AD9042
IF Sam pling, Using the AD 9042 as a Mix-D own 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.
RECEIVE CH AIN FO R D IGITAL BEAM-FO RMING
MED ICAL ULTRASO UND USING TH E AD 9042
T he AD9042 is an excellent digitizer for digital and analog
beam-forming medical ultrasound systems. T he 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. T he 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. T he AD604 receives its
input directly from the transducer, or from an external preamp
connected to the transducer. T he AD604 contains two separate
stages. T he first stage is a preamp with a fixed gain (14 dB to
20 dB) selected by a fixed resistor. T he second stage is a
variable gain amplifier with the gain set by the AD7226 DAC.
T he gain is increased over time to compensate for the attenu-
ation of signal level in the body.
f1NYQUISTS = fSAMPLE − fSIGNAL
f2NYQUISTS = abs ( fSAMPLE − fSIGNAL
f3NYQUISTS = 2 × fSAMPLE − fSIGNAL
f4NYQUISTS = abs (2 × fSAMPLE − 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.
PRE-AMP
VGA
14 TO 20dB
–14 TO 34dB
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.
TRANSDUCER/
PRE-AMP
INPUT
AD9042
AD8041
AD604
LPF
AD7226
0
ENCODE = 10.0 MSPS
AIN = 21.4MHz
Figure 58. Using the AD9042 in Ultrasound Applications
–20
Following the AD604, a low-pass filter is used to minimize the
amount of noise presented to the ADC. T he AD8041 is used to
buffer the filter from the AD9042 input. T his function may not
be required depending on the filter configuration and PC board
partitioning. T he digital outputs of the AD9042 are then
presented to the digital system for processing.
–40
–60
8
7
8
6
2
5
3
4
–80
–100
–120
dc
1.0
2.0
3.0
4.0
5.0
FREQUENCY – MHz
Figure 57. IF-Sam pling a 21.4 MHz Input
REV. A
–23–
ADI [ ADI ]
