Two-channel magnetic radiometric gradiometer for precision measurement
Called TLI5590-A6W, it has two TMR (tunnel magneto-resistance) Wheatstone bridges inside (see diagram), spaced 500μm apart so that they deliver sine and cosine voltages when moved relative to long magnets, or circular magnets, whose poles change every 500μm in the working direction.
“Qualified for industrial and consumer applications to JEDEC JESD47K, it has accuracy better than 10µm achievable by use of proper linear or rotary magnet encoder,” according to the company. “Channel matching is less than 5% over full magnetic input range of ±5mT.
Operating range is over 1 to 5.5V and -40 to +125°C, with 1mA consumed at its nominal working voltage of 3.3V – each bridge draws half.
The two bridges have different sensitivities: Bridge 1 (VP1 – VN1) is 12 to 25mV/V/mT (typically 18.5mV/V/mT) while bridge 2 (VP2 – VN2) is 6 to 12.5mV/V/mT (typically 9mV/V/mT).
Why do the bridges have different sensitivities, and why are there six elements in the second bridge.
It transpires that this is related to a third magnetically-sensitive patch mid-way between the two main sensors.
“TLI5590 has two differential bridges: one compares the ‘left-right’, and the second compares ‘center-(left+right)’, the company told Electronics Weekly. “Therefore the second bridge requires six elements while the first only requires four. To simplify signal processing TLI5590 was designed that changes in magnetic pole length do not impact the phase shift between the two output bridges, but affect the amplitude of the second output bridge instead. A trade off for this is that, for perfect magnet pitch size, the amplitude of Bridge2 = 0.5x Bridge1.”
Temperature coefficient of differential output voltage amplitude is typically -0.01%/K, differential offset is ±10mV/V (with ±10μV/V/K max, but typically 0μV/V/K).
If 1mm of movement (one magnetic cycle) is nominally 360°: residual angular error is typically ±2° (±4.1° max) between 1.5 and 5mT, improving to ±0.8° (±2.5° max) at or below 1.5mT.
The outputs are intended to be connected directly to analogue inputs on a microcontroller, with equal capacitors to 0V on each line forming low-pass filters.
“The value for the buffer capacitor has to be adjusted according to the speed of the magnetic input signal,” said Infineon. “This filter limits the bandwidth of the circuit but improves noise performance.”
With 4 x 5nF connected, bandwidth is ~5kHz.
The wafer-level BGA package measures 1 x 1.3 x 0.6mm.
Applications are foreseen sensing zoom lens position in cameras, as well and more general industrial and medical use.
TLI5590-A6W product page
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