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MC33077 Folha de dados(PDF) 11 Page - ON Semiconductor |
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MC33077 Folha de dados(HTML) 11 Page - ON Semiconductor |
11 / 12 page MC33077 11 MOTOROLA ANALOG IC DEVICE DATA occur, the amplifier’s phase will degrade severely causing the amplifier to become unstable. Effective source resistances, acting in conjunction with the input capacitance of the amplifier, should be kept to a minimum to avoid creating such a pole at the input (see Figure 31). There is minimal effect on stability where the created input pole is much greater than the closed loop corner frequency. Where amplifier stability is affected as a result of a negative feedback resistor in conjunction with the amplifier’s input capacitance, creating a pole near the closed loop corner frequency, lead capacitor compensation techniques (lead capacitor in parallel with the feedback resistor) can be employed to improve stability. The feedback resistor and lead capacitor RC time constant should be larger than that of the uncompensated input pole frequency. Having a high resistance connected to the noninverting input of the amplifier can create a like instability problem. Compensation for this condition can be accomplished by adding a lead capacitor in parallel with the noninverting input resistor of such a value as to make the RC time constant larger than the RC time constant of the uncompensated input resistor acting in conjunction with the amplifiers input capacitance. For optimum frequency performance and stability, careful component placement and printed circuit board layout should be exercised. For example, long unshielded input or output leads may result in unwanted input output coupling. In order to reduce the input capacitance, the body of resistors connected to the input pins should be physically close to the input pins. This not only minimizes the input pole creation for optimum frequency response, but also minimizes extraneous signal “pickup” at this node. Power supplies should be decoupled with adequate capacitance as close as possible to the device supply pin. In addition to amplifier stability considerations, input source resistance values should be low to take full advantage of the low noise characteristics of the amplifier. Thermal noise (Johnson Noise) of a resistor is generated by thermally–charged carriers randomly moving within the resistor creating a voltage. The rms thermal noise voltage in a resistor can be calculated from: Enr = 4k TR × BW / where: k = Boltzmann’s Constant (1.38 × 10–23 joules/k) T = Kelvin temperature R = Resistance in ohms BW = Upper and lower frequency limit in Hertz. By way of reference, a 1.0 k Ω resistor at 25°C will produce a 4.0 nV/ Hz √ of rms noise voltage. If this resistor is connected to the input of the amplifier, the noise voltage will be gained–up in accordance to the amplifier’s gain configuration. For this reason, the selection of input source resistance for low noise circuit applications warrants serious consideration. The total noise of the amplifier, as referred to its inputs, is typically only 4.4 nV/ Hz √ at 1.0 kHz. The output of any one amplifier is current limited and thus protected from a direct short to ground, However, under such conditions, it is important not to allow the amplifier to exceed the maximum junction temperature rating. Typically for ±15 V supplies, any one output can be shorted continuously to ground without exceeding the temperature rating. Figure 36. Voltage Noise Test Circuit (0.1 Hz to 10 Hzp–p) Note: All capacitors are non–polarized. + – 0.1 µF 10 Ω 100 k Ω 2.0 k Ω 4.7 µF Voltage Gain = 50,000 Scope × 1 Rin = 1.0 MΩ 1/2 MC33077 – + D.U.T. 100 k Ω 0.1 µF 2.2 µF 22 µF 24.3 k Ω 4.3 k Ω 110 k Ω |
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