. Image used courtesy of TE Connectivity
In the case of the temperature sensor, accuracy degrades at both lower and higher supply voltages, but the polarity of the error changes:
Temperature error vs. supply voltage of MS5839. Image used courtesy of TE Connectivity
Featured image (modified) used courtesy of TE Connectivity
Have you ever designed electronic devices for underwater applications? Feel free to leave a comment if you have any design tips or interesting experiences that you’d like to share.
