AFE5804 datasheet(51/60 Pages) TI

51 / 60 page

OVERLOAD RECOVERY

LNA

Cable

Probe

Transducer

From

Pulser

-5V

+5V

AFE5804

3kW

0.1 F

3kW

BAS40

0.1 F

AFE5804

www.ti.com ................................................................................................................................................ SBOS442A – JUNE 2008 – REVISED SEPTEMBER 2008

are largely determined by the biasing current of the

diodes, which can be set by adjusting the 3k

Ω

The AFE5804 is designed in particular for ultrasound

resistor values; for example, setting a higher current

applications where the front-end device is required to

level may lead to an improved switching characteristic

recover very quickly from an overload condition. Such

and reduced noise contribution. A typical front-end

an overload can either be the result of a transmit

protection circuitry may add in the order of 2nV/

√Hz

pulse feed-through or a strong echo, which can cause

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

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

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

earlier, the LNA inputs are internally protected by a

pair

back-to-back

diodes

prevent

severe

As Figure 100 shows, the front-end circuitry should

overload of the LNA. Figure 100 illustrates an

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

ultrasound receive channel front-end that includes

This coupling ensures that the LNA input bias voltage

typical external overload protection elements. Here,

of +2.4V is maintained and decoupled from any other

four high-voltage switching diodes are configured in a

biasing voltage before the LNA.

bridge configuration and form the transmit/receive

Within the AFE5804, overload can occur in either the

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

LNA or the PGA. LNA overload can occur as the

pulses from the pulser are applied to the transducer

result of T/R switch feed-through; and the PGA can

elements and the T/R switch isolates the sensitive

be driven into an overload condition by a strong echo

LNA input from being damaged by the high voltage

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

signal. However, it is common that fast transients up

case, the AFE5804 is optimized for very short

to several volts leak through the T/R switch and

recovery times, as shown in Figure 100.

potentially

overload

the

receiver.

Therefore,

additional pair of clamping diodes is placed between

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

±0.3V can significantly reduce the overall overload

recovery performance. The T/R switch characteristics

Figure 100. Typical Input Overload Protection Circuit of an Ultrasound System

Submit Documentation Feedback

Product Folder Link(s): AFE5804

AFE5804 Datasheet(PDF) 51 Page - Texas Instruments

AFE5804 Datasheet(HTML) 51 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Datasheet Download

Link URL