Datasheet

LNA
Cable
Probe
Transducer
From
Pulser
-5V
+5V
AFE5805
3kW
C
2
³ m0.1 F
C
1
3kW
R
T
BAS40
0.1 Fm
IN
V
BL
AFE5805
SBOS421D –MARCH 2008–REVISED MARCH 2010 www.ti.com
OVERLOAD RECOVERY ±0.3V can significantly reduce the overall overload
recovery performance. The T/R switch characteristics
The AFE5805 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 3kΩ
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 feed-through 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 of back-to-back diodes to prevent severe
depends on the value of the termination resistor (R
T
).
overload of the LNA. Figure 51 illustrates an
ultrasound receive channel front-end that includes As Figure 51 shows, the front-end circuitry should be
typical external overload protection elements. Here, capacitively coupled to the LNA signal input (IN). This
four high-voltage switching diodes are configured in a coupling ensures that the LNA input bias voltage of
bridge configuration and form the transmit/receive +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 AFE5805, 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 feed-through; and the PGA can
signal. However, it is common that fast transients up
be driven into an overload condition by a strong echo
to several volts leak through the T/R switch and
in the near-field while the signal gain is high. In any
potentially overload the receiver. Therefore, an
case, the AFE5805 is optimized for very short
additional pair of clamping diodes is placed between
recovery times, as shown in Figure 51.
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 51. Typical Input Overload Protection Circuit of an Ultrasound System
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