In practice the Nyquist bandwidth of an ADC is never fully used, and a fraction of it is usually dedicated to the roll-off of the anti-aliasing filter. This free band is exploited to inject a constrained calibration signal. A sine-wave is selected for calibration as it is easy to generate with high spectral purity on which two main constraints are imposed:
- The amplitude is kept small enough to avoid any impact on the dynamic range while providing enough estimation accuracy. Experiments show that -40 dBFS to -35 dBFS level range provides the best tradeoff for a 14-bit ADC.
- The frequency is limited to the following discrete values in order to reduce the complexity of the digital signal processing algorithms:
............................................................ (Equation 1)
Where Fs is the TIADC sampling frequency, P, K are unsigned integers and S=+-1 depending on the location of the calibration signal with relation to the edge of the Nyquist zone (see Figure 1). This signal can be easily generated on-chip with a fractional-N PLL using the clock of the ADC as a reference signal. By choosing K high enough, the harmonics of the calibration signal will alias outside the useful band which relaxes their filtering requirements. The swing adjustment can be achieved with a programmable attenuator placed at the output of the PLL.
Figure 1: Frequency plan showing the location of the calibration signal. Click image to enlarge.