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CLC418 Datasheet(PDF) 5 Page - National Semiconductor (TI) |
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CLC418 Datasheet(HTML) 5 Page - National Semiconductor (TI) |
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5 / 12 page  5 http://www.national.com CLC418 OPERATION The CLC418 has a current-feedback (CFB) architecture built in an advanced complementary bipolar process. The key features of current-feedback are: s AC bandwidth is independent of voltage gain s Inherently unity-gain stability s Frequency response may be adjusted with feedback resistor (Rf in Figures 1-3) s High slew rate s Low variation in performance for a wide range of gains, signal levels and loads s Fast settling Current-feedback operation can be explained with a simple model. The voltage gain for the circuits in Figures 1 and 2 is approximately: where: s Av is the DC voltage gain s Rf is the feedback resistor s Z(j ω) is the CLC418’s open-loop transimpedance gain s is the loop gain The denominator of the equation above is approximately 1 at low frequencies. Near the -3dB corner frequency, the interaction between Rf and Z(jω) dominates the circuit performance. Increasing Rf does the following: s Decreases loop gain s Decreases bandwidth s Reduces gain peaking s Lowers pulse response overshoot s Affects frequency response phase linearity CLC418 DESIGN INFORMATION Standard op amp circuits work with CFB op amps. There are 3 unique design considerations for CFB: s The feedback resistor (Rf in Figures 1-3) sets AC performance s Rf cannot be replaced with a short or a capacitor s The output offset voltage is not reduced by balancing input resistances The following sub-sections cover: s Design parameters, formulas and techniques s Interfaces s Application circuits s Layout techniques s SPICE model information DC Gain (non-inverting) The non-inverting DC voltage gain for the configuration shown in Figure 1 is: Figure 1: Non-Inverting Gain The normalized gain plots in the Typical Performance Characteristics section show different feedback resistors (Rf) for different gains. These values of Rf are recommended for obtaining the highest bandwidth with minimal peaking. The resistor Rt provides DC bias for the non-inverting input. For Av < 6, use linear interpolation on the nearest Av values to calculate the recommended value of Rf. For Av ≥ 6, the minimum recommended R f is 200Ω. Select Rg to set the DC gain: DC gain accuracy is usually limited by the tolerance of Rf and Rg. DC Gain (unity gain buffer) The recommended Rf for unity gain buffers is 3kΩ. Rg is left open. Parasitic capacitance at the inverting node may require a slight increase of Rf to maintain a flat frequency response. DC Gain (inverting) The inverting DC voltage gain for the configuration shown in Figure 2 is: The normalized gain plots in the Typical Performance Characteristics section show different feedback resistors (Rf) for different gains. These values of Rf are recommended for obtaining the highest bandwidth with minimal peaking. The resistor Rt provides DC bias for the non-inverting input. For |Av| < 6, use linear interpolation on the nearest Av values to calculate the recommended value of Rf. For |Av| ≥ 6, the minimum recommended Rf is 200Ω. + - 1/2 CLC418 418 Fig1 Rf 0.1 µF 6.8 µF Vo Vin VCC 0.1 µF 6.8 µF VEE 3(5) 2(6) 4 8 1(7) + + Rg Rt V V A 1 R Z j o in v f = + ( )ω Z j Rf ω ( ) A 1 R R v f g = + R R A 1 g f v = − A R R v f g = − |
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Similar Description - CLC418 |
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