I A Hall probe is a sensor which can be used to measure magnetic field strength.

Question

I

A Hall probe is a sensor which can be used to measure magnetic field strength. It requires a constant current to be maintained even if environmental changes affect the conductance of the sensor. The sensor itself is modeled as a simple resistor. When placed in a magnetic field, charge is segregated to the top and bottom of the sensor creating a potential difference proportional to the magnetic field strength. To utilize the sensor a constant current must be setup through the sensor, the circuit below performs that task. Describe how and why it works. What input voltage is required to generate a constant 100 mu A in the sensor? What limits apply to this design?

Explanation / Answer

The given circuit is an inverting amplifier with sensor probe in the feedback path.

Since the input resistance of the opamp is very high, the current entering it through the inverting input will be zero. Thus all the current from the voltage source Vin flows through the feedback sensor.

Current through R1 = Current through the sensor.

IR1 = Is

Due to virtual ground, the voltage at the inverting node will be equal to zero .

Thus IR1 = (Vin - V-) / R1 = (Vin - 0 )/R1 = Vin/R1

Is = Vin/R1

With R1 constant and by applying a constant voltage , the current through the sensor will be constant.

For Is = 100

Vin = IsR1 = (100)(10k) = 1000mV

Vin  = 1V

Is the input voltage required for 100A current through the sensor.

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