kriswiner
commented
8 years ago Any copper underneath the QFN package is undesirable for best magnetometer performance. In general for all MEMS sensors, avoid copper directly beneath the package (on the top layer). This means no pad underneath the chip. If you don't care about the mag performance, you may put a GND layer on the bottom-most layer but I would not recommend it. The performance of the mag and gyro is best with no copper on any layer directly beneath the QFN package outline.
You do want to make sure your pcb design has good ground planes around the chip and that the power ports are properly bypasses with good quality 100 nF 16 V capacitors. Route I2C or SPI traces away (i.e., bottom of board if possible) from the analog power pins (on top layer), keep vias away from the QFN package as well as buttons, switches, mounting holes, etc.
Best results are obtained with a four layer board design with a dedicated ground and power plane in the middle two layers, with signal layers on top and bottom with as much GND copper pour on top and bottom as you can muster.
See:
https://github.com/kriswiner/EM7180_SENtral_sensor_hub/wiki/B.-SENtral-Refer ence-Design
for an example of good design practice using the MPU9250.
Kris Pesky Products
-----Original Message----- From: empyre [mailto:notifications@github.com] Sent: February 26, 2016 4:32 PM To: kriswiner/MPU-9250 Subject: [MPU-9250] Ground plane under the chip (#43)
Hello kriswiner,
I find all your MPU-9250 board were designed without ground plane under the chip. In datasheet of MPU-9150, the Assembly Precautions section point out
The exposed center die pad(e-pad or thermal pad) is not required forheat sinking, and should not be soldered to the PCB.
But it also point out
For best performance, design a ground plane under the e-pad toreduce PCB signal noise from the board on which the gyro device is mounted. If the gyro device is stacked under an adjacent PCB board, design a ground plane directly above the gyro device to shield active signals from the adjacent PCB board.
Does it mean a ground plane under the e-pad is necessary but should not be soldered to the e-pad?
I recently contacted Invensense, they said the same design rules are still applicable to MPU-9250. Your mini MPU9250 Teensy 3.1 add-on shields are directly mounted under Teensy board. If without a ground plane between Teensy and MPU9250, would the performance of gyro in MPU9250 affected by Teensy board?
Thanks
Reply to this email directly or view it on GitHub https://github.com/kriswiner/MPU-9250/issues/43 . https://github.com/notifications/beacon/AGY1qnl3u2JLNXxeR2DtTscVwyKnpCCbks5 poOXvgaJpZM4HkTZi.gif
empyre
khalil50
Hi kriswiner !! 
Hello kriswiner,
I find all your MPU-9250 board were designed without ground plane under the chip. In datasheet of MPU-9150, the Assembly Precautions section point out
But it also point out
Does it mean a ground plane under the e-pad is necessary but should not be soldered to the e-pad?
I recently contacted Invensense, they said the same design rules are still applicable to MPU-9250. Your mini MPU9250 Teensy 3.1 add-on shields are directly mounted under Teensy board. If without a ground plane between Teensy and MPU9250, would the performance of gyro in MPU9250 affected by Teensy board?
Thanks