Datasheet

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OVERLOAD RECOVERY
LNA
Cable
Probe
Transducer
From
Pulser
-5V
+5V
VCA8500
3kW
C
2
³ m0.1 F
C
1
3kW
R
T
BAS40
0.1 Fm
VCA8500
SBOS390A – JANUARY 2008 – REVISED MARCH 2008
are largely determined by the biasing current of the
diodes, which can be set by adjusting the 3k Ω
The VCA8500 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. This slight increase also
overload of the LNA, the PGA, or both. As discussed
depends on the value of the termination resistor (R
T
).
earlier, the LNA inputs are internally protected by a
pair of back-to-back diodes to prevent severe As Figure 65 shows, the front-end circuitry should be
overload of the LNA. Figure 65 illustrates an capacitively coupled to the LNA signal input (IN). This
ultrasound receive channel front-end that includes coupling ensures that the LNA input bias voltage of
typical external overload protection elements. Here, +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 VCA8500, 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 VCA8500 is optimized for very short
to several volts leak through the T/R switch and
recovery times, as shown in Figure 65 .
potentially overload the receiver. Therefore, an
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 65. Typical Input Overload Protection Circuit of an Ultrasound System
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