Extreme low power sensor interface
VTI has achieved a remarkably low 1.5 µA continuous current consumption for a 3-axis capacitive accelerometer ΔΣ-front end circuit. The result is partly based on innovative circuit design by VTI's research partner.
It is as well based on VTI's 3D MEMS, which enables very high dynamic range and sensitivity for the sensor. The circuit can thus operate at a higher noise level and with less current.
Battery powered sensors require µA-scale current consumption
Portable electronics is powered by batteries. Those with re-chargeable batteries can live with the current consumption of today's inertial sensors when clever power management is applied. But lower would be better.
The HUT team* reports sensor front end with only 1.5 µA current draw at 1 V supply voltage. The front end is based on the well known self balanced bridge. This principle gives optimum linearity, differential cancellation of errors and zero net electrostatic force. The principle was extended to multi-axis operation and to conversion to a digital signal. The reported front end excludes the decimator part of the ΔΣ-converter, though. The total active area was less than 0.5 mm2 on the selected 0.25 µm CMOS technology.
Fig.1. Self balanced bridge principle is used to convert the capacitive signal into a digital one. All three axes of an accelerometer are handled by multiplexing to save area.
Advanced development in VTI
VTI has selected partners for advanced development in strategic areas of MEMS sensors: MEMS, packaging technologies and circuits. Some of these partnerships have existed for more than 10 years.
In the circuit design VTI's partner for many years has been the circuit design team of Helsinki University of Technology, located in Espoo, Finland. They recently published some world class results in the International Solid State Circuit Conference on February 3-7th 2008 in San Francisco.
The title of the talk was "A 1.5μW 1V 2nd-Order ΔΣ Sensor Front-End with Signal Boosting and Offset Compensation for a Capacitive 3-Axis Micro-Accelerometer" and it appears in the proceedings as paper 32.2. The authors are M. Kämäräinen, M. Paavola, M. Saukoski, E. Laulainen, L. Koskinen, M. Kosunen and prof. K. Halonen, the leader of the group.
Fig.2. The circuit was processed on 0.25 µm CMOS. The table summarizes the results.
*The circuit design team of Helsinki University of Technology