AFE5803ZCF datasheet(41/53 Pages) TI2

41 / 53 page

S0500-01

AFE

LNAx

ACTx

INPx

INMx

Input

4500

3600

1800

900

450

LNA

/ /C

/ /R

AFE5803

www.ti.com

SLOS763A

– JANUARY 2012 – REVISED JANUARY 2012

Under the no termination configuration, the input impedance of the AFE5803 is about 6 K

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 73 .

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:

(3)

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 4 where RIN (8 K) and CIN (20 pF) are the input resistance

and capacitance of the LNA.

(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.

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