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

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