July 6, 2023

# What is power supply rejection ratio?

Power supply rejection ratio (PSRR) is an important parameter in electronics that measures the ability of a device or circuit to reject variations in the power supply voltage. It is a crucial factor in determining the overall performance and reliability of electronic systems, particularly those that rely on precise and stable power sources.

## Understanding the Basics of Power Supply Rejection Ratio

Before delving into the details of PSRR, it is essential to have a clear understanding of its definition and significance. So, let's take a closer look at what PSRR really is and why it is crucial in electronic design.

Power supply rejection ratio, in simple terms, is a measure of how well a device can maintain its output voltage consistency, regardless of any changes or fluctuations in the input power supply voltage. It quantifies the device's ability to reject the impact of variations in the power supply, ensuring stable output voltages.

When it comes to electronic design, stability is of utmost importance. Without a stable power supply, electronic devices can malfunction or fail to perform their intended functions. This is where the power supply rejection ratio plays a vital role. By measuring the device's ability to reject power supply variations, engineers can ensure that the output voltage remains consistent, even when the input voltage fluctuates.

### Definition of Power Supply Rejection Ratio

Now that we have a general understanding of what PSRR is, let's dive a bit deeper into its definition. The power supply rejection ratio is usually expressed in decibels (dB) and is calculated by taking the ratio of the change in the power supply voltage to the change in the output voltage.

For example, if the input power supply voltage changes by 1 volt and the output voltage changes by 0.1 volts, the PSRR would be calculated as follows:

PSRR = 20 * log10(Î”Vin / Î”Vout)

Where Î”Vin is the change in the input power supply voltage and Î”Vout is the change in the output voltage.

A higher PSRR value indicates a device with better rejection capabilities, meaning it can maintain a stable output voltage even in the presence of significant power supply variations. On the other hand, a lower PSRR value indicates that the device is more susceptible to power supply fluctuations, which can result in distorted outputs and reduced performance.

### Importance of Power Supply Rejection Ratio

The importance of PSRR cannot be overstated. It directly affects the performance and reliability of various electronic devices, especially those used in sensitive applications such as audio equipment, power supply design, and precision instruments.

In audio equipment, for example, a high PSRR is crucial to ensure clean and distortion-free sound reproduction. Any fluctuations in the power supply can introduce noise and affect the overall audio quality. By having a high PSRR, audio devices can effectively reject power supply variations, resulting in a more accurate and enjoyable listening experience.

In power supply design, PSRR is essential to maintain stable output voltages regardless of any changes in the input voltage. This is particularly important in applications where a precise and consistent power supply is required, such as in critical medical equipment or industrial control systems. Without a high PSRR, these devices can experience voltage fluctuations that may lead to malfunctions or even pose a risk to human safety.

Furthermore, precision instruments heavily rely on stable power supplies to provide accurate measurements. Any variations in the power supply can introduce errors and compromise the reliability of the measurements. By ensuring a high PSRR, engineers can minimize the impact of power supply fluctuations on the instrument's performance, resulting in more precise and reliable measurements.

In conclusion, the power supply rejection ratio is a critical parameter in electronic design. It measures a device's ability to reject power supply variations and maintain stable output voltages. By understanding and optimizing the PSRR, engineers can ensure the performance, reliability, and accuracy of electronic devices used in various applications.

## Components of Power Supply Rejection Ratio

Power Supply Rejection Ratio (PSRR) is a crucial parameter in electronic systems that determines the ability of a circuit to reject variations in the power supply voltage. It is influenced by several factors, each playing a significant role in the overall rejection performance. Understanding these components is essential for designing circuits that exhibit excellent rejection of power supply variations.

### Input Voltage

The input voltage to a device or circuit is one of the primary factors that determine its PSRR. Devices with higher input voltage sensitivity are more prone to variations in the power supply voltage, resulting in lower rejection ratios. This sensitivity can be attributed to the internal circuitry and design choices made during the device's development. Conversely, devices characterized by lower input voltage sensitivity tend to exhibit higher rejection ratios, as they are less affected by changes in the power supply voltage.

For example, integrated circuits (ICs) with built-in voltage regulators often have lower input voltage sensitivity, allowing them to maintain stable operation even in the presence of power supply fluctuations. On the other hand, discrete components such as transistors or operational amplifiers (op-amps) may have higher input voltage sensitivity, making them more susceptible to power supply variations.

### Output Voltage

The stability of the output voltage is another critical factor affecting PSRR. Devices with precise voltage regulation and low output voltage drift will generally have higher rejection ratios. These devices are designed to maintain a constant output voltage regardless of fluctuations in the power supply voltage. On the other hand, devices with less accurate voltage control may suffer from reduced rejection capabilities, as their output voltage may vary along with changes in the power supply voltage.

For instance, voltage regulators with tight voltage regulation specifications, such as low-dropout regulators (LDOs), are known for their excellent rejection of power supply variations. These regulators employ feedback mechanisms and control loops to ensure that the output voltage remains stable even when the power supply voltage changes.

### Frequency

The frequency of the power supply variations also contributes to the power supply rejection ratio. Different electronic systems may exhibit different PSRR characteristics at different frequencies. It is crucial to consider the frequency range of interest to ensure the desired rejection performance across the spectrum.

At low frequencies, the PSRR of a circuit may be dominated by the inherent characteristics of the components used, such as the input and output voltage sensitivities. However, as the frequency increases, other factors come into play, such as the impedance of the power supply and the coupling between the power supply and the circuit. These factors can affect the ability of the circuit to reject power supply variations at higher frequencies.

Designers must carefully analyze the frequency range of interest for their specific application and consider the PSRR performance at those frequencies. This analysis allows for the selection of components and circuit topologies that provide the desired rejection performance across the entire frequency spectrum.

## How Power Supply Rejection Ratio Works

Now that we have a good understanding of the components of PSRR, let's explore how it works within electronic systems.

Power Supply Rejection Ratio (PSRR) is a critical parameter in electronic systems that measures the ability of a circuit or device to reject variations and disturbances in the power supply. It quantifies the extent to which changes in the input power supply voltage affect the output voltage of the system.

### The Role of Amplifiers

Amplifiers play a significant role in determining the PSRR of a circuit. They are responsible for amplifying and conditioning the input signals while maintaining stable output voltages despite any power supply variations. The design and characteristics of the amplifiers employed in a system have a direct impact on the overall rejection ratio.

There are various types of amplifiers used in electronic systems, such as operational amplifiers (op-amps) and differential amplifiers. These amplifiers are carefully designed to have high gain and low output impedance, which allows them to amplify the input signals effectively and provide a stable output voltage. The amplifiers' ability to reject power supply variations is crucial in maintaining the integrity of the output signal.

### The Impact of Noise

Noise, both external and internal, is a prevalent factor in electronic systems that affects the PSRR performance. Power supply noise can couple into a device or circuit, introducing unwanted disturbances into the output signal. Devices with high PSRR effectively reject such noise and ensure the output remains clean and stable.

External noise sources, such as electromagnetic interference (EMI) from nearby electronic devices or power lines, can induce fluctuations in the power supply voltage. These fluctuations can be detrimental to the performance of sensitive electronic components. However, with a high PSRR, the system can mitigate the impact of these external noise sources and maintain a stable output voltage.

Internal noise sources, such as thermal noise generated by the electronic components themselves, can also affect the PSRR. These noise sources can introduce random variations in the power supply voltage, which can degrade the overall performance of the system. By designing amplifiers and other components with low internal noise, the PSRR can be improved, resulting in a more robust and reliable system.

In conclusion, power supply rejection ratio is a crucial parameter in electronic systems that ensures stable output voltages despite power supply variations and noise. Amplifiers play a significant role in determining the PSRR, and their design and characteristics directly affect the rejection ratio. By understanding and optimizing the PSRR, engineers can design electronic systems that deliver clean and reliable signals, even in the presence of power supply disturbances.

## Measuring Power Supply Rejection Ratio

To accurately assess the power supply rejection ratio of a device or circuit, specific tools and equipment are required. Let's explore the measurement process in more detail.

### Tools and Equipment Needed

The measurement of PSRR typically involves the use of precision power supplies, signal sources, and test instruments such as oscilloscopes, spectrum analyzers, and distortion analyzers. These tools help capture and analyze the behavior of the circuit under varying power supply conditions.

### Step-by-Step Measurement Process

The measurement process involves applying a known variable voltage to the power supply while monitoring the output of the circuit. By observing the changes in the output voltage in response to the power supply variations, the rejection ratio can be calculated. This process is repeated at several frequencies to assess the circuit's behavior across different frequency ranges.

## Applications of Power Supply Rejection Ratio

The power supply rejection ratio has widespread applications across various domains in electronics. Let's take a look at some of its key applications.

### Use in Audio Equipment

In audio equipment, maintaining a clean and stable power supply is essential to ensure high-quality sound reproduction. By employing devices with high PSRR, audio systems can effectively reject power supply noise and deliver pristine audio output.

### Importance in Power Supply Design

In power supply design, PSRR determines the ability of the circuit to maintain stable output voltages despite fluctuations at the input. It is crucial for ensuring reliable power delivery to sensitive electronic components and avoiding potential malfunctions or damage.

In conclusion, power supply rejection ratio is a critical parameter that determines the ability of electronic devices and circuits to reject variations in the power supply voltage. By understanding its importance, components, measurement process, and applications, engineers can design robust and reliable systems that deliver stable and accurate performance in various applications.