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AD9020JE Arkusz danych(PDF) 6 Page - Analog Devices |
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AD9020JE Arkusz danych(HTML) 6 Page - Analog Devices |
6 / 12 page AD9020 –6– REV. A THEORY OF OPERATION Refer to the AD9020 block diagram. As shown, the AD9020 uses a modified “flash,” or parallel, A/D architecture. The ana- log input range is determined by an external voltage reference (+VREF and –VREF), nominally ±1.75 V. An internal resistor lad- der divides this reference into 512 steps, each representing two quantization levels. Taps along the resistor ladder (1/4REF, 1/2REF and 3/4REF) are provided to optimize linearity. Rated per- formance is achieved by driving these points at 1/4, 1/2 and 3/4, respectively, of the voltage reference range. The A/D conversion for the nine most significant bits (MSBs) is performed by 512 comparators. The value of the least signifi- cant bit (LSB) is determined by a unique interpolation scheme between adjacent comparators. The decoding logic processes the comparator outputs and provides a 10-bit code to the output stage of the converter. Flash architecture has an advantage over other A/D architec- tures because conversion occurs in one step. This means the performance of the converter is primarily limited by the speed and matching of the individual comparators. In the AD9020, an innovative interpolation scheme takes advantage of flash archi- tecture but minimizes the input capacitance, power and device count usually associated with that method of conversion. These advantages occur by using only half the normal number of input comparator cells to accomplish the conversion. In addi- tion, a proprietary decoding scheme minimizes error codes. In- put control pins allow the user to select from among Binary, Inverted Binary, Twos Complement and Inverted Twos Complement coding (see AD9020 Truth Table). APPLICATIONS Many of the specifications used to describe analog/digital con- verters have evolved from system performance requirements in these applications. Different systems emphasize particular speci- fications, depending on how the part is used. The following ap- plications highlight some of the specifications and features that make the AD9020 attractive in these systems. Wideband Receivers Radar and communication receivers (baseband and direct IF digitization), ultrasound medical imaging, signal intelligence and spectral analysis all place stringent ac performance require- ments on analog-to-digital converters (ADCs). Frequency do- main characterization of the AD9020 provides signal-to-noise ratio (SNR) and harmonic distortion data to simplify selection of the ADC. Receiver sensitivity is limited by the Signal-to-Noise Ratio of the system. The SNR for an ADC is measured in the frequency do- main and calculated with a Fast Fourier Transform (FFT). The SNR equals the ratio of the fundamental component of the sig- nal (rms amplitude) to the rms value of the noise. The noise is the sum of all other spectral components, including harmonic distortion, but excluding dc. Good receiver design minimizes the level of spurious signals in the system. Spurious signals developed in the ADC are the re- sult of imperfections in the device transfer function (non- linearities, delay mismatch, varying input impedance, etc.). In the ADC, these spurious signals appear as Harmonic Distortion. Harmonic Distortion is also measured with an FFT and is speci- fied as the ratio of the fundamental component of the signal (rms amplitude) to the rms value of the worst case harmonic (usually the 2nd or 3rd). Two-Tone Intermodulation Distortion (IMD) is a frequently cited specification in receiver design. In narrow-band receivers, third- order IMD products result in spurious signals in the pass band of the receiver. Like mixers and amplifiers, the ADC is charac- terized with two, equal-amplitude, pure input frequencies. The IMD equals the ratio of the power of either of the two input sig- nals to the power of the strongest third-order IMD signal. Un- like mixers and amplifiers, the IMD does not always behave as it does in linear devices (reduced input levels do not result in pre- dictable reductions in IMD). Performance graphs provide typical harmonic and SNR data for the AD9020 for increasing analog input frequencies. In choos- ing an A/D converter, always look at the dynamic range for the analog input frequency of interest. The AD9020 specifications provide guaranteed minimum limits at three analog test frequencies. Aperture Delay is the delay between the rising edge of the EN- CODE command and the instant at which the analog input is sampled. Many systems require simultaneous sampling of more than one analog input signal with multiple ADCs. In these situ- ations, timing is critical and the absolute value of the aperture delay is not as critical as the matching between devices. Aperture Uncertainty, or jitter, is the sample-to-sample variation in aperture delay. This is especially important when sampling high slew rate signals in wide bandwidth systems. Aperture un- certainty is one of the factors that degrade dynamic performance as the analog input frequency is increased. |
Podobny numer części - AD9020JE |
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Podobny opis - AD9020JE |
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