AFE5808A datasheet(50/66 Pages) TI1

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S0500-01

AFE

LNAx

ACTx

INPx

INMx

Input

4500Ω

3600Ω

1800Ω

900Ω

450Ω

LNA

/ /C

/ /R

AFE5808A

SLOS729B – OCTOBER 2011 – REVISED APRIL 2012

www.ti.com

Under the no termination configuration, the input impedance of the AFE5808A is about 6K

Ω (8 K//20 pF) at 1

MHz. Passive termination requires external termination resistor Rt, 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 RINs under different LNA gains. System designers can achieve fine tuning for different

probes.

The equivalent input impedance is given by Equation 7 where RIN (8 K) and CIN (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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