Application Cases

[Overview]

In this study, a broadband power amplifier was used to build a PSRR testing platform. As electronic devices impose higher demands on power-supply performance, low-dropout linear regulators (LDOs) are widely used due to their low-noise characteristics, and their power supply rejection ratio (PSRR) is a key metric for evaluating the ability to suppress input noise. Existing research mostly focuses on improvements to test methods and equipment, with insufficient discussion on the influence of peripheral configuration of the device under test. Therefore, based on a fixed test method, this study investigates how the sampling position of the test points and the presence or absence of input/output port capacitors affect PSRR measurement results, aiming to improve test accuracy from the perspective of peripheral-circuit optimization.

Experiment Name: Experiment on the Influence of Sampling Position and Capacitors on PSRR Test Results

Experiment Objective: This study designs multiple groups of comparative experiments to investigate the influence of nine conditions—combinations of different test positions and whether input/output port capacitors are connected—on the PSRR test results of an LDO. By comparing the PSRR curves under these conditions, especially the data differences in the mid-to-high frequency band, the optimal and worst test configurations are identified. The mechanism of capacitors and their equivalent series resistance (ESR) on measurement is analyzed, ultimately providing experimental basis and suggestions for improving the accuracy and reliability of PSRR testing.

Test Equipment:

1. Signal source

2. Venable-model loop analyzer

3. PC

4. ATA-1200C broadband power amplifier

5. LDO device under test: ADM7150

6. Chr63640 electronic load

Experiment Procedure: First, the experimental conditions were set as follows: input voltage 6.2 V, output voltage 5 V, load current 400 mA, with 10 μF input and output capacitors. Next, the test platform was built. By changing the input capacitor (present/absent), output capacitor (present/absent), and the measurement position of the input/output signals (left or right side relative to the capacitor), nine different experimental conditions were systematically constructed. Then, a loop analyzer and other equipment were used to perform PSRR frequency-sweep tests under each condition, obtaining complete frequency-response curves. Finally, the PSRR values at characteristic frequencies such as 100 kHz were compared and analyzed in groups. Combined with the filtering characteristics of capacitors and the ESR model, the study concluded that the input capacitor attenuates the test signal and leads to lower measured values, while the ESR of the output capacitor degrades high-frequency noise rejection. Based on these findings, suggestions for optimizing test position and capacitor selection were given.

Figure 1. PSRR test scheme diagram

Figure 2. Schematic of PSRR test points

Experiment Results: Based on a systematic study of the peripheral configuration of the test circuit, it was confirmed that the sampling position and the input/output capacitors have a decisive influence on PSRR measurement results. The results show that at 100 kHz, the best test condition (no input capacitor, test point on the right side of the output capacitor) yields a PSRR of about 98 dB, consistent with the device datasheet; while the worst test condition (with input and output capacitors, test point on the left side of the input capacitor and right side of the output capacitor) yields only about 60 dB, a significant difference. This confirms that unreasonable capacitor configuration and sampling position severely degrade measurement results and lead to incorrect evaluation of the LDO’s power-supply rejection capability. The conclusions provide a clear basis for circuit configuration and measurement-point selection to improve PSRR test accuracy.

Figure 3. Comparison of three groups of typical data curves

Table 1. PSRR results at different frequencies (unit: dB)

Product Advantages of Aigtek Amplifiers in This Application:

1. Wide bandwidth and low-distortion output — Precisely covers PSRR test requirements and ensures data reliability.

2. High output current drive capability — Effectively drives capacitive loads, ensuring stable injection of AC ripple under different capacitor configurations.

3. Pure output waveform — Can amplify and output AC/DC superimposed signals, guaranteeing the accuracy of simulating real operating conditions.

[Recommended Product]: ATA-1000 Series Broadband Amplifier

Figure: Specifications of the ATA-1000 series broadband amplifier

The experimental materials in this article were compiled and published by Xi'an Aigtek Electronics. Aigtek has become an instrumentation supplier with a broad product portfolio and considerable scale in the industry; demonstration units are available with free trial support. Xi'an Aigtek Electronics is a high-tech enterprise specializing in the R&D, manufacturing, and sales of electronic test instruments including power amplifiers, high-voltage amplifiers, power signal sources, preamplifiers (low-noise signal amplifiers), high-precision voltage sources, and high-precision current sources.