Power Supply Rejection Ratio (PSRR) is a crucial parameter in analog circuits that measures how effectively an electronic device can reject power supply noise. This parameter is particularly important in applications that require high accuracy and low noise, such as audio amplifiers and analog-to-digital converters. PSRR is expressed in decibels (dB) and is defined as the ratio of the change in output voltage to the change in input voltage of the power supply.
One of the main sources of power supply noise is voltage regulators, especially linear regulators, which have poor PSRR at high frequencies. As the frequency of the noise increases, the PSRR of the regulator decreases, allowing more noise to enter the output of the device. To improve PSRR, bypass capacitors can be added, or a better regulator with better PSRR can be used.
To measure PSRR, a sinusoidal signal is applied to the power supply input, and the output voltage is measured. The frequency of the input signal is swept over a range of frequencies, and the ratio of the output voltage to the input voltage is measured at each frequency. The PSRR can then be calculated by taking the ratio of the output voltage to the input voltage at a particular frequency and expressing it in decibels.
PSRR measurement can be performed using a network analyzer, a signal generator, and an oscilloscope. However, specialized PSRR measurement equipment is also available, such as the J2120A PSRR measurement instrument from Picotest, which simplifies the measurement process.
The design of the device can influence PSRR, particularly the input stage. The PSRR can be improved by using a differential input stage, which can reject common-mode noise. Additionally, a well-designed feedback network can improve PSRR.
When designing analog circuits, it is essential to consider PSRR. Many analog IC datasheets provide PSRR information to help designers select the appropriate components for their applications. Additionally, external components, such as bypass capacitors and voltage regulators, can improve PSRR.
To achieve high-performance analog circuits, it is critical to understand and improve PSRR. This parameter can be improved by using a better regulator, adding bypass capacitors, using a differential input stage, or using a well-designed feedback network. Specialized PSRR measurement equipment is available, such as the J2120A PSRR measurement instrument from Picotest, which simplifies the measurement process. By considering PSRR, designers can ensure the accuracy and reliability of their analog circuits in applications where noise and accuracy are critical.