PSRR and Measurement of PSRR in Class-D Audio Amplifiers and LDOs - An-135

APPLICATION NOTEAN-135 PSRR and Measurement of PSRR in Class-D Audio Amplifiers and LDOs Introduction This application note explains the importance of PSRR and shows how to calculate and measure it. Two examples using the AAT5101 Class D audio amplifier and the AAT2868 CABC Compatible 4 Channel Backlight Driver with Dual LDO Regulators are used to explain how to measure the PSRR of a Class D audio amplifier and a low dropout regulator (LDO). Additionally, a non-inverting amplifier adder circuit used in PSRR measurement is introduced at the end of this note. What is PSRR? Power Supply Rejection Ratio (PSRR) is the ratio of the output ripple voltage to the power supply ripple voltage; PSRR indicates how well a circuit rejects ripple coming from the power supply input at various frequencies. PSRR is an important parameter of amplifier and LDO performance in many applications, especially RF and wireless application. The Importance of PSRR In a typical portable RF application, the RF amplifier is turned on/off at a rate of 217Hz. At each of these events, a high current (typically, up to 1.7A) is drawn from the power supply, creating a sudden voltage drop (about 200mV) through the battery's equivalent series resistance (ESR)as shown in Figure 1. The battery also provides power to other devices such as an audio amplifier. The supply ripple caused by the RF amplifier will be injected into the audio amplifier, which then will appear at the same frequency as a voltage ripple in the audio amplifier output. Since 217Hz is within the audio bandwidth (20Hz to 20kHz) the ripple can be heard by the human ear as is observed as a fixed-frequency noise. The level of this noise depends on the PSRR performance of the audio amplifier. With excellent PSRR, the 217Hz noise will be rejected well and not disturb the audio performance. IRF ESR LDO V BAT RF Amplifier Audio In Audio Amplifier Figure 1: RF Subsystem Diagram. AN-134.2010.08.1.0 www.analogictech.com 1 The PSRR of the LDO can then be calculated using Equation 2: Eq. if there is 200mV peak-to-peak ripple in the power supply input and the same frequency voltage ripple in the amplifier output at 200μV peak-to-peak.APPLICATION NOTE AN-135 PSRR and Measurement of PSRR in Class-D Audio Amplifiers and LDOs Calculating PSRR The PSRR of the audio amplifier can be calculated using Equation 1: Eq. VRIPPLE(IN) VDD + Inputs VRIPPLE(IN) Input Output VRIPPLE(OUT) LDO Output VRIPPLE(OUT) Amplifier - a: Amplifier PSRR Measurement b: LDO PSRR Measurement Figure 2: PSRR Measurement.1. in an LDO.0 .2010. 1: PSRR = 20 · log VRIPPLE(OUT) VRIPPLE(IN) For example. when the amplitude of the input ripple and relative output ripple are known. The PSRR value can be expressed in dB directly by using the crosstalk or A/R function in the measurement instrument instead of calculating it. 2: PSRR = 20 · log 200 · 10-6 200 · 10-3 = -60dB However. An Audio Precision audio analyzer or a network analyzer is needed to perform the PSRR measurement. the PSRR is always expressed as a positive value.08. The common way to perform the PSRR measurement is to add a fixed amplitude sinusoid ripple on the power supply input and measure the amplitude of the same frequency ripple on the output. Measuring PSRR As shown previously. 2 www. the device PSRR can be calculated easily.analogictech.com AN-134. The PSRR can then be calculated using Equation 1 or 2. In this case. The output ripple can be measured by the Audio Precision (AP) analyzer through a 30kHz low pass filter (LPF).analogictech. Choose the crosstalk function and scan frequency from 20Hz to 20kHz. non-inverting amplifier adder. Connect the power supply input (VDD) to the AP’s channel A and connect the output of the low pass filter to the AP’s channel B.1. AN-134. A 200mV peak to peak sinusoid ripple generated from the Audio Precision analyzer output is added onto the power supply input through the non-inverting amplifier adder. 3. then a resulting curve of PSRR over the audio bandwidth will be measured as shown in Figure 4. an 8Ω load.APPLICATION NOTE AN-135 PSRR and Measurement of PSRR in Class-D Audio Amplifiers and LDOs PSRR Measurement of Class D Audio Amplifier Figure 3 shows the PSRR measurement set-up for the AAT5101 Class D audio amplifier. 30kHz low pass filter.5V. Adjust the power supply DC value to the intended system operating range. Figure 4 shows the AAT5101 PSRR measurement results for DC input values of VDD at 2. the DC power supply.0 www.08.2010.6V and 5V.com 3 . and Audio Precision analyzer AP-SYS2722 are needed. DC Input Voltage DC Supply VS + Adder + INA Audio Precision OUT (SYS2722) Sine Ripple VS - VDD INN OUTP LPF + Audio Precision (SYS2722) AAT5101 INP GND OUTN Load INB - Figure 3: AAT5101 PSRR Measurement Set-up. The inputs of the AAT5101 should be AC grounded as shown. Figure 4: AAT5101 PSRR Result. A 200mV peak-to-peak sinusoid ripple waveform generated from network analyzer output is added onto the power supply input (INLDO) through the non-inverting amplifier adder.APPLICATION NOTE AN-135 PSRR and Measurement of PSRR in Class-D Audio Amplifiers and LDOs PSRR Measurement of LDO Figure 5 shows the PSRR measurement set-up for the AAT2868 which contains two LDO regulators. Figure 6: AAT2868 LDO PSRR Measurement Result.analogictech. Connect the LDO input to the network port INA and connect the LDO output to the network port INR. non-inverting amplifier adder. DC Supply DC Input Voltage INA VS + AAT2868 INLDO LDOA INR Network Analyzer (4395A) Adder Network Analyzer (4395A) RF OUT Sine Ripple VS - GND eLoad Figure 5: AAT2868 LDO Part PSRR Measurement Set-up. In this example.08. Figure 6 shows the PSRR measurement result using the AAT2868 LDO for the condition: VIN = 3.com AN-134.0 . ILOAD =10mA. The output voltage can be measured by the network analyzer. Adjust the power supply DC value to the intended system operating range. the DC power supply. Chose the A/R function and a scan frequency from 100Hz to 100kHz. and a network analyzer such as the Agilent 4395A are needed. 4 www. VRIPPLE = 200mVpp.6V.2010.1. DC electronic load. a non-inverting amplifier adder is adopted. DC Input Voltage +12V C3 0.analogictech. The output of the adder will be DC+AC as shown in Figure 7.2010. DC Input Voltage Adder Sine Ripple DC Input Voltage Ripple Peak Value Figure 7: Adder for PSRR Measurement.0 www.08.1μF U1A Sine Ripple V+ C1 OUT 470μF C5 10μF 0Ω C4 10μF R1 20Ω R2 DC Input Voltage Ripple Peak Value OPA V- C2 0.com 5 .APPLICATION NOTE AN-135 PSRR and Measurement of PSRR in Class-D Audio Amplifiers and LDOs Design and Application of the Non-inverting Amplifier Adder To generate a DC power with a certain sinusoid ripple waveform added. Figures 8 and 9 show the schematic and board picture of the non-inverting amplifier adder used in the PSRR measurement.1μF -12V Figure 8: Schematic of the Non-inverting Amplifier Adder in PSRR Measurement. AN-134.1. The DC+AC voltage of VDD + In signal will be generated at OUT. Figure 10 shows the system connecting of the set-up described above.0 . The yellow curve on the scope is the sinusoid ripple waveform before the non-inverting amplifier adder and the blue curve is the DC+AC signal of the Adder output.APPLICATION NOTE AN-135 PSRR and Measurement of PSRR in Class-D Audio Amplifiers and LDOs Figure 9: Board Picture of the Non-inverting Amplifier Adder in PSRR Measurement.analogictech.08.2010. +5V DC power supply input. Yellow trace is the DC+AC output of the adder.1. Connect the power supply input voltage of the test device to VDD. Connect ±12V DC power to +12V and -12V on the board. Figure 10: Real Connect of the Non-inverting Amplifier Adder Board. Sine ripple fed into adder generated by Audio Precision or Network output. Power supply provides ±12V to the adder board.com AN-134. Connect the output of audio precision or network analyzer to In to supply the desire sinusoid ripple. 6 www. copyright or other intellectual property right. AnalogicTech and the AnalogicTech logo are trademarks of Advanced Analogic Technologies Incorporated.g. or infringement of any patent. PSRR Calculation Class D Audio Amplifier LDO PSRR = 20 · log VRIPPLE(OUT) VRIPPLE(IN) Measurement Equipment Audio Precision (e. the biggest challenge is that the output ripple amplitude is very small. Specific testing of all parameters of each device is not necessarily performed.1. AP-SYS2722). Table 1 shows the required equipment and calculations for the Class D audio amplifier and the LDO PSRR measurement. Testing and other quality control techniques are utilized to the extent AnalogicTech deems necessary to support this warranty.analogictech. No circuit patent licenses. copyrights.g.0 www. Santa Clara. Inc. merchantability.g. AnalogicTech reserves the right to make changes to their products or specifications or to discontinue any product or service without notice.2010. An Audio Precision analyzer (e.08. AnalogicTech assumes no liability whatsoever. 3230 Scott Boulevard. Advanced Analogic Technologies. or other intellectual property rights are implied. adequate design and operating safeguards must be provided by the customer to minimize inherent or procedural hazards. Agilent 4395A) is necessary to do the measurement correctly. Adder VRIPPLE(IN) PSRR = 20 · log V RIPPLE(OUT) Table 1: PSRR Calculation Equation and Measurement Equipment. This makes it difficult to measure accurately using common equipment such as an oscilloscope or multi-meter. AnalogicTech cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in an AnalogicTech product. mask work rights. Agilent-4395A).com 7 . AP-SYS2722) or a network analyzer (e. AN-134. Adder Network Analyzer (e. and AnalogicTech disclaims any express or implied warranty relating to the sale and/or use of AnalogicTech products including liability or warranties relating to fitness for a particular purpose. Except as provided in AnalogicTech’s terms and conditions of sale. In order to minimize risks associated with the customer’s applications. Inc.APPLICATION NOTE AN-135 PSRR and Measurement of PSRR in Class-D Audio Amplifiers and LDOs Conclusion The PSRR measurement indicates how well a circuit rejects ripple or noise from the power supply input. CA 95054 Phone (408) 737-4600 Fax (408) 737-4611 © Advanced Analogic Technologies.g. When making a PSRR measurement. All other brand and product names appearing in this document are registered trademarks or trademarks of their respective holders.