Datasheet

IN IN IN
LNA
R
Z / /C / /R
A
1
2
=
n
+
f
IN
LNA
R
Z
A
1
2
=
n
+
f
S0500-01
AFE
LNAx
ACTx
INPx
INMx
Input
4500Ω
3600Ω
1800Ω
900Ω
450Ω
AFE5808A
SLOS729B –OCTOBER 2011–REVISED APRIL 2012 www.ti.com
Under the no termination configuration, the input impedance of the AFE5808A is about 6KΩ (8 K//20 pF) at 1
MHz. Passive termination requires external termination resistor R
t
, which contributes to additional thermal noise.
The LNA supports active termination with programmable values, as shown in Figure 85 .
Figure 85. Active Termination Implementation
The AFE5808A has four pre-settings 50,100, 200 and 400 Ω which are configurable through the registers. Other
termination values can be realized by setting the termination switches shown in Figure 85. Register [52] is used
to enable these switches. The input impedance of the LNA under the active termination configuration
approximately follows:
(6)
Table 5 lists the LNA R
IN
s under different LNA gains. System designers can achieve fine tuning for different
probes.
The equivalent input impedance is given by Equation 7 where R
IN
(8 K) and C
IN
(20 pF) are the input resistance
and capacitance of the LNA.
(7)
Therefore the ZIN is frequency dependent and it decreases as frequency increases shown in Figure 10. Since 2
MHz~10 MHz is the most commonly used frequency range in medical ultrasound, this rolling-off effect doesn’t
impact system performance greatly. Active termination can be applied to both CW and TGC modes. Since each
ultrasound system includes multiple transducers with different impedances, the flexibility of impedance
configuration is a great plus.
Figure 32, Figure 33, and Figure 34 shows the NF under different termination configurations. It indicates that no
termination achieves the best noise figure; active termination adds less noise than passive termination. Thus
termination topology should be carefully selected based on each use scenario in ultrasound.
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