Digital waveform generator provides flexible frequency tuning for sensor measurement
Edited by Brad Thompson
Colm Slattery, Analog Devices, Limerick, Ireland -- EDN, December 17, 2004
Variable-resistance sensors convert a fixed dc excitation voltage or current into a current or voltage that's a straightforward function of the quantity undergoing measurement. In another class of sensors, moving objects or fluids produce a sensor signal by altering an LC circuit's inductance or capacitance. Figure 1 shows a basic ac-driven tuned-circuit proximity sensor, L and C, and sampling resistor, R. Under static conditions, L and C resonate and provide maximum impedance at one frequency. As an object approaches the sensor, the value of L or C varies and alters the circuit's resonant frequency. You can derive the object's position by exciting the sensor with a fixed frequency and measuring changes in the phase or amplitude of output voltage V2 with respect to excitation voltage V1. However, this approach limits the sensor's dynamic range and resolution.
As an alternative, you can drive the sensor with a swept-frequency ac source that tracks the sensor's resonant-frequency variation. Figure 2 shows one approach in which IC1, a DDS (direct-digital synthesis) device, produces a sine-wave excitation voltage. Lowpass filter IC2 removes clock artifacts and harmonics, and amplifier IC3 drives the sensor. Amplifier IC4 boosts the sensor's output voltage, V2, and drives IC5, a dual-channel, 12-bit ADC,which simultaneously samples and digitizes referencevoltage V1 and IC4's output. IC5, a DSP-capable microcontroller, analyzes the sensor output's amplitude and phase, setting the frequency of IC1 via alternate programming of either of IC1's dual frequency-control registers. One of IC6's serial ports delivers position data to an external controller. Using a DDS/DSP combination offers considerable flexibility when using various types of sensors. For example, certain sensors require a relatively narrow but high-resolution range of excitation frequencies, and others may work best with broadly swept excitation.
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