The spectral analysis tool is intended to evaluate the dynamic performance (SNR, THD, SFDR, SINAD, and ENOB) of the ADS8681W SAR ADC through single-tone sinusoidal signal FFT analysis using the 7-term Blackman-Harris window setting.

The spectral analysis corresponding to a 1kHz sinusoidal signal and 1Msps sampling rate is displayed on clicking the Capture button as shown in Figure 4-7.

The expected ADC input is a sinusoidal signal of peak-to-peak amplitude close to the ADC’s full-scale input range (FSR). The RMS power of the input signal normalized to FSR is shown in the Signal Power (dBFS) field and is about –0.5 dBFS (or about 95% × FSR) to avoid the input clipping.

The sampling rate of the ADC is adjusted by modifying the target sampling rate (sps) argument which is a global setting (it affects all tools). The achievable ADC sampling rate can differ from the target value depending on the applied SCLK frequency and PHI PLL settings. Note that the user is also required to specify a target SCLK frequency which the tool tries to match as exactly as possible by iteratively changing the PHI PLL settings until convergence.

Figure 4-7 Spectral Analysis Tool

Finally, the FFT tool includes windowing options that are required to mitigate the effects of non-coherent sampling (this discussion is beyond the scope of this document). The 7-Term Blackman Harris window is the default option and has sufficient dynamic range to resolve the frequency components of up to a 24-bit ADC. Note that the None option corresponds to not using a window (or using a rectangular window) and is not recommended.