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AD9020 Datasheet(PDF) 6 Page  Analog Devices 

AD9020 Datasheet(HTML) 6 Page  Analog Devices 
6 / 12 page AD9020 –6– REV. C THEORY OF OPERATION Refer to the AD9020 block diagram. As shown, the AD9020 uses a modified “flash,” or parallel, A/D architecture. The analog input range is determined by an external voltage refer ence (+VREF and –VREF), nominally ± 1.75 V. An internal resistor ladder divides this reference into 512 steps, each rep resenting two quantization levels. Taps along the resistor ladder (1/4REF, 1/2REF and 3/4REF) are provided to optimize linearity. Rated performance 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 sig nificant bit (LSB) is determined by a unique interpolation scheme between adjacent comparators. The decoding logic processes the comparator outputs and provides a 10bit 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 architecture 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 num ber 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, Two’s Complement and Inverted Two’s Complement coding (see Table I). APPLICATIONS Many of the specifications used to describe analog/digital converters have evolved from system performance require ments in these applications. 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 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 analogtodigital converters (ADCs). Frequency domain characterization of the AD9020 provides sig naltonoise ratio (SNR) and harmonic distortion data to simplify selection of the ADC. Receiver sensitivity is limited by the SignaltoNoise Ratio of the system. The SNR for an ADC is measured in the fre quency domain and calculated with a Fast Fourier Transform (FFT). The SNR equals the ratio of the fundamental compo nent 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 (nonlinearities, delay mismatch, vary ing input impedance, etc.) in the device transfer function. In the ADC, these spurious signals appear as Harmonic Dis tortion. 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 worstcase harmonic (usually the 2nd or 3rd). TwoTone Intermodulation Distortion (IMD) is a frequently cited specification in receiver design. In narrowband receiv ers, thirdorder IMD products result in spurious signals in the passband of the receiver. Like mixers and amplifiers, the ADC is characterized with two, equalamplitude, 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 AD9020 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 AD9020 specifications 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 sampletosample 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 degrade dynamic performance as the ana log 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 (ENOBs). ENOB is calculated with a sine wave curve fit and equals: ENOB = N – LOG2 [Error (measured)/Error (ideal)] N is the resolution (number of bits) of the ADC. The measured error is the actual rms error calculated from the converter out puts 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 band width is a good indicator of a converter’s stewing capabilities. 
Similar Part No.  AD9020_15 

Similar Description  AD9020_15 


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