Author: Hao Wang Shenzhen analog engineers PSRR (Power supply rejection ratio), also known as power supply rejection ratio, is an important parameter to measure the magnitude of ripple rejection in the input power supply. It is expressed as the logarithmic ratio of output ripple and input ripple in decibels (dB) [1] ], its calculation formula is: In the formula: : ripple peak-to-peak value in input voltage : ripple peak-to-peak value in output voltage It can be seen from the formula that the larger the PSRR, the smaller the ripple of the same input ripple at the output end, and the RFO with high PSRR is required for RF and wireless applications with high ripple requirements. So how is the PDRR of the LDO measured? This article summarizes the various measurement methods. The DC voltage Vin_DC input to the LDO is superimposed with a certain frequency and the peak-to-peak value is Rippleinput's AC voltage Vin_AC (the AC voltage peak-to-peak value is generally several hundred millivolts), and then the Vout_DC AC voltage Vout_AC peak-to-peak Rippleoutput is measured in the LDO output voltage, and finally utilized. Equation 1 calculates the PSRR at this frequency. The input voltage of the LDO needs to meet the following conditions during the test: The maximum input voltage must not exceed the maximum operating voltage of the LDO. The input voltage minimum is greater than the sum of the LDO output voltage and the voltage drop. The principle of PSRR measurement is very simple, but it is not easy to find in the actual measurement process, mainly reflected as: How to superimpose the AC voltage in the DC voltage? A signal generator with a bias voltage function seems to suffice, but the maximum output current of the signal generator is typically tens of milliamperes. If the LP5907 with an output of 150 mA is to be measured, it will not be sufficient. How to measure the peak-to-peak value of the AC voltage in the LDO output voltage? A typical oscilloscope can only measure millivolts. When the PSRR of the LDO is 60dB, the output ripple is usually less than 1mV, and the oscilloscope cannot accurately measure it. For the above two issues, this article will introduce the corresponding solutions. Use professional input injectors, such as J2120A, bandwidth 10Hz-10MHz, DC voltage maximum 50V, output current up to 5A, with network analyzer to measure the LDO input and output AC voltage, use software to draw LDO in the settings PSRR in the frequency range. Figure 1 Input injector and network analyzer test PSRR 2. Addition op amp circuit An operational amplifier is used to design the summing circuit to superimpose the DC voltage and the AC voltage on the output. The choice of op amps needs to meet the following basic conditions: 1) The bandwidth of the op amp meets the LDO test range. 2) The maximum output current of the op amp is not less than the maximum output current of the LDO. 3) The output voltage range of the op amp covers the input voltage range of the LDO. There are many operational amplifiers that meet the above requirements, such as OPA552, OPA564, THS3120, etc. The addition circuit diagram is shown in Figure 2 (R1=R2). The minimum cutoff frequency of this circuit is determined by C1 and R1 [2], the highest cutoff frequency. It is determined by the bandwidth of the op amp. Figure 2 Addition op amp circuit The op amp can also be designed as a voltage follower if the signal generator dc bias voltage maximum meets the measurement requirements. In this method, the input capacitance of the LDO should be removed when measuring the PSRR to avoid the instability of the operational amplifier. 3. LC node method The method of superimposing DC voltage and AC voltage by using inductor and capacitor is shown in Figure 3. The maximum frequency of the circuit is determined by L1 and C1, and the lowest frequency is determined by C1. Figure 3 LC node method For a general oscilloscope, the millivolt-level voltage can be measured. When the PSRR of the LDO is not higher than 40dB~50dB, if the input AC voltage peak-to-peak value is 1V, the peak-to-peak voltage of the same frequency AC voltage in the LDO output is 3mV~10mV. Direct measurement with an oscilloscope. 2. Amplifier and oscilloscope measurements When the PSRR of the LDO is greater than 50 dB, since the output ripple amplitude is usually less than 1 mV, it cannot be directly measured by the oscilloscope. At this time, consider using an operational amplifier to amplify the LDO output AC voltage by a factor of 100 or more. When designing an op amp, consider: 1) The LDO output has a DC voltage and the circuit needs to remove the DC voltage. 2) The noise generated by the amplifier circuit itself is much smaller than the amplified AC voltage. 3) The op amp input offset voltage should not be too large, otherwise it will output a large DC voltage after amplification by the amplifier circuit. 4) The bandwidth of the amplifier circuit satisfies the PSRR measurement frequency range of the LDO. Therefore, low noise, low input offset voltage and high bandwidth op amps such as OPA211, OPA228, OPA189, etc. can be selected during design. The amplifying circuit is shown in Figure 4. The lowest cutoff frequency of the circuit is determined by C1 and R1. The highest cutoff frequency is determined by the bandwidth of the op amp. Figure 4 amplifier circuit 3. Spectrum analyzer measurement The spectrum analyzer measures the microvolt-level voltage signal and can be used with a high-impedance input probe to measure the LDO output AC voltage. However, spectrum analyzer high-impedance input probes are usually expensive and generally not equipped in the lab. Consider using an op amp to build a high-impedance probe. See Steve Hageman's high-impedance FET probe for extending the range of RF spectrum analyzers. The circuit mentioned in [3], as shown in Figure 5, can be used with the OPA656. Figure 5 high resistance probe circuit The measured LDO is TPS7A4901, the output voltage of the TPS7A4901EVM is redesigned to 1.2V, and the output capacitance is changed to 10uF. The THS3120 is used as a DC voltage and AC voltage superposition circuit, and the THS3120EVM is used and changed to the circuit shown in FIG. The OPA211 is selected as the 100x amplification circuit shown in Figure 7. Figure 6 THS3120 DC voltage and AC voltage superposition circuit Figure 7 OPA211 amplifier circuit The THS3120 and OPA211 supply voltage is ±15V, the THS3120 DC voltage is 3.2V, the AC sinusoidal voltage is 1kHz and the peak-to-peak value is 1V. The TPS7A4901 has an output current of 150mA, and the NR/SS pin capacitor and feedforward capacitor are not connected. Figure 8 shows the output ripple and input ripple after LDO amplification. Figure 9 shows the output ripple FFT conversion after TPS7A4901 amplification. Figure 8 Output ripple (yellow line) and input ripple (blue line) after LDO amplification Figure 9 Output ripple FFT transform after amplification of TPS7A4901 It can be seen from Fig. 9 that the output ripple amplitude at 1 kHz is -26.46 dbV, which is converted to an unamplified LDO output voltage with a peak-to-peak value of 0.95 mV at 1 kHz. Using Equation 1, the PSRR is 60.4 dB, and the datasheet is obtained. The 62dB is relatively close, and changing the AC voltage frequency can also measure the PSRR at different frequencies. If the input injector and network analyzer are used, it is convenient to measure the PSRR curve of the LDO in the set frequency range. If you do not have an input injector and network analyzer, you can choose a combination of the inputs and outputs listed in the appeal, then set a frequency, measure the amplitude of the AC voltage in the input and output voltages, use Equation 1 to derive the PSRR, and then change The input AC signal frequency is repeatedly measured, and finally the PSRR curve is obtained over the entire frequency range. [1] Understanding power supply ripple rejection in linear regulators, http:// [2] LDO PSRR Measurement Simplified, http:// [3] High-impedance FET probe extends RF-spectrum analyzer's usable range, https://High-impedance-FET-probe-extends-RF-spectrum-analyzer-s-usable-range Touch Panel For Iphone,Touch Screen Digitizer Panel,Touch Screen Panel For Iphone,Phone Touch Screen Shenzhen Xiangying touch photoelectric co., ltd. , https://www.starstp.com
