How does the spurious free dynamic range of a DAC affect the spectral purity of a transmitted signal?
DAC SFDR
DAC output spectrum contains: the desired signal, harmonics of the signal, Nyquist images at n×f_clock ± f_signal, and the sinc roll-off envelope. The sinc response attenuates the output by sinc(π×f/f_clock), causing 3.9 dB loss at the Nyquist frequency. Digital pre-compensation (inverse sinc filter) corrects this roll-off for signals occupying a large fraction of the Nyquist band.
| Parameter | Pipeline ADC | SAR ADC | Sigma-Delta ADC |
|---|---|---|---|
| Sample Rate | 100 MS/s - 10 GS/s | 1-100 MS/s | 10 kS/s - 50 MS/s |
| Resolution | 8-14 bits | 10-20 bits | 16-24 bits |
| Latency | Several clock cycles | 1 conversion cycle | Many cycles (decimation) |
| Power | High | Low-moderate | Low |
| Typical RF Use | Direct sampling, DPD | Control, monitoring | Audio, baseband |
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
Frequently Asked Questions
How does SFDR affect EVM?
DAC spurs that fall within the signal bandwidth degrade EVM. For 64-QAM: EVM < -25 dB is required, so in-band SFDR must be > 30 dBc. For 256-QAM: EVM < -32 dB, requiring in-band SFDR > 37 dBc. Out-of-band spurs must meet the transmit spectrum emission mask.
What is the Nyquist image?
For a DAC running at f_clock producing a signal at f_signal: images appear at n×f_clock ± f_signal for all integers n. The first image at f_clock - f_signal is typically the strongest and must be attenuated by a reconstruction filter (typically 40-60 dB rejection required).