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LMH6515 bảng dữ liệu(PDF) 3 Page - Texas Instruments |
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LMH6515 bảng dữ liệu(HTML) 3 Page - Texas Instruments |
3 / 14 page 2 SIGNAL PATH designer Selecting Amplifiers, ADCs, and Clocks for High-Performance Signal Paths Ignoring the filter, the noise of the overall cascaded path shown is given by Frii’s equation: Where FLNA = noise factor of LNA FDRIVER = noise factor of driver stage FADC = noise factor of ADC GLNA = Gain of LNA GDRIVER = Gain of driver stage The noise of the ADC driver is divided by the gain of the LNA and consequently, it is best to select the lowest-noise LNA available and take as much gain as possible at this first stage. Since the noise of the driver is divided by the LNA gain, it becomes less critical to the overall noise performance. In fact, the further along the signal path, the less critical the noise performance of each stage becomes. The building block after the LNA is the ADC-driver stage. In a system that responds down to signals at 0 Hz, a DC-coupled amplifier is the only choice, while in an AC-coupled system, a transformer can also be used. However, transformers are limited in their frequency range of operation and can have poor differential output balance, which is important when driving differential-input ADCs. When providing gain, transformers also multiply the source impedance driving the ADC by the transformer turns ratio squared. This reduces the pole frequency formed with the ADC-input capacitance, thereby reducing system bandwidth. Even though amplifiers can add more noise than a transformer, they have better gain flatness and can provide a range of desired gains by setting external resistors. The gain of a trans- former is limited by achievable turns ratios. Amplifiers have lower output impedance which is not significantly affected by the choice of gain. The signal path between each stage may be single- ended or differential, depending on the initial signal source. For a source with a single-ended output, a “single-to-diff stage” can be used to create differential-drive signals. Differential signal paths are higher performance, but the drawbacks include an increase in the number of components, board area, cost, and complexity of the filter. Types of Data Acquisition Systems Sampled-data systems can be split into two main types. The simplest is the baseband system also known as the “1st-Nyquist-zone” system. The second is the more complex under-sampled system, often referred to as bandpass, narrow band, sub-sampled, or Intermediate Frequency (IF)-sampled system. Baseband-system signal paths are generally DC- coupled while IF-bandpass signal paths tend to be AC-coupled. In a conventional 1st-Nyquist-zone system, the ADC samples the input at sample rate, fS, which is at least twice the highest signal frequency, fH, present at the ADC input (Figure 2a). To avoid aliasing of input frequencies above fS/2 back down into the 1st Nyquist zone as shown in Figure 2b, the ADC input is normally band-limited to the 1st Nyquist zone by a low-pass channel filter. Frequency fs fs/2 3f s/2 2f s 1st Nyquist Zone 2nd Nyquist Zone 3rd Nyquist Zone 4th Nyquist Zone Input Signal Input Image Input Image Input Image Input Image 5th Nyquist Zone fH Wanted signal band ADC Dynamic Range fs fs/2 3f s/2 2f s 1st Nyquist Zone 2nd Nyquist Zone 3rd Nyquist Zone 4th Nyquist Zone Frequency Input Signal Input Image Input Image 5th Nyquist Zone Input Signal ‘Aliased’ by Spur Image Unwanted Input Signal Spur Input Image Input Image Spur Image Spur Image Spur Image Wanted signal band ADC Dynamic Range fH FCASCADE FLNA FDRIVER - 1 GLNA FADC - 1 GLNA x GDRIVER + = + Figure 2a. 1st Nyquist baseband sampling where (fs>2fH) Figure 2b. 1st Nyquist sampling with no ADC input filter showing input spur >fs/2 aliasing back into 1st Nyquist zone to interfere with input< fs/2 SignalPathDesigner.indd 2 SignalPathDesigner.indd 2 9/5/07 3:24:30 PM 9/5/07 3:24:30 PM |
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Mô tả tương tự - LMH6515 |
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