Maintain low impedance for power into integrated circuits (ICs) through power supply decoupling
Analog integrated circuits such as amplifiers and converters have at least two or more power supply pins. For single-supply devices, one of the pins is usually connected to ground. Mixed-signal devices such as ADCs and DACs can have analog and digital supply voltages and I/O voltages. Digital ICs like FPGAs can also have multiple supply voltages, such as core voltages, memory voltages, and I/O voltages.
Regardless of the number of power pins, IC data sheets detail the allowable ranges for each power supply, including recommended operating ranges and absolute maximum values, and these limits must be adhered to in order to maintain proper operation and prevent damage.
However, small changes in supply voltage due to noise or power supply ripple—even within the recommended operating range—can cause device performance degradation. In amplifiers, for example, small power supply changes produce small changes in the input and output voltages, as shown in Figure 1.
Figure 1. The power supply rejection of an amplifier shows the sensitivity of the output voltage to changes in the supply rails.
The sensitivity of an amplifier to supply voltage changes is usually quantified by the power supply rejection ratio (PSRR), which is defined as the ratio of supply voltage changes to output voltage changes.
Figure 1 shows how the PSR of a typical high-performance amplifier (OP1177) decreases with frequency by approximately 6dB/8 octave (20dB/10 octave). The figure shows the curves for both cases with positive and negative power supplies. Although the PSRR is 120dB at dc, it decreases rapidly at higher frequencies, where more and more unwanted energy on the power line is coupled directly to the output.
If the amplifier is driving a load and there is unwanted impedance on the supply rail, the load current modulates the supply rail, increasing noise and distortion in the AC signal.
Although the actual PSRR may not be given in the data sheet, the performance of data converters and other mixed-signal ICs can also degrade with noise on the power supply. Power supply noise can also affect digital circuits in a number of ways, including reducing logic-level noise margins and creating timing errors due to clock jitter.
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