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AD9060 Arkusz danych(PDF) 6 Page - Analog Devices |
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AD9060 Arkusz danych(HTML) 6 Page - Analog Devices |
6 / 12 page REV. B AD9060 –6– THEORY OF OPERATION Refer to the Functional Block Diagram. As shown, the AD9060 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 ladder 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 (MSB) is performed by 512 comparators. The value of the least significant bit (LSB) is determined by a unique interpolation scheme between adjacent comparators. The decoding logic processes the com- parator outputs and provides a 10-bit code to the output stage of the converter. Flash architecture has an advantage over other A/D architectures because conversion occurs in one step. This means the perfor- mance of the converter is limited primarily by the speed and matching of the individual comparators. In the AD9060, an innova- tive interpolation scheme takes advantage of the flash architecture but minimizes the input capacitance, power, and device count usually associated with that method of conversion. These advantages occur because of using only half the normal number of input comparator cells to accomplish the conversion. In addition, a proprietary decoding scheme minimizes error codes. Input control pins allow the user to select from among binary, inverted binary, twos complement, and inverted twos complement coding (see Table I, the AD9060 Truth Table). APPLICATIONS Many of the specifications used to describe A/D converters have evolved from system performance requirements in these applica- tions. Different systems emphasize particular specifications, depending on how the part is used. The following applications highlight some of the specifications and features that make the AD9060 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 domain characterization of the AD9060 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 (SNR) of the system. The SNR for an ADC is measured in the frequency domain and calculated with a Fast Fourier Transform (FFT). The SNR equals the ratio of the fundamental component of the signal (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 result of imperfections in the device transfer function (non- linearities, delay mismatch, varying input impedance, and so on). In the ADC, these spurious signals appear as Harmonic Distortion. Harmonic Distortion is also measured with an FFT and is specified as the ratio of the fundamental component of the signal (rms amplitude) to the rms value of the worst- case harmonic (usually the second or third). 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 signals to the power of the strongest third order IMD signal. Unlike mixers and amplifiers, the IMD does not always behave as it does in linear devices (reduced input levels do not result in predictable reductions in IMD). Performance graphs provide typical harmonic and SNR data for the AD9060 for increasing analog input frequencies. In choosing an A/D converter, always look at the dynamic range for the analog input frequency of interest. The AD9060 speci- fications provide guaranteed minimum limits at three analog test frequencies. Aperture Delay is the delay between the rising edge of the ENCODE 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 situations 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 uncertainty is one of the factors that degrades dynamic performance as the analog input frequency is increased. Digitizing Oscilloscopes Oscilloscopes provide amplitude information about an observed waveform with respect to time. Digitizing oscilloscopes must accurately sample this signal without distorting the information to be displayed. One figure of merit for the ADC in these applications is Effective Number of Bits (ENOB). ENOB is calculated with a sine wave curve fit and equals ENOB N Error measured Error ideal = () ( ) [] – log 2 N is the resolution (number of bits) of the ADC. The measured error is the actual rms error calculated from the converter outputs with a pure sine wave input. The Analog Bandwidth of the converter is the analog input fre- quency at which the spectral power of the fundamental signal is reduced 3 dB from its low frequency value. The analog bandwidth is a good indicator of a converter’s slewing capabilities. The Maximum Conversion Rate is defined as the encode rate at which the SNR for the lowest analog signal test frequency tested drops by no more than 3 dB below the guaranteed limit. Imaging Visible and infrared imaging systems each require similar char- acteristics from ADCs. The signal input (from a CCD camera or multiplexer) is a time division multiplexed signal consisting of a series of pulses whose amplitude varies in direct proportion to the intensity of the radiation detected at the sensor. These vary- ing levels are then digitized by applying ENCODE commands at the correct times, as shown in Figure 1. |
Podobny numer części - AD9060_15 |
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Podobny opis - AD9060_15 |
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