Choosing External Component Values

There are three key parameters to consider in the design:

Noise, signal bandwidth, and gain. Refer to

Figure 2.

The best noise performance for a given transmit/receive

switch R

200

Ω; selecting the matching termination resistance R

by reducing I

which optimizes the Noise Figure (NF). For this circuit, with

R

is set to 50

Ω and R

The signal bandwidth is determined by the selection of L

L

the first stage. R

that introduces an additional highpass pole. The filter values

can be chosen to create a sharper high frequency roll off of

the closed loop gain. For R

nal (V

served. By increasing C

put series resistance with a small resistor, the stability and

harmonic distortion performance can be improved for large

signals. This filter can also be designed as a multi-pole But-

terworth filter but care must be taken to ensure stability with

the desired load over the operating temperature range.

The passband gain is customer selected by setting R

R

for minimal interaction with other parameters.

Capacitor C

tion of the gm input stage with values of C

= 1k for the applications described below.

Calculating and Measuring the Noise

The circuit input referred noise is best calculated using a

SPICE model where the external components can be opti-

mized

for

the

transducer

source

impedance

and

transmit/receive switch impedance. The SPICE model for

the CLC5509 is available on the NSC web site. Refer to the

figures for total noise performance over temperature and

supply at 3mA. Once the noise is modeled and circuit param-

eters chosen the evaluation board can be used to measure

actual noise performance.

To measure the CLC5509 input referred noise vs. other

noise sources, several key steps should be followed. The

bench setup is fairly simple using the evaluation board and a

spectrum analyzer. (If a noise figure meter is available that is

even easier yet.) The procedure requires calibrating out the

spectrum analyzer background noise, and other noise

sources from the CLC5509 noise. Since the thermal noise of

a resistor is well known, add a series resistor R

signal source V

noise measurements. Several R

’’reference’’ noise sources. The values chosen depend on

the R

of 0, 12.5, 25, 50

Ω should be used. If R

(R

Start by connecting the analyzer input to the evaluation

board output. Remove R

nect R

erage them for each R

the result by the analyzer and circuit gain to make the noise

input referred. Subtract the R

each value. Compare the result to the theoretical noise val-

ues. They should agree closely over the R

This verifies the test method. The CLC5509 noise is the R

0

Ω data point. A similar procedure can be used to remove

the T/R switch noise by varying the T/R bias current I

The total circuit noise performance can now be optimized for

R

Evaluation Board

Evaluation boards are available for customer product evalu-

ation for the 8-pin SOIC. Evaluation kits that contain an

evaluation board and CLC5509 samples can be obtained by

calling National Semiconductor’s Customer Service Cen-

ter. The evaluation kit number is CLC730101. The evalua-

tion board utilizes surface mount components. The corner

frequencies are set to

∼ 0.9MHz to 12.5MHz with a pass-

band gain set at 20dB. The highpass filter is set at R

C

Increasing C

proves distortion for large signals. R

tor that terminates the output and isolates cable capaci-

tance, for minimum distortion, over the frequency band of

interest. An R

source resistor R

3mA. The expected input referred noise based on bench

measurements on similar boards is

∼ 0.6nV

oraNFof

2dB. The noise can be optimized with slight variations in R

R

evaluation board both the voltage drop and R

compensated for. The R

the addition of the T/R switch. The V

moved, to a first order, by adding a second switch in series

with the feedback gain setting resistor R

restore the input DC level to

∼ 0V. This T/R switch diode

should be biased with a resistor (

∼ R

passed with a 0.1µF cap to maintain the same AC perfor-

mance as the evaluation board without the switches.

CLC5509 Applications

The signal path for a typical ultrasound transceiver is shown

in

Figure 3.

DS101304-32

FIGURE 2. Complete Circuit

DS101304-33

FIGURE 3.

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