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AS8413 Datasheet(PDF) 8 Page - ams AG |
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AS8413 Datasheet(HTML) 8 Page - ams AG |
8 / 15 page High Performance Automotive Sonar Intrusion – Data Sheet AS8413 March 2001 Page 8 of 15 Analog Conditioning The analog front-end, composed of a preamplifier and a filter centered at 40 kHz, increases the signal level and removes noise outside the bandpass. It is followed by a digitally-controlled AGC amplifier, that keeps signal level at the VCAP output within prescribed levels. Finally, an envelope detector extracts the information embedded in the amplified echo signal. The front-end needs proper bias during power-up. That can be provided by an RC series circuit to VDD, as shown at Fig. 2, or alternatively, by the pre-amplifier of Fig. 4. VDD 100n 100k To RX-Pin Fig. 2 - Series-RC circuit at RX The AS8413 has a wide dynamic range, to follow the signal fluctuations that occur in a large variety of vehicles, sensors and environmental conditions. Only under extreme conditions, like in a larger vehicle, an external pre-amplifier at RX may help to improve performance. A practical way to verify if a pre-amplifier might be useful, is by monitoring the echo waveform at VCAP. For measuring that voltage a FET-input buffer (input impedance at least 10 9 ohm) should be used, as the output impedance at VCAP is very high. A pre-amplifier that satisfies the bias requirements is shown at the Fig. 4. Its gain is around 8 dB. 0 10 20 30 40 T(ms) Period = 44,4 ms VCAP(V) 3,0V-4,2V 1,3V-1,8V Fig. 3 - Waveform at VCAP |
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