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AD9022AZ Arkusz danych(PDF) 7 Page - Analog Devices |
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AD9022AZ Arkusz danych(HTML) 7 Page - Analog Devices |
7 / 12 page AD9022 REV. B –7– is present when the unit is strobed with an ENCODE com- mand. The conversion process begins on the rising edge of this pulse, which should conform to the minimum and maximum pulsewidth requirements shown in the specifications. Operation below the recommended encode rate (4 MSPS) may result in excessive droop in the internal T/H devices–leading to large dc and ac errors. The held analog value of the first track-and-hold is applied to a 5-bit flash converter and a second T/H. The 5-bit flash con- verter resolves the most significant bits (MSBs) of the held analog voltage. These five bits are reconstructed via a 5-bit DAC and subtracted from the original T/H output signal to form a residue signal. A second T/H holds the amplified residue signal while it is en- coded with a second 5-bit flash ADC. Again the five bits are reconstructed and subtracted from the second T/H output to form a residue signal. This residue is amplified and encoded with a 4-bit flash ADC to provide the three least significant bits (LSBs) of the digital output and one bit of error correction. Digital Error Correction logic aligns the data from the three flash converters and presents the result as a 12-bit parallel digi- tal word. The output stage of the AD9022 is TTL. Output data may be strobed on the rising edge of the ENCODE command. AD9022 IN RECEIVER APPLICATIONS Advances in semiconductor processes have resulted in low cost digital signal processing (DSP) and analog signal processing which can help create cost effective alternative receiver designs. Today, an all-digital receiver allows tuning, demodulation, and detection of receiver signals in the digital domain. By digitizing IF signals directly, and utilizing digital techniques, it becomes possible to make significant improvements in receiver design. For high frequency IFs, the ADC is the key to the receiver’s performance. Unfortunately, the specifications frequently used by receiver designers and analog-to-digital (ADC) manufactur- ers are often very different. Noise Figure and Intercept Point are common measures of noise and linearity in analog RF system design. ADCs are more frequently specified in terms of SNR and harmonic distortion. Noise Noise figure (NF) is a measure of receiver sensitivity and is defined as the degradation of signal-to-noise ratio (SNR) as a signal passes through a device. In equation form: NF = SNR (in) – SNR (out) Noise figure is a bandwidth invariant parameter for reasonably narrow bandwidths in most devices. The system noise figure for a combination of amplifiers and mixers, for instance, can be analyzed without regard to the information bandwidth. Thermal noise contribution from the ADC behaves in a similar fashion; however, the spectral density of quantization noise is a function of the sample rate. In addition, the spectral density of the quantization noise is flat only in an ADC with perfect linear- ity, i.e., perfect 1 LSB step sizes. To analyze the system noise performance, ADC noise figure is calculated by normalizing the SNR of the ADC output to a 1 Hz bandwidth. This result is given by: SNR (/Hz) = SNR + 10 log10 (FS/2) where FS is the sample rate. THEORY OF OPERATION Refer to the block diagram. The AD9022 employs a three-pass subranging architecture and digital error correction. This combination of design techniques ensures 12-bit accuracy at relatively low power. Analog input signals are immediately attenuated through a resistor divider and applied directly to the sampling bridge of the track-and-hold (T/H). The T/H holds whatever analog value 0 –100 –70 –90 –80 –40 –60 –50 –30 –20 –10 FREQUENCY – MHz 10 0 AIN = 1.2MHz AIN = –1.0dBFS SNR = 66.7dB THD = 77.51dB SFDR = 79.49dBFS Figure 8. FFT Plot 0 –100 –70 –90 –80 –40 –60 –50 –30 –20 –10 FREQUENCY – MHz 10 AIN = 9.6MHz AIN = –1.0dBFS SNR = 66.05dB THD = 74.28dB SFDR = 75.32dBFS 0 Figure 9. FFT Plot 10.0 0.0 8.0 6.0 4.0 2.0 AIN1 = 8.9MHz AIN2 = 9.8MHz AIN1 = 7.0dBFS AIN2 = 7.0dBFS SFDR = 80.62dBFS 0 100 80 40 60 20 FREQUENCY – MHz 120 Figure 10. Two-Tone FFT |
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