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AD9042STPCB Datasheet(PDF) 23 Page - Analog Devices |
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AD9042STPCB Datasheet(HTML) 23 Page - Analog Devices |
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23 / 24 page  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. f 1NYQUISTS = fSAMPLE − fSIGNAL f 2NYQUISTS = abs ( f SAMPLE − fSIGNAL ) f 3NYQUISTS = 2 × fSAMPLE − fSIGNAL f 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 0 –80 –120 –40 –100 –20 –60 dc 5.0 1.0 2.0 3.0 4.0 ENCODE = 10.0 MSPS AIN = 21.4MHz 8 7 8 6 25 34 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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