AD9042STSLASHPCB datasheet(23/24 Pages) AD

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AD9042

REV. A

–23–

IF Sampling, Using the AD9042 as a Mix-Down Stage

Since performance of the AD9042 extends beyond the baseband

region into the second and third Nyquist zone, the converter

may find many uses as a mix down converter in both narrowband

and wideband applications. Many common IF frequencies exist

in this range of frequencies. If the ADC is used to sample these

signals, they will be aliased down to baseband during the sampling

process in much the same manner that a mixer will down-convert a

signal. For signals in various Nyquist zones, the following

equation may be used to determine the final frequency after

aliasing.

1NYQUISTS = fSAMPLE − fSIGNAL

2NYQUISTS = abs ( f SAMPLE − fSIGNAL )

3NYQUISTS = 2 × fSAMPLE − fSIGNAL

4NYQUISTS = abs (2 × f SAMPLE − fSIGNAL )

Using the converter to alias down these narrowband or wideband

signals has many potential benefits. First and foremost is the

elimination of a complete mixer stage, along with amplifiers,

filters and other devices, reducing cost and power dissipation.

One common example is the digitization of a 21.4 MHz IF

using a 10 MSPS sample clock. Using the equation above for

the fifth Nyquist zone, the resultant frequency after sampling is

1.4 MHz. Figure 57 shows performance under these conditions.

Even under these conditions, the AD9042 typically maintains

better than 80 dB SFDR.

FREQUENCY – MHz

–80

–120

–40

–100

–20

–60

5.0

1.0

2.0

3.0

4.0

ENCODE = 10.0 MSPS

AIN = 21.4MHz

Figure 57. IF-Sampling a 21.4 MHz Input

RECEIVE CHAIN FOR DIGITAL BEAM-FORMING

MEDICAL ULTRASOUND USING THE AD9042

The AD9042 is an excellent digitizer for digital and analog

beam-forming medical ultrasound systems. The price/

performance ratio of the AD9042 allows ultrasound designers

the luxury of using state-of-the-art ADCs without jeopardizing

their cost budgets. ADC performance is critical for image

quality. The high dynamic range and excellent noise

performance of the AD9042 enable higher image quality

medical ultrasound systems.

Figure 58 shows the AD9042 used in one channel of the receive

chain of a medical ultrasound system. The AD604 receives its

input directly from the transducer, or from an external preamp

connected to the transducer. The AD604 contains two separate

stages. The first stage is a preamp with a fixed gain (14 dB to

20 dB) selected by a fixed resistor. The second stage is a

variable gain amplifier with the gain set by the AD7226 DAC.

The gain is increased over time to compensate for the attenu-

ation of signal level in the body.

AD9042

AD7226

AD604

LPF

AD8041

VGA

–14 TO 34dB

PRE-AMP

14 TO 20dB

TRANSDUCER/

PRE-AMP

INPUT

Figure 58. Using the AD9042 in Ultrasound Applications

Following the AD604, a low-pass filter is used to minimize the

amount of noise presented to the ADC. The AD8041 is used to

buffer the filter from the AD9042 input. This function may not

be required depending on the filter configuration and PC board

partitioning. The digital outputs of the AD9042 are then

presented to the digital system for processing.

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