ADA4896-2ACPZ-R7 datasheet(23/28 Pages) AD

23 / 28 page

Data Sheet

ADA4896-2/ADA4897-1/ADA4897-2

Rev.

| Page 23 of 28

ADA4896-2/ADA4897-1/ADA4897-2 in the Ultrasound System

LNA

AD7982

18-BIT ADC

2.5V

4nF

50Ω

CHANNEL H

CHANNEL A

GENERATION

CWI+

CWI–

AD9279

CFILT

RFILT

1.5V

RFILT

AD7982

18-BIT ADC

2.5V

4nF

50Ω

CWQ+

CWQ–

CFILT

RFILT

1.5V

RFILT

ADA4896-2/

ADA4897-1/

ADA4897-2

ADA4896-2/

ADA4897-1/

ADA4897-2

ADA4896-2/

ADA4897-1/

ADA4897-2

ADA4896-2/

ADA4897-1/

ADA4897-2

Figure 55. Using the ADA4896-2/ADA4897-1/ADA4897-2 as Filters, I-to-V Converters, Current Summers, and ADC Drivers After the I/Q Outputs of the AD9279

The ADA4896-2/ADA4897-1/ADA4897-2 are used in the CW

Doppler path in the ultrasound application after the I/Q demod-

ulators of the AD9279. Doppler signals can be typically between

100 Hz to 100 kHz. The low noise floor and high dynamic range

of the ADA4896-2/ADA4897-1/ADA4897-2 make them excellent

choices for processing weak Doppler signals.

The rail-to-rail output and the high output current drive of

the ADA4896-2/ADA4897-1/ADA4897-2 make them suitable

candidates for the I-to-V converter, current summer, and ADC

driver.

Figure 55 shows an interconnection block diagram of all

eight channels of the AD9279. Two stages of the ADA4896-2

amplifiers are used. The first stage performs an I-to-V conver-

sion and filters the high frequency content that results from

the demodulation process. The second stage of the ADA4896-2

amplifiers is used to sum the output currents of multiple AD9279

devices, to provide gain, and to drive the AD7982 device, an

18-bit SAR ADC.

The output-referred noise of the CW signal path depends on

the LNA gain, the selection of the first stage summing amplifier,

and the value of RFILT. To determine the output-referred noise, it

is important to know the active low-pass filter (LPF) values RA,

RFILT, and CFILT, as shown as Figure 55. Typical filter values for

all eight channels of a single AD9279 are 100 Ω for RA, 500 Ω

for RFILT, and 2.0 nF for CFILT; these values implement a 100 kHz,

single-pole LPF.

The gain of the I-to-V converter can be increased by increasing

the filter resistor, RFILT. To keep the corner frequency unchanged,

decrease the filter capacitor, CFILT, by the same factor. The factor

limiting the magnitude of the gain is the output swing and drive

capability of the op amp selected for the I-to-V converter, in this

example, the ADA4896-2/ADA4897-1/ADA4897-2. Because

any amplifier has limited drive capability, a finite number of

channels can be summed.

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