Datasheet

LNA
Cable
Probe
Transducer
From
Pulser
-5V
+5V
AFE5804
3kW
C
2
³ m0.1 F
C
1
3kW
R
T
BAS40
0.1 Fm
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 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 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 of back-to-back diodes to prevent severe
depends on the value of the termination resistor (R
T
).
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, an
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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