AFE5803 Datasheet, PDF (41/52 Pages) Texas Instruments – Fully Integrated, 8-Channel Ultrasound Analog Front End

AFE5803
www.ti.com
SLOS763A – JANUARY 2012 – REVISED JANUARY 2012
Under the no termination configuration, the input impedance of the AFE5803 is about 6 KΩ (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 73 .
450Ω
900Ω
ACTx
1800Ω
3600Ω
4500Ω
Input
INPx
INMx
LNAx
AFE
S0500-01
Figure 73. Active Termination Implementation
The AFE5803 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 73. Register [52] is used
to enable these switches. The input impedance of the LNA under the active termination configuration
approximately follows:
Z_IN
=
1+
R_f
An_LNA
2
(3)
Table 5 lists the LNA R_INs under different LNA gains. System designers can achieve fine tuning for different
probes.
The equivalent input impedance is given by Equation 4 where R_IN (8 K) and C_IN (20 pF) are the input resistance
and capacitance of the LNA.
R
Z
IN
=
f
1+ ^AnLNA
/ /C / /R
IN
IN
2
(4)
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. Since each ultrasound system includes multiple transducers with different
impedances, the flexibility of impedance configuration is a great plus.
Figure 29, Figure 30, and Figure 31 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.
Copyright © 2012, Texas Instruments Incorporated
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