Frequency Dithering Method to Enhance ADC Performance

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Frequency Dithering Method to Enhance ADC Performance [Copy link]

Since the late 1970s, designers have successfully improved the effective resolution and parasitic performance of A/D converters by adding dither (uncorrelated noise) to the converter input and then using DSP techniques to neutralize the noise from the converted data. The most common dithering method is to add random amplitude noise to the A/D converter input signal. Although this method is practical, the added noise contains large random peak signals. To prevent the A/D converter input port from saturating, the designer must know the peak signal and the peak dither level. Even short-term saturation will add more nonlinear components to the A/D converter, which will exceed the range that dither can eliminate.
　　Another method is to add a signal with a constant amplitude while dithering the frequency. Figure 1 shows a possible implementation, which uses a Linear LTC1799 programmable oscillator IC2 operating in VCO (voltage-controlled oscillator) mode, in which the center frequency is modulated by an applied voltage. The LTC1799's center frequency can be set from 1 kHz to 33 MHz, making it suitable as a dither generator for most existing A/D converters. Since the LTC1799 output contains a square wave, its peak output amplitude is deterministic.
　　The center frequency of the random jitter can be set below or above the signal frequency of interest. For narrow-band IF conversion, center frequencies above or below the signal frequency work well. For A/D converters that must operate at DC, the only available position is above the signal frequency of interest. One approach is to place the jitter frequency at half the sampling frequency or Nyquist frequency. In this case, random noise will generally not interfere with the desired signal, and any aliasing will only involve random frequency noise around it and not into the desired signal band.
　　The circuit in Figure 1 uses a 20 MHz sampling A/D converter to generate random noise around a center frequency of about 10 MHz. Random noise can be generated using any technology, including digital shift registers and semiconductor junctions biased in the breakdown region. In this case, a 12V Zener diode D1 is used to generate the noise, and a two-stage amplifier performs amplification and frequency shaping. If necessary, more complex active filtering circuits (IC _1A and IC _1B ) can be used to further shape the noise distribution. After filtering, the noise modulates the LTC1799. Make sure that the LTC1799's supply voltage is pure DC and has no ripple, because power supply noise will produce nonrandom AM sidebands.

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Figure 1. A Zener diode, two-stage amplifier, and an FM voltage-controlled oscillator form a constant-amplitude jitter generator.
　　Figure 2 shows the amplitude-frequency curve for the limited spectrum of the design in Figure 1. Depending on the circuit configuration, the jitter can be added to the A/D converter using a small coupling capacitor or a more complex active summing circuit. Although Zener diode noise generators are simple in theory, they do not perform well in a production environment because their noise output varies greatly. Even within the same production batch of diodes, so-called popcorn noise, irregularly distributed noise histograms, amplitude drift, and frequency-weighted noise can be observed. In high-volume applications, well-specified noise diodes, such as those from Micronetics, may be more cost-effective than Zener diodes. Once the noise diode is selected, the gain of the amplifier stage can be selected so that no significant clamping noise peaks appear at the circuit output. If the application requires it, the frequency response of the amplifier can also be changed to change the noise spectrum. Finally, the frequency setting resistors R6 and R7 of the LTC1799 can be adjusted to make the noise spectrum similar to that shown in Figure 2. Any clamping along the amplifier path will add peaks at the edges of the spectrum, indicating clamping of the amplitude and degradation of the random nature of the noise.

In Figure 2, the broad bell-shaped curve shows a random frequency jitter spectrum superimposed on the 10 MHz unmodulated output of the LTC1799.
　　A filter can be added between the noise output and the A/D converter summing input to limit the in-band noise or remove any periodic modulation caused by power supply ripple. In modern high-performance A/D converters, even a small amount of periodic noise will show up as a -80 dBc (decibels below carrier) spurious response.