Analyze how to reduce noise in op amp circuits from multiple angles, all the solutions are here!

In the full-wave rectified linear regulated power supply circuit, 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's high-voltage amplifier OP77. It can be seen that the PSRR of OP77 is about 76dB at 100Hz. 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.

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 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 pin.

Figure 3: RC decoupling of op amp power supply

Any change in power supply voltage will cause a change in the input bias current of the op amp. In Figure 1, the PSRR of OP77 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.

The switching power supply is a very serious noise source. The following figure is a typical voltage waveform at the output of the switching power supply:

Figure 4. Voltage waveform at the output of the switching power supply

It can be seen that the noise spectrum includes not only the switching frequency and its harmonic components, but also the high-frequency components of 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 a few hundred Hz, and is almost zero at a few hundred kHz. At this time, the power supply noise appearing at the output end will be very serious.

In addition to paying attention to the selection of op amp PSRR or CMRR parameters and strengthening op amp power supply decoupling (such as using RC decoupling), the following aspects should also be noted in the design of switching power supply:

• 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 filtering the output of the voltage reference source. For PCB leakage, add ground wire protection between the signal input lead and the power supply line;

• Noise may be directly coupled to the amplifier input through the distributed capacitance between PCB traces, causing interference. When PCB wiring, be careful not to run the power line and weak signal line close to each other and in parallel. The line clearance should be greater than 3 times the line width (3W principle), and ground wires should be added between the power line or digital signal line and the analog small signal line for isolation;

• Improper grounding can cause noise to affect sensitive circuit parts through common impedance. In order to prevent common impedance from introducing power supply noise into the signal loop, the following points should be noted: Avoid large current with noise flowing through the small signal ground of the previous stage; Single-point grounding, separate grounding for power supply, analog, and digital circuits; Use ground plane layer for layout to minimize ground impedance; The output of the switching power supply should lead the power supply ground from the ground terminal of the last filter capacitor, and avoid leading it from the ground terminal of the capacitor before the filter inductor.

  
  

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.

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 common-mode chokes to enhance common-mode filtering of the output of the switching power supply;
   

3. Use isolation techniques to minimize common-mode currents in the loop.

   
   

Figure 6. Common-mode current loop in a switching power supply.

The spatial magnetic field is coupled to the signal loop or ground loop with a certain loop area, causing an impact 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 spatial stray capacitance.

• Reduce the ripple and noise components of the power supply output through decoupling, filtering and other measures

• Improve the design and increase the power supply voltage regulation

• Reasonable circuit structure, sophisticated PCB wiring, and reasonable routing technology

• Select devices with higher PSRR or CMRR in the sensitive noise band