AD8331 datasheet(26/32 Pages) AD

26 / 32 page

AD8331/AD8332

Rev. C | Page 26 of 32

COMM

VIP

LOP

COML

LMD

LON

VPS

INH

COMM

ENBV

ENBL

GAIN

0.1

µF

HILO

+5V

CFB

0.018

µF

VOH

VOL

VOUT

VPOS

+5V

VCM

RCLMP

VIN

0.1

µF

AD8331

MODE

GAIN

MODE

VCM

CLMP

HILO

VIN

Figure 74. Disabling the LNA

INH

LNA

Ω

LMD

LOP

ADG736

LON

200

Ω

0.1

µF

18nF

SELECT RFB

280

Ω

1.13k

Ω

AD8332

Ω

Figure 75. Accommodating Multiple Sources

MEASUREMENT CONSIDERATIONS

Figure 51 through Figure 55 show typical measurement

configurations and proper interface values for measurements

with 50 Ω conditions.

Short-circuit input noise measurements are made using

Figure 53. The input-referred noise level is determined by

dividing the output noise by the numerical gain between Point

A and Point B and accounting for the noise floor of the

spectrum analyzer. The gain should be measured at each

frequency of interest and with low signal levels since a 50 Ω

load is driven directly. The generator is removed when noise

measurements are made.

ULTRASOUND TGC APPLICATION

The AD8332 ideally meets the requirements of medical and

industrial ultrasound applications. The TGC amplifier is a key

subsystem in such applications, since it provides the means for

echolocation of reflected ultrasound energy.

Figure 76 through Figure 78 are schematics of a dual, fully

differential system using the AD8332 and AD9238 12-bit high

speed ADC with conversion speeds as high as 65 MSPS. In this

example, the VGA outputs are dc-coupled, using the reference

output of the ADC and a level shifter to center the common-

mode output voltage to match that of the converter. Consult the

data sheet of the converter to determine whether external CMV

biasing is required. AC coupling is recommended if the CMV

of the VGA and ADC are widely disparate.

Using the circuit shown, and a high speed ADC FIFO

evaluation kit connected to a laptop PC, an FFT can be

performed on the AD8332. With the on-board clock of 20 MHz,

and minimal low-pass filtering, and both channels driven with a

1 MHz filtered sine wave, the THD is –75 dB, noise floor –93 dB

and HD2 –83 dB.

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