FEATURE COVER STORY
CHOOSING THE IDEAL DAC FOR SIGNAL GENERATION
Clarence Mayott, Applications Engineer, Mixed Signal Products at Linear Technology Corporation explores the critical requirements in high speed signal generation applications
n high speed signal generation applications bandwidth and resolution are the critical requirements. Modern signal generation applications use high speed digital-to-analogue converters (DACs) to produce various types of waveforms - from single tones to complex multichannel waveforms with several hundred megahertz of bandwidth. These applications demand high speed DACs that are fast enough to produce these waveforms without sacrificing analogue performance. In many signal generation applications phase noise will limit the number of channels and the spacing of the channels that are possible. Phase noise is traditionally set by the
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clock that is driving the DAC clock inputs, but any phase noise added by the DAC will show up in the output spectrum and can limit the signals that are possible to generate. The ideal DAC for any general purpose signal generation application should be as fast as possible, with low noise, high linearity, and very low additive phase noise. If any one of these specifications is lacking, then the generated waveform will not be adequate to meet the application’s needs.
14 OCTOBER 2014 | ELECTRONICS
between bandwidth and sample rate (fs) was defined by Harry Nyquist and describes how signals behave in sampled systems. While it is possible to generate a signal that spans from DC to the fs/2, it is often not practical to do so due to the images of the generated signal that appear in the output spectrum. In practice, reconstruction filters are required to attenuate any images of the generated signal that may appear in the output spectrum. Even if the generated signal does not extend up to fs/2 but closely approaches it, the images will be difficult to filter out due to constraints on the filter. The closer the bandwidth of the signal is to fs/2, the higher order the filter must be to attenuate the images produced by the sampling process. This higher order filter requires more components, and will have more insertion loss and passband ripple. By using a DAC with a faster sample rate,
In any signal generation application the
most important design criteria is bandwidth. The designer may need a certain amount of bandwidth for a particular signaling protocol or for a particular application. The bandwidth required can only be generated with a DAC that is at least twice as fast as the bandwidth of interest. This relationship
Figure 1:
LTC2000 and LTC6946 phase noise Fout = 80MHz
the usable bandwidth will increase, which will ease the requirements of the filter. The LTC2000 from Linear Technology Corporation, which is a high performance, 16-bit, 2.5Gsps high speed DAC, for example, has a sample rate of 2.5Gsps, so the fs/2 frequency is 1.25GHz. Thus, for a signal bandwidth of 800MHz there will be an image of the signal beginning at 1.7GHz. There will be 900MHz between the frequency band of interest and the image frequency. With 900MHz of guard band the image can easily be filtered out with a simple low pass filter. Another issue with generating a signal that extends out to fs/2 is that with any DAC there will be a SINC (sin(x)/x) roll-off that will attenuate the generated signal as the frequency increases. For practical applications, about 60% of the Nyquist zone (DC to fs/2) can be used without much SINC attenuation. If 0dB is the signal level at DC, then at 60% of Nyquist the signal level would be down by 6dB. Using a higher speed DAC will reduce the roll-off of the SINC function as the output frequency of the DAC increases.
/ ELECTRONICS
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