APPLICATION NOTE
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
RF
Amplifier
V BAT
Audio In
Audio
Amplifier
Figure 1: RF Subsystem Diagram.
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APPLICATION NOTE
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. 1: PSRR = 20 · log
VRIPPLE(OUT)
VRIPPLE(IN)
For example, 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.
The PSRR of the LDO can then be calculated using Equation 2:
Eq. 2: PSRR = 20 · log
200 · 10-6
200 · 10-3 = -60dB
However, in an LDO, the PSRR is always expressed as a positive value.
Measuring PSRR
As shown previously, when the amplitude of the input ripple and relative output ripple are known, the device PSRR can
be calculated easily.
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. The PSRR can then be calculated using
Equation 1 or 2.
VRIPPLE(IN)
+
Inputs
VRIPPLE(IN)
VDD
Amplifier
Output
Input
VRIPPLE(OUT)
LDO
Output
VRIPPLE(OUT)
-
a: Amplifier PSRR Measurement
b: LDO PSRR Measurement
Figure 2: PSRR Measurement.
An Audio Precision audio analyzer or a network analyzer is needed to perform the PSRR measurement. 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
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APPLICATION NOTE
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. In this case, the DC power supply, non-inverting amplifier adder, an 8Ω load, 30kHz low pass filter, and Audio Precision analyzer AP-SYS2722 are
needed.
DC Supply
DC Input
Voltage
VS +
Adder
Audio
Precision OUT
(SYS2722)
Sine Ripple
+
INA
VS -
LPF
VDD
INN
+
OUTP
AAT5101
INP
-
Load
OUTN
Audio
Precision
(SYS2722)
INB
-
GND
Figure 3: AAT5101 PSRR Measurement Set-up.
The inputs of the AAT5101 should be AC grounded as shown. 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. The
output ripple can be measured by the Audio Precision (AP) analyzer through a 30kHz low pass filter (LPF). 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. Choose
the crosstalk function and scan frequency from 20Hz to 20kHz, then a resulting curve of PSRR over the audio bandwidth
will be measured as shown in Figure 4. 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 2.5V, 3.6V and 5V.
Figure 4: AAT5101 PSRR Result.
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APPLICATION NOTE
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. In this example,
the DC power supply, non-inverting amplifier adder, DC electronic load, and a network analyzer such as the Agilent
4395A are needed.
DC Supply
DC Input
Voltage
VS +
Adder
Network
Analyzer
(4395A)
RF
OUT
Sine Ripple
VS -
INA
Network
Analyzer
(4395A)
AAT2868
INLDO
LDOA
INR
GND
eLoad
Figure 5: AAT2868 LDO Part PSRR Measurement Set-up.
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. The output voltage can be measured by the network
analyzer. Connect the LDO input to the network port INA and connect the LDO output to the network port INR. Chose
the A/R function and a scan frequency from 100Hz to 100kHz. Adjust the power supply DC value to the intended system operating range.
Figure 6 shows the PSRR measurement result using the AAT2868 LDO for the condition: VIN = 3.6V, VRIPPLE = 200mVpp,
ILOAD =10mA.
Figure 6: AAT2868 LDO PSRR Measurement Result.
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APPLICATION NOTE
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, a non-inverting amplifier adder is adopted. The
output of the adder will be DC+AC as shown in Figure 7.
DC Input
Voltage
DC Input
Voltage
Adder
Ripple
Peak Value
Sine Ripple
Figure 7: Adder for PSRR Measurement.
Figures 8 and 9 show the schematic and board picture of the non-inverting amplifier adder used in the PSRR measurement.
DC Input
Voltage
+12V
U1A
Sine Ripple
C3
C4
R1
0.1μF
10μF
20Ω
V+
OPA
C1
OUT
VC2
0.1μF
470μF
R2
DC Input
Voltage
Ripple
Peak Value
0Ω
C5
10μF
-12V
Figure 8: Schematic of the Non-inverting Amplifier Adder in PSRR Measurement.
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APPLICATION NOTE
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.
Connect ±12V DC power to +12V and -12V on the board. Connect the power supply input voltage of the test device to
VDD. Connect the output of audio precision or network analyzer to In to supply the desire sinusoid ripple. 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.
Yellow trace is
the DC+AC output
of the adder.
Power supply provides
±12V to the adder board.
+5V DC power
supply input.
Sine ripple fed into adder
generated by Audio Precision
or Network output.
Figure 10: Real Connect of the Non-inverting Amplifier Adder Board.
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.
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APPLICATION NOTE
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. When making
a PSRR measurement, the biggest challenge is that the output ripple amplitude is very small. This makes it difficult to
measure accurately using common equipment such as an oscilloscope or multi-meter. An Audio Precision analyzer (e.g.
AP-SYS2722) or a network analyzer (e.g. Agilent 4395A) is necessary to do the measurement correctly.
Table 1 shows the required equipment and calculations for the Class D audio amplifier and the LDO PSRR measurement.
PSRR Calculation
Class D Audio Amplifier
PSRR = 20 · log
VRIPPLE(OUT)
VRIPPLE(IN)
VRIPPLE(IN)
PSRR = 20 · log V
RIPPLE(OUT)
LDO
Measurement Equipment
Audio Precision (e.g. AP-SYS2722), Adder
Network Analyzer (e.g. Agilent-4395A), Adder
Table 1: PSRR Calculation Equation and Measurement Equipment.
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