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ADRF6510 데이터시트(PDF) 18 Page - Analog Devices |
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ADRF6510 데이터시트(HTML) 18 Page - Analog Devices |
18 / 32 page ADRF6510 Data Sheet Rev. B | Page 18 of 32 –100 –135 0.5 1.0 1.5 2.0 2.5 3.0 FREQUENCY (MHz) –130 –125 –120 –115 –110 –105 BANDWIDTH = 1MHz GAIN = 40dB GAIN = 20dB GAIN = 0dB Figure 48. Total Output Noise with a 1 MHz Corner Frequency for Three Different Gain Settings Note that the noise spectral density outside the filter bandwidth is limited by the fixed VGA output noise. It may be necessary to use an external, fixed-frequency, passive filter prior to an analog- to-digital conversion to prevent noise aliasing from degrading the signal-to-noise ratio. The higher the sampling rate relative to the maximum ADRF6510 corner frequency setting to be used, the lower the order of the external filter. DISTORTION CHARACTERISTICS The distortion performance of the ADRF6510 is similar to its noise performance. The filters and the VGAs contribute to the overall distortion and signal handling capabilities. Furthermore, the front end must also cope with out-of-band signals that can be larger than the in-band signals. These out-of-band signals are filtered before reaching the VGA. It is important to understand the signals presented to the ADRF6510 and to match these signals with the input and output characteristics of the part. When the gain is low, the distortion is typically limited by the input section because the output is not driven to its maximum capacity. When the gain is high, the distortion is likely limited by the output section because the input is not driven to its maximum capacity. An exception to this is when the input is driven with a small desired signal in combination with a large out-of-band signal. In this case, the out-of-band signal may drive the input to distort. As long as the input is not overdriven, the out-of-band signal is removed by the filter. A high VGA gain is still needed to raise the small desired signal to a higher level at the output. The overall distortion introduced by the part depends on the input drive level, including the out-of-band signals, and the desired output signal level. As noted in the Input Buffers section, the input section can handle a total signal level of 1 V p-p for a 6 dB preamplifier and 500 mV p-p for a 12 dB preamplifier with >50 dBc harmonic distortion. This includes both in-band and out-of-band signals. To distinguish and quantify the distortion performance of the input section, two different IP3 specifications are presented. The first is called in-band IP3 and refers to a two-tone test where the signals are inside the filter bandwidth. This is exactly the same figure of merit familiar to communications engineers in which the third-order intermodulation level, IM3, is measured. To quantify the effect of out-of-band signals, a new out-of-band (OOB) IIP3 figure of merit is introduced. This test also involves a two-tone stimulus; however, the two tones are placed out-of- band so that the lower IM3 product lands in the middle of the filter pass band. At the output, only the IM3 product is visible because the original two tones are filtered out. To calculate the OOB IP3 at the input, the IM3 level is referred to the input by the overall gain. The OOB IIP3 allows the user to predict the impact of out-of-band blockers or interferers at an arbitrary signal level on the in-band performance. The ratio of the desired input signal level to the input-referred IM3 at a given blocker level represents a signal-to-distortion limit imposed by the out-of-band signals. MAXIMIZING THE DYNAMIC RANGE The role of the ADRF6510 is to increase the level of a variable in-band signal while minimizing out-of-band signals. Ideally, this is achieved without degrading the SNR of the incoming signal or introducing distortion to the incoming signal. The first goal is to maximize the output signal swing, which can be defined by the ADC input range or the input signal capacity of the next analog stage. For the complex waveforms often encoun- tered in communication systems, the peak-to-average ratio, or crest factor, must be considered when choosing the peak-to-peak output. From the chosen output signal and the maximum gain of the ADRF6510, the minimum input level can be defined. Lower signal levels do not yield the maximum output and suffer a greater degradation in SNR. As the input signal level increases, the VGA gain is reduced from its maximum gain point to maintain the desired fixed output level. The output noise, initially dominated by the filter, follows the gain reduction, yielding a progressively better SNR. At some point, the VGA gain drops sufficiently that the constant VGA noise becomes dominant, resulting in a constant SNR from that point. From the perspective of SNR alone, the maximum input level is reached when the VGA reaches its minimum gain. Distortion must also be considered when maximizing the dynamic range. At low and moderate signal levels, the output distortion is constant and assumed to be adequate for the selected output level. At some point, the input signal becomes large enough that distortion at the input limits the system. The maximum tolerable input signal depends on whether the input distortion becomes unacceptably large or the minimum gain is reached. |
유사한 부품 번호 - ADRF6510_17 |
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유사한 설명 - ADRF6510_17 |
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