AFE5808ZCF datasheet(50/68 Pages) TI1

50 / 68 page

S0500-01

AFE

LNAx

ACTx

INPx

INMx

Input

4500Ω

3600Ω

1800Ω

900Ω

450Ω

LNA

/ /C

/ /R

AFE5808

SLOS688C – SEPTEMBER 2010 – REVISED APRIL 2012

www.ti.com

Under the no termination configuration, the input impedance of the AFE5808 is about 6K

Ω (8K//20pF) 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 84 .

Figure 84. Active Termination Implementation

The AFE5808 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 the above figure. 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 (8K) and CIN (20pF) 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

2MHz to approximately 10MHz 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, andFigure 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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