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For a given supply voltage, a ratiometric sensor’s output signal level is a proportion of the supply voltage. Ratiometric error is the deviation in this proportion resulting from any change in the supply voltage. Usually expressed as a % of full-scale output (FSO). For example, if the signal level changes 2% with a 4% supply voltage change, the ratiometric error will be 2%.
Ratiometric – the sensor’s zero point and sensitivity is proportional to it’s voltage supply. A ratiometric sensor is designed to share the same voltage rail as the analog-to-digital converter (ADC) to mitigate errors from voltage supply variation.
Non-Ratiometric – the sensor has a fixed sensitivity so voltage supply variations don’t affect the sensor’s output. An ADC with a stable reference can be paired with a non-ratiometric sensor on a different voltage rail as long as a voltage reference (VREF) is shared between them to maintain accuracy over temperature. Crocus TMR sensors have temperature stability and extremely low noise such that a VREF connection is not necessary.
Unipolar – only one field polarity (DC current) is sensed and the VREF is set to minimum voltage (0.5V for 5V device and 0.65V for 3.3V device) to double the voltage output range.
Bipolar – both positive and negative polarities (AC current) are sensed and VREF is set to VCC/2 where VCC can be 5V or 3.3V.
Common Mode Field Rejection Ratio (CMFRR) is a parameter to quantify the ability of the sensor to reject (cancel out) external in-phase (common mode) magnetic fields. TMR current sensors with CMFRR capability will have two sense elements. A current carrying conductor must be routed such that it creates a magnetic field near one sense element and an equal but opposite field near the other element, any external in-phase field imposed on the two sensors will be rejected.
Tunnel Magneto Resistor (TMR) is a special sense element that exhibits a resistance value dependent on the strength of an external magnetic field. A TMR can be used to measure magnetic field presence, strength, and direction.
A magnetic tunnel junction (MTJ) consists of two layers of magnetic material, separated by an ultrathin insulating film with a thickness of about 1 nm. If the insulating layer is thin enough, electrons can tunnel from one ferromagnet into the other. A TMR sensor is composed of MTJ elements.
The Hall effect uses the Hall principle. A current is driven through thin silicon plates. When a magnetic flux perpendicular (z-axis) to the flow is applied, a voltage proportional to the magnetic flux density is output.
TMR uses magneto resistors in a Wheatstone Bridge configuration. In the absence of a magnetic field, the bridge is in a balanced state and no voltage is output. A planar (x/y axis) magnetic field will unbalance the Wheatstone resistor bridge leading to a voltage proportional to the magnetic flux density.
A TMR element can obtain a larger signal amplitude with 1/5 less signal distortion compared with a Hall element. An amplifier can increase the Hall output voltage but this adds to its signal distortion.
Yes, every Hall or shunt/opamp current sense solution can be replaced by a TMR sensor. Crocus TMR sensors have pin and package options compatible with popular Hall sensors. Shunt solutions for low current sensing are not practical for TMR replacement. High current shunt solutions benefit from the high isolation, decreased PCB area and simplicity of a TMR solution.
If VREF is not used, the pin should not be DC grounded, it should be AC grounded by adding a 47nF decoupling capacitor from this pin to ground. DC grounding the VREF pin will affect the output signal
When interfacing to the sensor’s voltage reference (VREF) add a series 10kΩ resistor to limit the current draw and avoid VREF output collapse.
To reduce noise, there is an integrated series resistor on the FILTER output which requires an external capacitor to adjust the 3dB cut-off frequency. At minimum a 5pF capacitor must be connected to this pin to reduce output pulses on the Vout, it cannot be left unconnected. A second order low pass filter can be implemented to further adjust the cut-off frequency.
When temperature increases, the magnetoresistance decreases. Crocus TMR devices are temperature compensated to enable accurate and stable sensing across the -40°C to +125°C operating range.
TMR devices will have a permanent offset shift of about 5 mV/V if a field greater than 70 mT to 90 mT is applied to it. One way to compensate for offset shift is to have the MCU compare recent readings with stored look up table (LUT) performance values. Every time the sensor is enabled, the MCU will do a calibration to compare the original LUT offset values with the recent measured values. If there is a difference the MCU will adjust the readings.
Yes, components containing ferrous materials or current carrying conductors, if they are closer than 2cm can affect TMR readings.
TMR sensors respond to any external magnetic field therefore precautions need to be taken to avoid contaminating the sensor readings. Lowest cost solution is separation distance since the magnetic field strength decreases based on the inverse square law. Next best is to use magnetic shielding to divert any external fields. Best solution is use TMR sensor with CMFR such as the CT452 and CT453.
Widely used magnets include ferrite, neodymium, and samarium-cobalt. TMR’s high sensitivity enables the use of smaller, lower cost magnets.
Yes, the CTC4000 is a calibration unit that can optimize the total error performance of the assembled CT45x sensor module. It can program the gain and offset of the CT45x to achieve a total module error of less than ±1.0%FS.
Crocus has online magnetic field calculators that can help select the appropriate TMR sensor (CT100, CT220 or CT450). Both the PCB trace and busbar field calculators are found here, tools.crocus-technology.com:5010/MF_CAL.
Yes, there Crocus has two online permanent magnet calculators, a basic calculator, https://crocus-technology.com/basic-magnetic-field-calculator/ and a more advanced version with graphical representation of the magnetic vector field, https://tools.crocus-technology.com:5010/Cal