What is the target impedance for the power distribution network of a high speed digital IC?
PDN Target Impedance
The target impedance concept is the foundation of modern PDN design. Violating the target impedance at any frequency can cause supply voltage droop or overshoot, leading to: timing margin reduction, logic errors, and increased jitter on clock and data signals.
| Parameter | Option A | Option B | Option C |
|---|---|---|---|
| Performance | High | Medium | Low |
| Cost | High | Low | Medium |
| Complexity | High | Low | Medium |
| Bandwidth | Narrow | Wide | Moderate |
| Typical Use | Lab/military | Consumer | Industrial |
Sampling and Quantization
When evaluating the target impedance for the power distribution network of a high speed digital ic?, engineers must account for the specific requirements of their target application. The optimal choice depends on the frequency range, power level, environmental conditions, and cost constraints of the overall system design.
- 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
Dynamic Range Considerations
When evaluating the target impedance for the power distribution network of a high speed digital ic?, engineers must account for the specific requirements of their target application. The optimal choice depends on the frequency range, power level, environmental conditions, and cost constraints of the overall system design.
Frequently Asked Questions
What happens if the target is exceeded?
If the PDN impedance exceeds Z_target at any frequency: the supply voltage will exhibit ripple or droop exceeding the IC's voltage tolerance during transient current events. Consequences: for digital logic (FPGA, ASIC): timing margin shrinks (reduced setup and hold times), potentially causing logic errors. For PLLs and clocks: power supply noise couples to the clock through the PLL's supply rejection, increasing jitter. The PLL's PSRR (Power Supply Rejection Ratio) attenuates the noise by 20-40 dB, but: 100 mV of supply ripple with 30 dB PSRR results in 3.2 mV of clock jitter, which can be significant for high-speed serial links. For ADC/DAC: supply noise degrades the SFDR and SNR.
How do I measure the actual PDN impedance?
Measurement methods: VNA with a 2-port probe (the standard method): connect a VNA to the PDN using two probes (one for port 1, one for port 2) soldered to the power and ground planes. Measure S21 and convert to impedance: Z = 2Z0/(S21/(1-S21)). This gives the PDN impedance vs. frequency from 100 kHz to several GHz. Equipment: any VNA covering 100 kHz-3 GHz (Keysight E5061B is commonly used for PI). Probing: Picotest P2102A 2-port probe tip or custom SMA probe soldered near the IC. Time-domain measurement: supply a known current step (using a pulse generator and a FET) and measure the resulting voltage transient. The voltage droop divided by the current step gives the PDN impedance at the step frequency.
What is anti-resonance?
Anti-resonance is a peak in the PDN impedance that occurs at the frequency where the inductance of one decoupling capacitor resonates with the capacitance of another. At the anti-resonance frequency: the PDN impedance spikes to a high value (potentially 10-100× above the target impedance). This spike can cause supply voltage oscillation at that specific frequency. Mitigation: use capacitors with overlapping impedance characteristics (multiple values of ceramic capacitors) to smooth out the anti-resonance peaks. Add lossy components (ferrite beads, ESR-controlled capacitors) to damp the resonance. Simulate the PDN impedance with all capacitors and planes to identify and eliminate anti-resonance peaks.