In ‘Dither’ series A, I mentioned that the quantization will introduce large harmonics to a low-level sinusoidal signal. Let’s look at another example in Figure 1 [1].

Adding dither to the circuit means adding noise to the circuit. Now let’s look at Figure 2, which shows the effects of dither on the situation of Figure 1. The dither noise causes the ADC to make transitions, and the sine wave can be recovered with short-term averaging.

With no dither, each analog input voltage is assigned one and only one code; with dither, each analog input voltage is assigned a probability distribution for being in one of several digital codes [2]. In [2], it was also mentioned that

• the optimum dither is white noise at a voltage level of about 1/3 LSB rms
• dither is very effective in reducing harmonic distortion for signal levels up to about 10 LSB.

What do you pay for a reduction in harmonic distortion by adding dither?

Answer: A slightly degraded signal-to-noise ratio and, if one uses time averaging, an increase in the effective conversion time. (SFDR/THD is traded for SNR.)

Reference

1. J. Vanderkooy and S. P. Lipshitz, Resolution below the least siginificant bit in digital systems with dither, J. Audio Eng. Soc., 1984.
2. Leon Melkonian, Improving A/D converter performance using dither, National Semiconductor (now TI), AN-804, 1992.