AFE5804ZCF datasheet(52/63 Pages) TI1

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LNA

Cable

Probe

Transducer

From

Pulser

-5V

+5V

AFE5804

3kW

0.1 F

3kW

BAS40

0.1 F

AFE5804

SBOS442C

– JUNE 2008 – REVISED OCTOBER 2011

www.ti.com

OVERLOAD RECOVERY

±0.3V can significantly reduce the overall overload

recovery performance. The T/R switch characteristics

The AFE5804 is designed in particular for ultrasound

are largely determined by the biasing current of the

applications where the front-end device is required to

diodes, which can be set by adjusting the 3k

Ω

recover very quickly from an overload condition. Such

resistor values; for example, setting a higher current

an overload can either be the result of a transmit

level may lead to an improved switching characteristic

pulse feedthrough or a strong echo, which can cause

and reduced noise contribution. A typical front-end

overload of the LNA, PGA, and ADC. As discussed

protection circuitry may add in the order of 2nV/

√Hz

earlier, the LNA inputs are internally protected by a

of noise to the signal path. The increase in noise also

pair

back-to-back

diodes

prevent

severe

depends on the value of the termination resistor (RT).

overload of the LNA. Figure 102 illustrates an

ultrasound receive channel front-end that includes

As Figure 102 shows, the front-end circuitry should

typical external overload protection elements. Here,

be capacitively coupled to the LNA signal input (IN).

four high-voltage switching diodes are configured in a

This coupling ensures that the LNA input bias voltage

bridge configuration and form the transmit/receive

of +2.4V is maintained and decoupled from any other

(T/R) switch. During the transmit period, high voltage

biasing voltage before the LNA.

pulses from the pulser are applied to the transducer

Within the AFE5804, overload can occur in either the

elements and the T/R switch isolates the sensitive

LNA or the PGA. LNA overload can occur as the

LNA input from being damaged by the high voltage

result of T/R switch feedthrough; and the PGA can be

signal. However, it is common that fast transients up

driven into an overload condition by a strong echo in

to several volts leak through the T/R switch and

the near-field while the signal gain is high. In any

potentially

overload

the

receiver.

Therefore,

case, the AFE5804 is optimized for very short

additional pair of clamping diodes is placed between

recovery times, as shown in Figure 102.

the T/R switch and the LNA input. In order to clamp

the over-voltage to small levels, Schottky diodes

(such as the BAS40 series by Infineon®) are

commonly used. For example, clamping to levels of

Figure 102. Typical Input Overload Protection Circuit of an Ultrasound System

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