What is power supply rejection ratio

The Power Supply Rejection Ratio (PSRR) is a measure of how well an electronic device, such as an operational amplifier (op-amp), can reject variations in its power supply voltage and prevent these variations from affecting its output. PSRR is a critical parameter in the design and selection of op-amps, especially in applications requiring high precision and low noise.

Key Points about PSRR:

1. Definition:

   • PSRR is defined as the ratio of the change in the power supply voltage to the corresponding change in the output voltage, typically expressed in decibels (dB).
   • Mathematically, PSRR is given by: PSRR (dB)=20log?(ΔVPSΔVout)\text{PSRR (dB)} = 20 \log \left( \frac{\Delta V_{\text{PS}}}{\Delta V_{\text{out}}} \right)PSRR (dB)=20log(ΔVout?ΔVPS??) where ΔVPS\Delta V_{\text{PS}}ΔVPS? is the change in power supply voltage and ΔVout\Delta V_{\text{out}}ΔVout? is the resulting change in output voltage.

2. Importance:

   • High PSRR indicates that the device is less affected by noise or fluctuations in the power supply, making it more suitable for precision applications.
   • Low PSRR can lead to significant variations in the output voltage due to power supply noise, which can degrade the performance of the circuit.

3. Frequency Dependence:

   • PSRR is frequency-dependent, meaning it can vary with the frequency of the power supply noise. Manufacturers typically provide PSRR specifications at various frequencies (e.g., at DC, 1 kHz, 10 kHz).
   • At higher frequencies, the PSRR tends to decrease, making the circuit more susceptible to high-frequency power supply noise.

4. Typical Values:

   • PSRR values can range from 60 dB to over 100 dB for high-performance op-amps. A higher value indicates better rejection of power supply variations.

Example:

Consider an op-amp with a PSRR of 80 dB. If the power supply voltage varies by 100 mV (ΔVPS=100 mV\Delta V_{\text{PS}} = 100 \text{ mV}ΔVPS?=100 mV), the resulting change in the output voltage (ΔVout\Delta V_{\text{out}}ΔVout?) can be calculated as follows:

First, convert PSRR to a linear scale:

PSRR (linear)=10PSRR (dB)20=108020=104=10000\text{PSRR (linear)} = 10^{\frac{\text{PSRR (dB)}}{20}} = 10^{\frac{80}{20}} = 10^4 = 10000PSRR (linear)=1020PSRR (dB)?=102080?=104=10000

Now, use the PSRR to find the change in output voltage:

ΔVout=ΔVPSPSRR (linear)=100 mV10000=0.01 mV\Delta V_{\text{out}} = \frac{\Delta V_{\text{PS}}}{\text{PSRR (linear)}} = \frac{100 \text{ mV}}{10000} = 0.01 \text{ mV}ΔVout?=PSRR (linear)ΔVPS??=10000100 mV?=0.01 mV

Practical Considerations:

• Design Implications: Designers often choose op-amps with high PSRR for applications such as audio amplification, sensor signal conditioning, and precision analog measurements to ensure stable performance despite power supply variations.
• Decoupling and Filtering: Additional decoupling capacitors and power supply filtering techniques can be used to improve the overall PSRR of the system by reducing power supply noise before it reaches the op-amp.

In summary, PSRR is a vital parameter that quantifies an op-amp's ability to reject power supply noise, ensuring stable and accurate performance in various electronic applications.