Power ripple
In a full-wave rectified linear regulated power supply circuit, the 100Hz ripple is the main power supply noise. For op amp circuits, the 100Hz noise level is usually required to be controlled within 10nV-100nV (RTI), depending on three factors: The power supply rejection ratio (PSRR) at 100 Hz operation, the ripple rejection ratio of the regulator, and the input filter capacitance of the regulator. Figure 1 shows the PSRR-frequency curve of the OP77. It can be seen that the PSRR of the OP77 is approximately 76dB at 100Hz. To obtain a performance of less than 100nV (RTI), the ripple of the power supply must be less than 0.6mV. Commonly used three-terminal regulators typically provide approximately 60dB of ripple rejection. In this case, the regulator's input filter capacitor must be large enough to limit the input ripple below 0.6V.
Power supply decoupling
A typical series regulator supplies power with 150uV amplitude and 100Hz-100KHz noise. The switching power supply is more severe. The PSRR of the op amp is reduced by 20dB/Decade at high frequency and passed through the power supply. The foot plus RC or LC decoupling network can filter out most of the noise. The circuit is shown in Figure 3. When using RC decoupling, it should be noted that changes in the load current can cause modulation of the voltage on the power pin.
Power regulation
Any change in the supply voltage will cause the input bias current of the op amp to change. In Figure 1, the PSRR of the OP77 is 126dB (0.5uV/V) at DC. The change in the supply voltage is a potential low-frequency noise source. In a low-noise op amp application, it is important to reduce the ripple on the power supply and to increase the regulation of the power supply. Inadequate power supply regulation often causes unpleasant low-frequency noise.
Switching power supply
The switching power supply is a very serious noise source. The following figure shows the typical voltage waveform at the output of the switching power supply:
It can be seen that the noise spectrum includes both the switching frequency and its harmonic components, as well as the high-frequency components of the ringing caused by the switching loop resonance, which extends from several tens of kHz to several tens of MHz while the ordinary operation is stored at several hundred Hz. The above PSRR begins to drop sharply, it is nearly zero to a few hundred KHz, at this moment, the power noise appearing on the output end will be very serious.
Impact Approaches and Countermeasures
In addition to paying attention to the choice of op amp PSRR or CMRR parameters and enhancing the power supply decoupling (such as using RC decoupling), in the switching power supply design, the following aspects should also be noted:
Noise in the power supply may be directly coupled to the input of the amplifier through the reference source or the leakage of the PCB. Pay attention to the filtering of the voltage reference source output. For PCB leakage, ground protection can be added between the signal input lead and the power supply trace.
Noise can be directly coupled to the amplifier input through the distributed capacitance between the PCB traces, causing interference. When PCB layout, pay attention to the power line and the weak signal line should not be close to the parallel line, the line spacing is more than 3 times the line width (3W principle), and add the ground between the power line or digital signal line and analog small signal line Line isolation.
Grounding is handled improperly, noise affects the sensitive circuit part through common impedance. In order to prevent the common impedance from introducing power noise into the signal loop, attention should be paid to the following points: Avoid high-noise current flowing through the small-signal ground of the previous stage on the ground; ground the single point, separate the power, analog, and digital circuits from ground; The plane layer minimizes the impedance of the ground line; the output of the switching power supply leads from the ground terminal of the last filter capacitor to the power ground to avoid drawing from the ground of the capacitor before the filter inductor.
The displacement current driven by the switch drain switch voltage, through the primary and secondary distributed capacitance, the secondary circuit, the secondary to ground and stray capacitance, the stray capacitance between the ground and the primary ground form a loop, the secondary analog circuit flows through The common-mode current flows through the unbalanced impedance and is converted into a differential mode, causing interference to the amplifier circuit (see Figure 6). The interference introduced by the common mode is generally the high-frequency component (more than a few MHz) in the switching noise. The measures mainly include the following three points: providing a low-impedance noise bypass channel returning from the switching power supply secondary ground to the primary ground, usually using a 1000p~2200p safety capacitor; using a common mode choke coil to strengthen the common mode of the output of the switching power supply Filtering; use isolation techniques to minimize common-mode currents in the loop.
The spatial magnetic field is coupled into a signal loop or a ground loop having a certain loop area, causing an influence on the signal. In addition, high-frequency interference from the switching power supply or mains network may be directly coupled to the signal loop through the stray capacitance in the space. Considerations in the design include: a reasonable layout, adjust the placement direction of the inductor or transformer, optimize the wiring, reduce the loop area of ​​the critical signal, avoid the formation of a ground loop to reduce interference; double-sided or single-panel wiring, pay attention to the signal line and Ground line, power line and ground must be close to the parallel line; use 1000p capacitor RF multi-point grounding, can take into account the EMC and low-frequency signal-to-noise ratio requirements; add shielding to the sensitive circuit, pay attention to the shielding layer connected to the reference signal being protected Ground; cable design attention to the power cord and signal lines do not bundle together.
summary
The main measures to reduce power supply noise in op amp circuit design include:
Through decoupling, filtering and other measures to reduce the power supply output ripple and noise components.
Improve design and increase power supply voltage regulation.
Reasonable circuit structure, elegant PCB layout, and reasonable wiring process.
Choose a device with a higher PSRR or CMRR in the sensitive noise band.
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