Design Article
Automotive ultrasonic ranging: Increasing gain may not improve detection distance
Arun T. Vemuri, Texas Instruments
6/11/2012 1:33 PM EDT
Observations
Equation 8 can be used to analyze the smallest detectable distance by considering noise from components upstream and downstream from the amplifier. The components upstream from the amplifier include environment noise, transducer noise, noise from any current limiting resistors, and noise the amplifier itself. Downstream components include the ADC quantization noise and filter calculations errors.
Here are three different scenarios that illustrate distance detection.
Conclusions
Figure 2 shows that the effect of increasing the amplifier gain really depends on the relative noise magnitudes of the components both upstream and downstream from the amplifier. If noise from upstream components dominates the overall noise, then increasing the gain will not help detect objects at farther distances.
Reference:
Datasheet for the PGA450-Q1
About the author:
Arun Tej Vemuri is a systems architect with the Mixed-Signal Automotive group at Texas Instruments where he is responsible for product definition of automotive sensor signal conditioners. Arun received his Ph.D. in Electrical Engineering from the University of Cincinnati, his MS in Systems Science from IISc Bangalore, India, and BSEE in Electrical Engineering from IIT Roorkee, India. Arun can be reached at ti_arunvemuri@list.ti.com.
Equation 8 can be used to analyze the smallest detectable distance by considering noise from components upstream and downstream from the amplifier. The components upstream from the amplifier include environment noise, transducer noise, noise from any current limiting resistors, and noise the amplifier itself. Downstream components include the ADC quantization noise and filter calculations errors.
Here are three different scenarios that illustrate distance detection.
Figure 2: Effect of increasing the gain does not produce a change in the smallest possible threshold value or largest measurable distance.
Conclusions
Figure 2 shows that the effect of increasing the amplifier gain really depends on the relative noise magnitudes of the components both upstream and downstream from the amplifier. If noise from upstream components dominates the overall noise, then increasing the gain will not help detect objects at farther distances.
Reference:
Datasheet for the PGA450-Q1
About the author:
Arun Tej Vemuri is a systems architect with the Mixed-Signal Automotive group at Texas Instruments where he is responsible for product definition of automotive sensor signal conditioners. Arun received his Ph.D. in Electrical Engineering from the University of Cincinnati, his MS in Systems Science from IISc Bangalore, India, and BSEE in Electrical Engineering from IIT Roorkee, India. Arun can be reached at ti_arunvemuri@list.ti.com.
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T.ORR
6/13/2012 1:16 PM EDT
Hi
Did I miss something - like how do you make it better.
A bigger TX signal is about the best method.
Regards
Tim Orr
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cdhmanning
6/14/2012 9:40 PM EDT
There are other ways too.
As an example, consider process gain achieved by coded signals. That's used in GPS signals to give many dB of gain and noise rejection.
Just yelling louder doesn't help. Remember there are many vehicles out there and if they're all yelling, you still end up not hearing properly.
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Arun V
6/19/2012 1:03 PM EDT
Agreed and that is the point of this article. A stronger echo (i.e., yelling louder) or large gain in the signal chain may not help detect objects at longer distance. One has to address the noise aspects also to detect objects at longer distances.
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WKetel
6/15/2012 9:32 PM EDT
The minimum detectable signal (MDS) determines the maximum range, and the MDS in turn is determined by the signal to noise ratio,(SNR). Of course the most obvious way to raise the SNR is to increase the transmitted signal. Unfortunately there are often a number of challenges to that approach. Of course there are quite a fewq other ways, but they are mostly not simple and not cheap, and sometimes they don't work very fast, either. Of course, speed is always relative, so it may be that they work fast enough.
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Arun V
6/19/2012 1:05 PM EDT
Agreed that this is a challenge. To your point, one has to consider many aspects and not just the gain of the signal chain to improve MDS.
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GREAT-Terry
7/9/2012 9:43 PM EDT
This is the same in almost all kind of signal chain processing, increasing gain sometimes won't help if the noise in front of the gain stage (including the noise from the amplifier itself) is dominating.
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