AS8412 View Datasheet(PDF) - austriamicrosystems AG
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AS8412 Datasheet PDF : 14 Pages
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High Performance Automotive Sonar Intrusion – Data Sheet
AS8412
all the vehicle interior. Each pillar may have either a transmitter/receiver pair or two sensors of
the same kind. The first arrangement is recommended, as it allows a single box at each pillar
containing the AS8412 and the transducer pair controlled by it, thus decreasing cabling.
The outputs TX1 and TX2 drive the transmitter in a push-pull configuration with 10 V peak-to-
peak. As shown in Fig. 1, the transmission duty-cycle is very short (around 1/75), reducing the
average current needed to generate the ultrasonic bursts to about 0.05 mA per IC.
Shielded cable is mandatory for the receiver and recommended for the transmitter, unless they
are adjacent to the IC. The shield at the receiver cable must be grounded and connected to the
RXGND pin.
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, which keeps signal level at the VCAP output within prescribed levels. Finally, an en-
velope 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 AS8412 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. A FET-input buffer (input impedance at least 109 ohm) should be used, as the output
impedance at VCAP is very high.
VCAP(V)
3,0V-4,2V
1,3V-1,8V
0
10
20
30
Period = 44,4 ms
Fig. 3 - Waveform at VCAP
40
T(ms)
March 2001
Page 7 of 14
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