Chimaera

Sensor Unit (SU-16)

The circuitry for a single sensor unit is pretty simple, it’s 16 A1304 linear hall-effect sensors (S0-S15) multiplexed to one output by a 16:1 multiplexer (CD74HC4067). The analog output of each sensor is connected to one input of the multiplexer. The multiplexer has one analog output whose connection to a given input is controlled by the four digital channel selects (CS0-CS3) driven by the DSP unit.

The analog output of the multiplexer is the positive input (Vin) of the OpAmp (U1), which acts as a shifting non-inverting amplifier. It amplifies the signal at its positive end (Vin) relative to a reference voltage (Vref). The output of the OpAmp (Vout) can be sent over the wire as low-impedance input for an analog-to-digital converter. The OpAmp’s is embedded in a single package (MCP601).

The analog signal thus can be influenced at two points, e.g. at the trim potentiometers RV1, RV3:

• RV3 acts as a voltage divider, where the reference voltage (Vref) can be set from Vee to Vcc. This is needed because the hall-effect sensors respond to both north and south polarized magnetic fields. With no applied magnetic field, the sensors output is roughly half of its input voltage, this is called the quiescent voltage of the sensor. In a south polarized magnetic field, the voltage linearly approaches Vcc and in a north polarized magnetic field, the voltage linearly approaches Vee. In the case where we want to amplify output voltage in both north and south polarized magnetic fields, the reference voltage (Vref) needs to be set to the mean quiescent voltage of all 16 sensors. This is the default setup of the sensor units in the Chimaera. But there may be usage scenarios where you are interested in only one of the two possible polarizations of the magnetic field and you would like to map it to the whole voltage range for higher resolved ADC performance. In this case you would set the reference voltage (Vref) to either Vcc or Vee.
• RV1 acts as a varistor, setting the amplification factor in conjunction with the fixed resistor R1. The amplification factor is given by 1 + RV1/R1 and can vary from x1 to x11 when using the default components.

Each sensor unit has its own 3.3V low-drop-out voltage regulator to drive the whole circuitry (MCP1703). This makes the unit modular, reduces noise considerably and helps to distribute the generated heat over the whole array of interconnected units. The board’s dimensions are 80x30 mm.