VCA8500_0803 Datasheet, PDF (27/40 Pages) Texas Instruments – 8-Channel, Ultralow-Power, Variable Gain Amplifier with Low-Noise Pre-Amp

VCA8500
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OVERLOAD RECOVERY
The VCA8500 is designed in particular for ultrasound
applications where the front-end device is required to
recover very quickly from an overload condition. Such
an overload can either be the result of a transmit
pulse feed-through or a strong echo, which can cause
overload of the LNA, the PGA, or both. As discussed
earlier, the LNA inputs are internally protected by a
pair of back-to-back diodes to prevent severe
overload of the LNA. Figure 65 illustrates an
ultrasound receive channel front-end that includes
typical external overload protection elements. Here,
four high-voltage switching diodes are configured in a
bridge configuration and form the transmit/receive
(T/R) switch. During the transmit period, high voltage
pulses from the pulser are applied to the transducer
elements and the T/R switch isolates the sensitive
LNA input from being damaged by the high voltage
signal. However, it is common that fast transients up
to several volts leak through the T/R switch and
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
+5V
SBOS390A – JANUARY 2008 – REVISED MARCH 2008
are largely determined by the biasing current of the
diodes, which can be set by adjusting the 3kΩ
resistor values; for example, setting a higher current
level may lead to an improved switching characteristic
and reduced noise contribution. A typical front-end
protection circuitry may add in the order of 2nV/√Hz
of noise to the signal path. This slight increase also
depends on the value of the termination resistor (R_T).
As Figure 65 shows, the front-end circuitry should be
capacitively coupled to the LNA signal input (IN). This
coupling ensures that the LNA input bias voltage of
+2.4V is maintained and decoupled from any other
biasing voltage before the LNA.
Within the VCA8500, overload can occur in either the
LNA or the PGA. LNA overload can occur as the
result of T/R switch feed-through; and the PGA can
be driven into an overload condition by a strong echo
in the near-field while the signal gain is high. In any
case, the VCA8500 is optimized for very short
recovery times, as shown in Figure 65.
3kW
Cable
C₁
C₂
³ 0.1mF
Probe
From
Transducer Pulser
BAS40
3kW
-5V
R_T
0.1mF
LNA
VCA8500
Figure 65. Typical Input Overload Protection Circuit of an Ultrasound System
Copyright © 2008, Texas Instruments Incorporated
Product Folder Link(s): VCA8500
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