How do I design the analog front end between the mixer output and the ADC input?
ADC Front End Design
The ADC input network must handle the ADC's switched-capacitor input impedance, which varies with frequency and sampling phase. A series resistor (10-50 Ω) and shunt capacitor (1-10 pF) at the ADC input form a charge-sharing network that isolates the FDA from the ADC's switching transients and provides a charge reservoir. The component values are specified in the ADC data sheet for optimal performance. Deviating from recommended values can degrade SFDR and SNR.
| 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
Active vs. passive front end?
Active (FDA): provides gain, DC offset control, and single-to-differential conversion. Best when signal levels are low or when single-ended to differential conversion is needed. Adds noise (NF = 10-20 dB) and nonlinearity. Passive (transformer): lower noise (< 1 dB insertion loss), wider bandwidth, no power consumption, and inherent differential output. Best when the signal level is adequate and impedance transformation is the primary need.
How do I set the full-scale voltage?
The signal level at the ADC input should use 70-90% of the full-scale range for maximum SNR while avoiding clipping. For an ADC with 1V p-p full scale: target signal level = 0.7-0.9V p-p (-3 to -1 dBFS). Leave headroom for peak-to-average ratio of the modulated signal (6-12 dB for OFDM).