PSRR Explained Properly — Why Clean Power Alone Is Not Enough

Many engineers believe that using a low-noise regulator guarantees clean analog performance. Then they see ripple at the output of an amplifier, unexplained jitter in an ADC, or spurs in a PLL — even with a “perfect” supply. The missing concept is PSRR.

Power Supply Rejection Ratio (PSRR) defines how much of the supply noise actually leaks into your signal path. Understanding PSRR is essential for designing robust analog systems — especially in mixed-signal environments.

1. What is PSRR?

PSRR (Power Supply Rejection Ratio) measures how well a circuit rejects variations on its power supply.

Formally:

PSRR = ΔV_supply / ΔV_output

In decibels:

PSRR(dB) = 20 · log₁₀(ΔV_supply / ΔV_output)

Higher PSRR means better immunity to supply noise.

2. Why PSRR Matters More Than You Think

Power rails are never quiet:

• Switching regulators inject ripple
• Digital circuits create current spikes
• Clock edges modulate ground and supply
• Load transients cause droop and ringing

PSRR determines whether this noise stays on the supply — or corrupts your signal.

3. DC PSRR vs AC PSRR

This is where many engineers get misled.

DC PSRR

Measured at low frequency (often near DC). Datasheets may show values like 90–120 dB.

AC PSRR

Measured across frequency. At higher frequencies, PSRR almost always degrades.

Important: A circuit with 100 dB DC PSRR may have only 30–40 dB PSRR at 1 MHz.

4. Why PSRR Degrades With Frequency

• Finite gain of internal amplifiers
• Parasitic capacitances coupling supply to output
• Reduced loop gain at high frequency
• Package and bond-wire inductance

At high frequency, the circuit can no longer “fight” supply noise.

5. PSRR in Common Analog Blocks

Op-Amps

Supply noise couples through:

• Input differential pair biasing
• Current mirrors
• Output stage

Negative feedback improves PSRR — but only within loop bandwidth.

Bandgap References

PSRR defines how much supply ripple appears directly in the reference voltage — critical for ADC accuracy.

PLLs & Oscillators

Supply noise modulates VCO frequency, directly increasing phase noise and jitter.

6. Improving PSRR — What Actually Works

• Local decoupling: Place capacitors as close as possible to supply pins
• RC / LC filtering: Especially effective at high frequency
• Cascode devices: Reduce supply sensitivity of current sources
• Separate analog and digital supplies
• Increase loop gain (carefully — stability matters)
• Good layout: Short supply paths, solid ground planes

Key rule: PSRR is a system property — not a single block property.

7. Measuring PSRR in Validation

In the lab, PSRR is measured by:

• Injecting AC ripple onto the supply
• Measuring output response using FFT or spectrum analysis
• Sweeping frequency to generate PSRR vs frequency plots

This reveals weaknesses that DC datasheet numbers completely hide.

8. Common Interview Questions

• What is PSRR and why does it degrade with frequency?
• How does feedback affect PSRR?
• Why is PSRR important in ADCs and PLLs?
• How would you improve PSRR in a noisy system?
• Why is decoupling not enough on its own?

Conclusion

PSRR is the silent guardian of analog performance. Clean power helps — but rejection is what saves you when power is never truly clean. Engineers who understand PSRR don’t just design circuits that work in the lab — they design circuits that survive reality.