The MEMS Gyro’s offered by InvenSense are really cool to work with, and yield great results. But remember that these devices are none standard IC’s and care must be taken when using these parts. There are some special precautions that should be noted with these types of IC’s. While most of these precautions can be found on the datasheet’s for all of the parts, Component Distributors FAE dept wanted to spell out some of the common problems our customers have been having with these devices.
Before I get into the caution involving the InvenSense Gyro’s, if this is the first time working with MEMS gyros’ or if need to quickly prototype a circuit using a gyro, it is strongly suggested that you purchase the Evaluation board. These boards have come down in pricing over the last few months (all are now less than $40 each) and come ready to plug and play. Almost all of the gyros’ made have an associated evaluation board.
Note: Do not get put off by the appeared complexity of the InvenSense Evaluation boards when you receive it. The circuit is the exact same circuit shown on all of the evaluation boards, however InvenSense has made one board to work with all of its’ sensors, so the actual board has some components that may or may not be used in your circuit.
Caution Number 1: The Gyro back Plane
All of the Gyro’s have a metal back plan. DO NOT SOLDER THE GYRO BACK PLANE TO THE CIRCUIT BOARD GND PLANE. In fact do not solder the gyro back plane to anything at all. If the gyro back plan is solder to the circuit board ground plane, the gyro will most likely be beyond repair and have to be discarded. If you have done this and you want to check and see the gyro is still functioning, check the CPOUT pin under power. If the part is operating correctly, this point should be at a voltage of about 20V.
Caution Number 2:
The gyro is most susceptible to power supply noise (ripple) at frequencies less than 100Hz. At less than 100Hz, the PSRR is determined by the overall internal gain of the gyroscope. Above 100Hz, the PSRR is determined by the characteristics of the on-chip low-pass filter. Above 1kHz, the PSRR is relatively constant except for two narrow frequency ranges corresponding to the resonant frequencies of the X and Y gyroscopes. Unless you are sure that power supply is clean, it’s suggested you use a quality low noise LDO.
Caution Number 3: Amplitude control capacitor connection:
The scale factor of the gyroscope depends on the amplitude of the mechanical motion and the trim setting of the internal programmable gain stages. The oscillation circuit precisely controls the amplitude to maintain constant sensitivity over the temperature range. Any coating, glue or epoxy on these pins or on the capacitors connected to these pins, will affect part performance and should be avoided.
Caution Number 4: Trace Routing
Testing indicates that 3-Volt peak-to-peak signals run under the gyro package or directly on top of the package of frequencies from DC to 1MHz do not affect the operation of the MEMS gyro. However, routing traces or vias under the MEMS gyro package such that they run under the exposed die pad should not be done.
Caution Number 5: AGC Nodes
The gyro pins marked XAGC and ZAGC are high impedance nodes that are sensitive to current leakage, which can impact gyroscope performance. Care should be taken to ensure that these nodes are not contaminated by residue such as flux and are clean.
Caution Number 6: MEMS Handling Instructions
MEMS (Micro Electro-Mechanical Systems) are a time-proven, robust technology used in hundreds of millions of consumer, automotive and industrial products. MEMS devices consist of microscopic moving mechanical structures. They differ from conventional IC products even though they can be found in similar packages. Therefore, MEMS devices require different handling precautions than conventional ICs prior to mounting onto printed circuit boards (PCBs). InvenSense’s gyroscopes have a shock tolerance of 10,000g. InvenSense packages its gyroscopes as it deems proper for protection against normal handling and shipping. It recommends the following handling precautions to prevent potential damage.
Caution Number 7: Gyroscope Surface Mount Guidelines
When using MEMS gyroscope components in plastic packages, package stress due to PCB mounting and assembly could affect the output offset and its value over a wide range of temperatures. This is caused by the mismatch between the Coefficient Temperature Expansion (CTE) of the package material and the PCB. Care must be taken to avoid package stress due to mounting.