The IC Insider looks at a MEMS-based gyroscope in microscopic detail, and finds that the ingenuity in this sophisticated sensor goes far beyond the process technology used to sculpt its mechanical features.
By Randy Torrance, Chipworks -- EDN, December 1, 2008
In recent years people have realized that a gyroscope would be a great addition to many devices that can benefit from information about their orientation. For instance, I'd like one in my coffee cup so that when I'm walking around the office, it can notify me when I tilt it too far. OK, that may be a bit of a stretch, but gyroscopes are now showing up in many applications. How does your car's ESC (electronic stability control) system know that your car is spinning out of control—and how does it figure out how to adjust the brakes and power distribution to correct the situation? A gyroscope. How do some of today's more advanced digital cameras adjust for a user's shaky hand to improve photo quality? A gyroscope.
Interlocking fingers are micromachined in between and connected to the proof-mass system and the support silicon, such that they can measure capacitance changes (Figure 3).
The design uses a bulk micromachining process, vertically integrated electronics, and wafer-scale packaging. The gyro design includes an integrated dual-mass electrostatically driven actuating mechanism with capacitance sensing. The two linked proof masses are driven into an anti-phase oscillation by electrostatic actuators placed beneath them. This out-of-plane resonating proof-mass system senses the rate of rotation in either the X or Y axis.
The two MEMS structures including the vibrating proof masses can be seen in the center of the die. Each MEMS device has its own dedicated drive circuitry (located below the MEMS sensors), and signal-detection and measurement circuitry (above the MEMS sensors). At the top of the die is a large amount of trimming and calibration circuitry, which includes nonvolatile memory arrays. The bottom of this die photo is mainly occupied by voltage- and bias-generation circuitry.
However this still leaves voltage and temperature variations to handle. InvenSense starts with a bandgap reference to generate a supply- and temperature-independent voltage reference to the oscillator. However, the resonant frequency of the proof mass is a function of temperature.
Hence the oscillator actually needs a temperature-dependent reference in order to maintain the proof-mass resonance. We believe InvenSense has modified the bandgap reference circuit to add in this characteristic.
Referring to Figure 6, we see the heart of the bandgap reference
X1167 can be clearly seen to be eight unity PNPs, compared with the two of X1619. What is of interest is just how much larger these devices are than a standard MOS device in this process, one of which is highlighted by the small white rectangle in the upper left of the bipolar devices. This is likely done for matching reasons, to ensure the best temperature response possible. It is also interesting to note that the feedback path to the op-amp actually travels through two more PNPs that are also scaled to allow for a large Vbe difference.
Another interesting feature of this circuit is the method by which it creates the bias currents through these two feedback PNP transistors of the bandgap core. The circuit shown in Figure 8 is used to create these currents.
After analysis, one can see that the current generated by this circuit is proportional to absolute temperature (PTAT) voltage divided by R1257. Assuming that the temperature dependence of the polysilicon resistor is much smaller than the bandgap voltage temperature dependence, the output current is also PTAT. This may be used to compensate for the temperature dependence of the feedback PNPs in the bandgap core of Figure 6. A more likely scenario, however, is that this PTAT current, along with appropriate selection of component values in the bandgap core, creates a temperature-dependent characteristic that will compensate for the temperature dependence of the proof mass resonant frequency. Taken all together, this is one of the most complex bandgap circuits we have ever seen.