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Automotive ultrasonic ranging: Increasing gain may not improve detection distance

June 12, 2012 | Arun T. Vemuri | 222902302
Automotive ultrasonic ranging: Increasing gain may not improve detection distance Arun T. Vemuri of Texas Instruments focuses on the subject of automotive ultrasonic ranging and explains why increasing gain may not improve detection distance.
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As ultrasonic-based distance ranging becomes more prevalent in applications such as blind spot detection, objects at distances greater than six meters (20 ft) have to be detected. The amplitude of the echo signal reflected by objects at far distances is very small. So there is a temptation to increase the amplifier gain, K, in order to detect objects at such distances. In this article, we show that increasing the amplifier gain, K, may not always result in the ability to detect objects at farther distances.

Background
One application for advanced driver assistance systems (ADAS) in a passenger car is ultrasonic-based distance ranging. Ultrasonic sound wave time-of-flight (TOF) is used to calculate distances to objects to assist the driver in parking the car, identifying parking spots, or detecting objects in the drivers blind spot.

In ultrasonic-based ADAS, piezoelectric transducers typically are used to convert the ultrasonic waves into electrical signals. The receiver sensitivity of piezoelectric ultrasonic transducers usually is small, resulting in very small voltages. Figure 1, below, shows a typical signal chain used to process the echo voltage. (For an example of an integrated automotive ultrasonic signal conditioner for automotive park assist systems, see TIs PGA450-Q1)


Figure 1: Using ultrasonic-based echo processing to detect objects deals with noiseboth external (shown) and internal.

This echo signal, which is an AM signal, is corrupted with noise. The noise in Figure 1 is input-referred noise and is the sum of noise from external environment and from all components in the signal chain. This corrupted signal is then amplified by an amplifier with gain K. The amplified signal is digitized using an analog-to-digital converter (ADC). The digitized AM signal is bandpass-filtered.

The bandpass filter (BPF) primarily is used to improve the signals signal-to-noise ratio (SNR). The filtered signal level is compared against a threshold,
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