Honeywell has developed a novel ASIC design that enables inductive proximity sensor/switches to be easily programmed without the need for test and calibration equipment — after full assembly in production or at the customer's location. In addition frequency independent temperature compensation, along with a quenching oscillator, results in sensing distances as much as three times farther than competing technology. As a result, smaller products can be sensed. Sensors using this unique ASIC contain fewer electronic variants — resulting in fewer parts/designs, less inventory, and increased reliability.

Though the ASIC has not as yet been commercialized, key circuits have been built within a device test structure to confirm suitability for the foundry process.

Honeywell's ASIC design is useful for inductive proximity switches having a digital output. Traditionally proximity switch ICs use an oscillator with a frequency defined by the resonance of an LC parallel tank circuit and a demodulator that detects a change of oscillator amplitude. When a metallic object enters the electromagnetic field of the oscillator coil, eddy currents are induced in the coil that quenches the amplitude of oscillation. This reduction in Q — and hence reduction in peak amplitude — is tracked by a demodulator such that if the signal fails to exceed the sense distance threshold, the output stage is activated.

The Q of the inductor head will vary with temperature. If left uncompensated, the change in Q will create large changes in Nominal Sense Distance (Sn) with temperature. Therefore, it is necessary to have positive temperature compensation in the oscillator.

This programmable ASIC automatically trims internally for Sn following power-up. It is necessary — in this preprogrammed state — to locate the proximity switch at the required sense distance to the target in order to achieve the optimum internal setting. During the programming cycle, a lock bit is also set such that the IC will set up a normal mode, where it cannot be programmed. Therefore, the ASIC can operate in two modes: a) programming mode, and b) normal mode.

Over-temperature protection is provided by a band gap reference that can be used with additional circuitry to automatically shutdown the sensor if an over-temperature condition is reached.