I Volts I=0.5mA 00001000c IGURE 1. 8 The total resistor noise is the sum of current noise and thermal noise. The current noise approaches the thermal noise at higher frequencies.( Source: Phillips Components, Discrete Products Division, 1990-91 Resistor/Capacitor Data Book, 1991. with permission. over one decade bandwidth to the average voltage caused by a specified constant current passed through the resistor at a specified hot-spot temperature [ Phillips, 1991] Noise voltage 1.10) dc vo V×10 (1.11) f where N.I. is the noise index, Va is the dc voltage drop across the resistor, and fi and f represent the frequency range over which the noise is being computed. Units of noise index are uV/V. At higher frequencies, the current noise becomes less dominant compared to Johnson noise. Precision film resistors have extremely low noise. Composition resistors show some degree of noise due internal electrical contacts between the conducting particles held together with the binder Wire-wound resistors re essentially free of electrical noise unless resistor terminations are faulty Power Rating and Derating Curves Resistors must be operated within specified temperature limits to avoid permanent damage to the materials. The temperature limit is defined in terms of the maximum power, called the power rating, and derating curve. The power rating of a resistor is the maximum power in watts which the resistor can dissipate. The maximum power rating is a function of resistor material, maximum voltage rating, resistor dimensions, and maximum allowable hot-spot temperature. The maximum hot-spot temperature is the temperature of the hottest part on the resistor when dissipating full-rated power at rated ambient temperature. The maximum allowable power rating as a function of the ambient temperature is given by the derating curve Figure 1.9 shows a typical power rating curve for a resistor. The derating curve is usually linearly drawn from the full-rated load temperature to the maximum allowable no-load temperature. A resistor may be operated at ambient temperatures above the maximum full-load ambient temperature if operating at lower an full-rated power capacity. The maximum allowable no-load temperature is also the maximum storage temperature for the resistor. e 2000 by CRC Press LLC© 2000 by CRC Press LLC over one decade bandwidth to the average voltage caused by a specified constant current passed through the resistor at a specified hot-spot temperature [Phillips, 1991]. (1.10) (1.11) where N.I. is the noise index, Vdc is the dc voltage drop across the resistor, and f1 and f2 represent the frequency range over which the noise is being computed. Units of noise index are mV/V. At higher frequencies, the current noise becomes less dominant compared to Johnson noise. Precision film resistors have extremely low noise. Composition resistors show some degree of noise due to internal electrical contacts between the conducting particles held together with the binder.Wire-wound resistors are essentially free of electrical noise unless resistor terminations are faulty. Power Rating and Derating Curves Resistors must be operated within specified temperature limits to avoid permanent damage to the materials. The temperature limit is defined in terms of the maximum power, called the power rating, and derating curve. The power rating of a resistor is the maximum power in watts which the resistor can dissipate. The maximum power rating is a function of resistor material, maximum voltage rating, resistor dimensions, and maximum allowable hot-spot temperature. The maximum hot-spot temperature is the temperature of the hottest part on the resistor when dissipating full-rated power at rated ambient temperature. The maximum allowable power rating as a function of the ambient temperature is given by the derating curve. Figure 1.9 shows a typical power rating curve for a resistor. The derating curve is usually linearly drawn from the full-rated load temperature to the maximum allowable no-load temperature. A resistor may be operated at ambient temperatures above the maximum full-load ambient temperature if operating at lower than full-rated power capacity. The maximum allowable no-load temperature is also the maximum storage temperature for the resistor. FIGURE 1.8 The total resistor noise is the sum of current noise and thermal noise. The current noise approaches the thermal noise at higher frequencies. (Source: Phillips Components, Discrete Products Division, 1990–91 Resistor/Capacitor Data Book, 1991. With permission.) N.I. Noise voltage dc voltage = Ê Ë Á ˆ ¯ ˜ 20 10 log E V f f RMS dc N.I. = ¥ Ê Ë Á ˆ ¯ ˜ 10 20 2 1 / log