TDK Sensor Shows How Capacitive MEMS and Low-power IoT Go Hand in Hand
TDK recently announced the so-called "lowest power waterproof MEMS barometric pressure sensor." Why did TDK choose capacitive MEMS as the go-to architecture for low power?
Of all design concerns for IoT devices, power consumption reigns supreme. Optimizations in power consumption can come from a variety of sources—the onboard processing units, RF modules, or sensors. TDK Corporation is specifically tackling the issue of sensor power consumption with its newest release: a low power consumption pressure sensor based on a proprietary capacitive MEMS technology.
TDK says the ICP-10125 is the "lowest power waterproof MEMS barometric pressure sensor" on the market. Image used courtesy of TDK
In this article, we’ll explore TDK's new product and discuss both capacitive and resistive MEMS pressure-sensing technology.
Resistive MEMS Sensors
The first type of MEMs pressure sensor that was commercially available was a resistive, or piezoresistive, MEMS sensor.
This type of pressure sensor leverages strain-dependent resistors in a voltage divider configuration to measure changes in pressure. These resistors vary their resistance value based on the amount of pressure that they’re under; the strain they experience elongates the component and varies its resistance value.
Piezoresistive pressure sensor example. Image used courtesy of AVNET
These resistors are then arranged in a voltage divider-type configuration and tested with an excitation voltage. The measured output voltage varies directly with the resistor values, allowing the pressure being applied to be measured electronically.
Capacitive MEMS Sensors
Capacitive MEMS pressure sensors take advantage of the properties of parallel plate capacitors to measure atmospheric pressures. These sensors bank on the fact that a parallel plate capacitor’s capacitance is a function of the spacing between the plates.
To leverage this feature, this type of pressure sensor consists of a conducting layer deposited onto a diaphragm, which creates a capacitor between the conducting layer and another electrode. Atmospheric pressure will cause deformation in the diaphragm, decreasing the spacing between the parallel plates and increasing the capacitance (and vice versa).
MEMS capacitive sensor cross-section. Image used courtesy of AVNET
While the change in capacitance can be on the order of picofarads, it is still measurable by multiple techniques. One way that this change in capacitance can be measured is with a tuned RC circuit, where the variable capacitance will be detectable by the circuit’s frequency response. Another method can measure the time it takes for the capacitor to charge directly from a known current source.
A New MEMS Barometric Pressure Sensor
Generally speaking, the capacitive MEMS solution tends to be a much lower power solution than piezoresistive solutions. With this in mind, TDK released its newest pressure sensor for IoT based on the same technology.
According to the datasheet, the sensor, dubbed the ICP-10125, is rated to work for a VDD range of -0.3 V–2.16 V and draws a maximum current of 10.4 μA in ultra-low noise mode. This comes to a worst-case ~25 μW power consumption, making it suitable for low-power IoT.
Typical application circuit using the ICP-10125. Image used courtesy of TDK
Along with low power, the new sensor offers other features including being waterproof to 10 ATM, a temperature coefficient of ±0.5 Pa/°C, and a pressure noise of 0.4 Pa, which TDK claims is the lowest on the market.
Low Power for IoT
For IoT devices, low power is arguably the most important design concern for electrical engineers, and new low-power sensors like the ICP-10125 from TDK may be a step in the right direction. With its waterproof features, TDK's sensor is marketed toward fitness, smartwatch, and portable device markets.