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ADC083000 Datasheet(PDF) 4 Page - Texas Instruments |
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ADC083000 Datasheet(HTML) 4 Page - Texas Instruments |
4 / 14 page 3 signalpath.national.com/designer SIGNAL PATH designer fs 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 Unwanted Input Signal Spur Input Image Input Image Low-Pass Filter at ADC Input fs/2 Wanted Signal Band ADC Dynamic Range fH To use the full ADC dynamic range, ensure that any undesired, out-of-band signal components are filtered to less than the ADC Least Significant Bit (LSB) level. This requires high-order filters to obtain a sufficiently sharp roll off if the wanted and unwanted input- signal components approach too close to fS/2 (Figure 2c). One solution is to increase the ADC sample rate and over-sample the input signal. This spreads the Nyquist zones further out in frequency and relaxes the channel-filter design (Figure 2d). High-speed baseband sampling is found in many test and measurement applications requiring data conversion from DC to GHz. An under-sampled system employs an ADC with a full-power bandwidth much higher than fS/2. For example, it is not unusual to find a 1 GHz-input bandwidth on a 100 MHz-sampling ADC. This allows a narrowband in- put centered at a frequency >fS/2 to be under-sampled at a rate much lower than the conventional Nyquist fS rate, and aliased or “folded” back down to the 1st Nyquist zone. This is shown in Figure 3a where signal A is the desired signal being converted. At higher input frequencies, the input stage of the ADC becomes slew- rate limited. For optimum distortion performance from the ADC, it is recommended to keep the center frequency of the under-sampled signal to no more than 10% to 30% of the ADC’s full-power bandwidth depending on the performance of the ADC. In an under-sampled system, the channel filter is the key to ensuring that the desired signal is optimally recovered at baseband and separated from all the 2f s 1st Nyquist Zone 2nd Nyquist Zone Frequency Input Signal Unwanted Input Signal Spur Input Image Input Image Low-Pass Filter at ADC Input fs/2 Wanted Signal Band Input Signal Spur Attenuated by Filter ADC Dynamic Range fH 2f s fs 3f s 4f s 1st Nyquist Zone Wanted Input Signal A 5f s 2nd Nyquist Zone 3rd Nyquist Zone 4th Nyquist Zone 5th Nyquist Zone 6th Nyquist Zone 7th Nyquist Zone 8th Nyquist Zone 9th Nyquist Zone 10th Nyquist Zone 11th Nyquist Zone 12th Nyquist Zone Frequency Image of A Image of A Image of A Image of A Image of A Image of A Alias of A Image of A Image of A Image of A Image of A Image of A Figure 2d. 1st Nyquist baseband >2x over-sampling with ‘relaxed’ low-pass filter requirement Figure 3a. Wanted signal A >fs under-sampled from 8th Nyquist zone back to 1st Nyquist zone Figure 2c. 1st Nyquist baseband sampling with low-pass filter other aliased components. A bandpass filter is used to remove all interfering frequencies and noise from the ADC input which might otherwise alias back to baseband with the wanted signal. Figure 3b shows the effects of a second unwanted signal B folding back from the 7th Nyquist zone to interfere with s s s s 1st s Frequency 2f f 3f 4f Nyquist Zone Wanted Input Signal A 5f 2nd Nyquist Zone 3rd Nyquist Zone 4th Nyquist Zone 5th Nyquist Zone 6th Nyquist Zone 7th Nyquist Zone 8th Nyquist Zone 9th Nyquist Zone 10th Nyquist Zone 11th Nyquist Zone 12th Nyquist Zone Unwanted Signal B Alias of B Inteferes with Alias of A Image of B Image of B Image of B Image of B Image of B Image of B Image of B Image of B Image of B Figure 3b. Failure to bandpass filter unwanted signal B allows it to alias back to the 1st Nyquist zone and interfere with the recovery of wanted signal A SignalPathDesigner.indd 3 SignalPathDesigner.indd 3 9/5/07 3:24:31 PM 9/5/07 3:24:31 PM |
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