Controlling Noise in a Power Source

• 1). Obtain the datasheet for the circuit power source. Often, a voltage regulator such as a five-volt LM7805 is used in digital circuits. For this example, National Semiconductor's "LM340/LM78XX..." datasheet will be used.
  
  
• 2). Locate the section of the LM7805 datasheet titled "Typical Applications." Refer to the diagram that is labeled "Fixed Output Regulator." For the LM7805 regulator, the datasheet suggests using two capacitors, C1 and C2. On datasheets and schematics, capacitors are symbolized by two horizontal lines and are labeled with "C" and a number.
  
  
• 3). Locate the suggested capacitor type for C1 and C2. On the LM7805 datasheet, the second note under the "Fixed Output Regulator" diagram suggests a ceramic disc capacitor for C2. A ceramic disc capacitor can also be used for C1. Ceramic disc capacitors are effective for controlling noise in power sources/regulators because they have a low ESR, or Equivalent Series Resistance. Low-ESR is a quality to look for in decoupling/bypass capacitors.
  
  
• 4). Locate any polarity indications for C1 and C2. On the LM7805 datasheet there are no polarity indications for C1 and C2. If a capacitor is polarized, it will have a "+" next to one side of the capacitor symbol. Also, one side of the capacitor symbol may be curved, which designates the negative or "-" side. A polarized capacitor has to be connected in a specific way, like a battery. Polarity indications also mean that the manufacturer wants you to use an electrolytic or tantalum capacitor.
  
  
• 5). Convert the values of C1 and C2 to capacitor codes. Use a code reference such as "Capacitor Code Information" in the "Resources" section. The LM7805 datasheet lists .22uF for C1 and .1uF for C2. The three-digit capacitor code for C1 is 224 and the code for C2 is 104. Most disc-type capacitors use a three-digit code for the value, which is printed on the surface of the capacitor. Converting the capacitor value into the correct code is essential.
  
  
• 6). Examine the size of the connections for C1 and C2. One side of C1 is connected to the LM7805 input pin and the other side of C1 is connected to the LM7805 gnd pin. On side of C2 is connected to the LM7805 output pin and the other side of C2 is connected to the LM7805 gnd pin. Typically, the LM7805 is a through-hole (has pin/legs) component. A through-hole disc capacitor is a good match and it can accommodate the length of the required connections.
  
  
• 7). Calculate the overall space requirements for the capacitor. This may dictate the packaging-style (through-hole or surface-mount) of the capacitor. In general, a decoupling/bypass capacitor is most effective when it is placed close to the power supply and the capacitor leads/legs are kept as short as possible. Again, through-hole, ceramic disc capacitors are a proper match for the LM7805.
  
  
• 8). Verify maximum voltage ratings for C1 and C2. It is important to choose a capacitor with a voltage rating that is above the highest voltage level of the circuit. Ceramic disc voltage ratings are printed on the capacitor or are listed on the manufacturer's specifications sheet. For the 5V, LM7805 circuit, a safe capacitor rating is 25V. Most ceramic disc capacitors are actually rated at 50V or above.
  
  

Controlling Noise in an IC

• 1). Obtain the manufacturer's datasheet for the IC. Analog Devices' datasheet for the AD7376 digital potentiometer will be used for this example. The datasheet is often the best source for capacitor information because it usually explains how to use the IC in a circuit.
  
  
• 2). Locate the section of the AD7376 datasheet titled "Layout And Power Supply Biasing." Four bypass capacitors (C1, C2, C3 and C4) are pictured in the diagram.
  
  
• 3). Locate the suggestions for capacitor type. The AD7376 datasheet states that tantalum or electrolytic capacitors should be used for C3 and C4. Tantalum is the best choice for noise suppression because of the low-ESR rating. C2 and C1 are not specified, but low-ESR ceramic disc capacitors are ideal.
  
  
• 4). Locate any polarity indications for the capacitors. The AD7376 datasheet has a "+" on one side of C3 and C4. The other two capacitors, C1 and C2, have no polarity indications. C3 and C4 are tantalum capacitors, which are polarity-sensitive. C1 and C2 are ceramic disc, which are not polarized.
  
  
• 5). Examine the size of the capacitor connections. For Vdd and gnd, the positive side of C3 is connected to Vdd and the negative side of C3 is connected to gnd or ground. One side of C1 is connected to Vdd and the other side of C1 is connected to gnd. Since the AD7376 is a small IC and the pins are tightly spaced, surface-mount ceramic and tantalum capacitors can be used. Surface-mount have flat terminals instead of pins/legs.
  
  
• 6). Calculate the overall space requirements for the capacitors. The AD7376 datasheet states that a remote connection to the gnd pin is desirable. This means that the gnd connection of the capacitors will be a wire or circuit trace that travels a short distance to the gnd pin of the AD7376 chip. Because the smallest circuit pathway and shortest connections are most effective, surface-mount capacitors are the best choice for the AD7376.
  
  
• 7). Convert the values of C3 and C4 to capacitor codes. They are listed as 10uF but the datasheet states that they can be between 1uF and 10uF. On a surface-mount tantalum capacitor, the printed code for 10uF will be 106.
  
  
• 8). Convert the values of C1 and C2 to capacitor codes. They are both listed as .1uF. Ceramic capacitors will have a three-digit code printed on the surface. For both C1 and C2, the printed code will be "104."
  
  
• 9). Verify the voltage ratings for the capacitors. Again, make sure the rating is higher than the maximum circuit voltage. The voltage rating is printed on the capacitor or listed on the manufacturer's specifications sheet.