High Performance Automotive Sonar Intrusion – Data Sheet

AS8412

March 2001

Page 7 of 14

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 10

impedance at VCAP is very high.

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