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AD815 Datasheet(PDF) 11 Page - Analog Devices |
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AD815 Datasheet(HTML) 11 Page - Analog Devices |
11 / 16 page AD815 REV. B –11– DC ERRORS AND NOISE There are three major noise and offset terms to consider in a current feedback amplifier. For offset errors refer to the equation below. For noise error the terms are root-sum-squared to give a net output error. In the circuit below (Figure 42), they are input offset (VIO) which appears at the output multiplied by the noise gain of the circuit (1 + RF/RG), noninverting input current (IBN × R N) also multiplied by the noise gain, and the inverting input current, which when divided between RF and RG and subsequently multiplied by the noise gain always appear at the output as IBI × R F. The input voltage noise of the AD815 is less than 2 nV/ √Hz. At low gains though, the inverting input current noise times RF is the dominant noise source. Careful layout and device matching contribute to better offset and drift specifications for the AD815 compared to many other current feedback amplifiers. The typical performance curves in conjunction with the equations below can be used to predict the performance of the AD815 in any application. V OUT = VIO × 1 + R F R G ± I BN × RN × 1 + R F R G ± I BI × RF I BI I BN RG RN RF VOUT Figure 42. Output Offset Voltage POWER CONSIDERATIONS The 500 mA drive capability of the AD815 enables it to drive a 50 Ω load at 40 V p-p when it is configured as a differential driver. This implies a power dissipation, PIN, of nearly 5 watts. To ensure reliability, the junction temperature of the AD815 should be maintained at less than 175 °C. For this reason, the AD815 will require some form of heat sinking in most applications. The thermal diagram of Figure 43 gives the basic relationship between junction temperature (TJ) and various components of θ JA. TT P AIN A JJ =+ θ Equation 1 θA (JUNCTION TO DIE MOUNT) θ B (DIE MOUNT TO CASE) θ A + θ B = θ JC CASE TA TJ θ JC θ CA TA θ JA TJ PIN WHERE: PIN = DEVICE DISSIPATION TA = AMBIENT TEMPERATURE TJ = JUNCTION TEMPERATURE θ JC = THERMAL RESISTANCE – JUNCTION TO CASE θ CA = THERMAL RESISTANCE – CASE TO AMBIENT Figure 43. A Breakdown of Various Package Thermal Resistances Figure 44 gives the relationship between output voltage swing into various loads and the power dissipated by the AD815 (PIN). This data is given for both sine wave and square wave (worst case) conditions. It should be noted that these graphs are for mostly resistive (phase < ±10°) loads. When the power dissipation requirements are known, Equation 1 and the graph on Figure 45 can be used to choose an appropriate heat sinking configuration. 4 3 10 20 30 40 2 1 VOUT – Volts p-p RL = 50 RL = 100 RL = 200 f = 1kHz SQUARE WAVE SINE WAVE Figure 44. Total Power Dissipation vs. Differential Output Voltage Normally, the AD815 will be soldered directly to a copper pad. Figure 45 plots θ JA against size of copper pad. This data pertains to copper pads on both sides of G10 epoxy glass board connected together with a grid of feedthroughs on 5 mm centers. This data shows that loads of 100 ohms or less will usually not require any more than this. This is a feature of the AD815’s 15-lead power SIP package. An important component of θ JA is the thermal resistance of the package to heatsink. The data given is for a direct soldered connection of package to copper pad. The use of heatsink grease either with or without an insulating washer will increase this number. Several options now exist for dry thermal connec- tions. These are available from Bergquist as part # SP600-90. Consult with the manufacturer of these products for details of their application. COPPER HEAT SINK AREA (TOP AND BOTTOM) – mm2 35 30 10 0 2.5k 0.5k 1k 1.5k 2k 25 20 15 AD815AVR, AY (θ JC = 2 C/W) Figure 45. Power Package Thermal Resistance vs. Heat Sink Area |
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