Solutions to reduce noise in op amp circuits

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Solutions to reduce noise in op amp circuits [Copy link]

Noise can be a random or repetitive signal, generated internally or externally, in the form of voltage or current with wideband or broadband, high frequency or low frequency. (Here, we define noise as any useless signal at the output of the op amp)
　　Noise usually includes the inherent noise of the device and external noise. The inherent noise includes: thermal noise, shot noise and low-frequency noise (1/f noise), etc.; external noise usually refers to power supply noise, spatial coupling interference, etc., which can usually be avoided or reduced through reasonable design. Reducing the impact of external noise is crucial to the performance of low-noise op amps.

　　Common external noise sources
　　Power supply ripple
　　In the circuit of full-wave rectified linear voltage regulator power supply, 100Hz ripple is the main power supply noise. For the op amp circuit, the 100Hz noise level is usually required to be controlled within 10nV-100nV (RTI), which depends on three factors: the power supply rejection ratio (PSRR) of the op amp at 100Hz, the ripple rejection ratio of the regulator and the size of the input filter capacitor of the regulator.
　　Figure 1 is the PSRR-frequency curve of ADI high-voltage amplifier OP77. It can be seen that the PSRR of OP77 at 100Hz is about 76dB. To obtain a performance of no more than 100nV (RTI), the ripple of the power supply must be less than 0.6mV. Commonly used three-terminal voltage regulators can generally provide about 60dB of ripple suppression capability. In this case, the input filter capacitor of the regulator must be large enough to limit the ripple at the input to less than 0.6V.

　　Power supply decoupling
　　A typical series regulator power supply contains noise with an amplitude of 150uV and a frequency range of 100Hz-100KHz. The switching power supply is more serious. The PSRR of the op amp decreases at a rate of 20dB/Decade at high frequencies. By adding an RC or LC decoupling network to the power supply pin, most of the noise can be filtered out. The circuit form is shown in Figure 3. When using RC decoupling, it should be noted that changes in load current will cause modulation of the voltage on the power supply pin.

　　Figure 3: RC decoupling of op amp power supply
　　Power supply regulation
　　Any change in power supply voltage will cause a change in the op amp input bias current. The PSRR of OP77 in Figure 1 is 126dB (0.5uV/V) at DC. The change in power supply voltage is a potential source of low-frequency noise. In the application of low-noise op amps, it is important to reduce the ripple of the power supply and improve the regulation rate of the power supply. Insufficient power supply regulation usually causes annoying low-frequency noise.
　　Switching power supply
　　Switching power supply is a very serious noise source. The following figure is a typical voltage waveform at the output of a switching power supply:

　　Figure 4. 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 damped oscillation caused by the resonance of the switching loop, which extends from tens of kHz to tens of MHz. The PSRR of ordinary op amps begins to drop sharply above hundreds of Hz, and is almost zero at hundreds of kHz. At this time, the power supply noise appearing at the output will be very serious.
　　Influencing channels and countermeasures:
　　In addition to paying attention to the selection of op amp PSRR or CMRR parameters and strengthening the op amp power supply decoupling (such as using RC decoupling), in the design of switching power supply, the following aspects should also be noted:
　　- The noise in the power supply may be directly coupled to the input of the amplifier through the reference source or PCB leakage. Pay attention to the filtering of the voltage reference source output. For PCB leakage, ground wire protection can be added between the signal input lead and the power supply line;
　　- The noise may be directly coupled to the input of the amplifier through the distributed capacitance between the PCB lines, causing interference. When PCB wiring, be careful not to run the power line and weak signal line parallel to each other, the line net distance should be greater than 3 times the line width (3W principle), and add ground wire isolation between the power line or digital signal line and the analog small signal line;
　　- Improper grounding, noise affects sensitive circuit parts through common impedance. In order to prevent the common impedance from introducing power supply noise into the signal loop, pay attention to the following points: avoid large current with noise flowing through the front stage small signal ground; single point grounding, separate grounding for power supply, analog and digital circuits; use ground plane layer for layout to minimize ground impedance; the switching power supply output leads the power ground from the ground end of the last filter capacitor, and avoids leading from the ground end of the capacitor before the filter inductor.

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　　Figure 5: Schematic diagram of common-mode impedance noise coupling
　　The displacement current driven by the switch voltage at the drain of the switch tube forms a loop through the primary and secondary distributed capacitance, the secondary circuit, the secondary to the ground and the stray capacitance, and the stray capacitance between the ground and the primary ground. The common-mode current flowing in the secondary analog circuit flows through the unbalanced impedance and is converted into a differential mode, causing interference to the amplifier circuit (as shown in Figure 6). The interference introduced by the common-mode method is generally the high-frequency component (above several MHz) in the switching noise.
　　The measures mainly include the following three points:
　　1. Provide a low-impedance noise bypass channel from the secondary ground of the switching power supply back to the primary ground, usually using a 1000p~2200p safety capacitor;
　　2. Use a common-mode choke to enhance the common-mode filtering of the output of the switching power supply;
　　3. Use isolation technology to minimize the common-mode current in the loop.

　　Figure 6. Common-mode current loop in switching power supply
　　The signal is affected by coupling to the signal loop or ground loop with a certain loop area through the spatial magnetic field. In addition, high-frequency interference from the switching power supply or the mains network may be directly coupled to the signal loop through spatial stray capacitance.
　　Considerations in the design include
　　- Reasonable layout, adjustment of the placement direction of the inductor coil or transformer, optimization of wiring, reduction of the loop area of key signals, and avoidance of the formation of ground loops to reduce interference;
　　- Double-sided or single-sided wiring, pay attention to the signal line and the ground line, the power line and the ground line must be closely parallel to the line; use 1000p capacitor RF multi-point grounding to take into account the requirements of EMC and low-frequency signal-to-noise ratio;
　　- Shield sensitive circuits, and pay attention to the shielding layer connected to the reference ground of the protected signal;
　　- In the wiring design, pay attention to the power line not being bundled with the signal line.
　　Summary: The main measures to reduce power supply noise in op amp circuit design include:
　　- Reducing the ripple and noise components of the power supply output through decoupling, filtering and other measures
　　- Improving the design and improving the power supply voltage regulation rate
　　- Reasonable circuit structure, sophisticated PCB wiring, and reasonable routing technology
　　- Selecting devices with higher PSRR or CMRR in sensitive noise bands