Research Lab Technologist
2014 Micro 3D Magnetometer
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NOTICE: This webpage and associated files are provided for reference only. This is not a kit site! It is a collection of my work here at the University of Toronto in the Physics department. If you are considering using any schematics, designs, or anything else from here then be warned that you had better know something of what you are about to do. No design is guaranteed in any way, including workable schematic, board layout, HDL code, embedded software, user software, component selection, documentation, webpages, or anything.
All that said, if it says here it works then for me it worked. To make the project work may have involved undocumented additions, changes, deletions, tweaks, tunings, alterations, modifications, adjustments, waving of a wand while wearing a pointy black hat, appeals to electron deities and just plain doing whatever it takes to make the project work.
Started March 2014 for
Stefan Trotsky in
Joseph's lab. A small 3D magnetometer is squished onto a small PCB, which is connected via a flex cable to
the amplifier board and then a ribbon to the breakout box.
Power comes from a 12V wall wart. The 12V goes directly into an isolated ±15V supply. Those voltages are filtered by CLC pi filters before being launched into a 0.025" 20-conductor ribbon cable to the amplifier board. The amplifier board drops the +15V to a regulated +5V reference, which is buffered and inverted for tracking ±5V. That ±5V is launched into a 16-conductor FFC (Flat Flexible Cable, AKA FPC Flat Printed Cable) and directly powers the three magnetometer bridges in the Honeywell HMC1043(L) 3D magnetometer. The differential outputs from the three bridges are sent back across the FFC to three sets of amplifiers. Each amplifier is an Analog Devices AD8523 instrumentation amplifiers with fixed gain of 100, followed by Texas Instruments OPA209 opamps with a gain of 4, giving a total differential gain of 400. Those three opamp outputs are sent back up the 20-conductor ribbon cable to three BNC connectors. Due to magnetometer imbalance and amplifier offsets, a small offset can be added to each channel from trimpots on the ±5V supplies.
The HMC1043 magnetometer requires a set/reset pulse to work properly. This starts with an opto-isolated TTL input being sent down the ribbon cable, into a Fairchild NC7S14M Schmitt input inverter. That invertor output is sent down the FFC to the magnetometer board. There, an Analog Devices ADP3654 fast dual FET driver is used to instead directly drive the capacitive coupled set/reset bar in the magnetometer. The set and reset pulses are exponential decays with a 1uS time constant. If a TTL rising edge is considered to be the set pulse, then the TTL falling edge is the reset pulse. The supply voltage for the FET driver comes down the FFC from a simple trimpot and BJT emitter follower. That supply ranges from +5V to +13V. The set/reset pulses will peak approximately 2V below the supply (e.g. +7.5V will result in a +5.5V peak).
The last feature of the HMC1043 magnetmeter are the two offset bars: one for X- and Y-axis, one for Z-axis. They share a common connection. The three wires are sent up the FFC, through the amplifier board then up the ribbon cable to two BNCs in the breakout box.
The temperature monitor shows the temperature right beside the magnetometer IC. The voltage is related to the temperature, at 25° the voltage change is +48.3mV/°.
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