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CLC446AJE Datasheet(PDF) 8 Page - National Semiconductor (TI) |
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CLC446AJE Datasheet(HTML) 8 Page - National Semiconductor (TI) |
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8 / 12 page  http://www.national.com 8 The CLC446 noise model in Figure 9 is used to develop this equation for NF: where: s RS is the source resistance at the non- inverting input s There is no matching resistor from the input to ground s eni, ibn, and ibi are the voltage and current noise density terms (see the Electrical Characteristics section) s s Rf is the feedback resistor, and Rg is the gain-setting resistor To achieve a low Noise Figure while matching the source, use a matching transformer or the Low Noise Composite Amp With Input Matching circuit found in the CLC446 Applications section. Dynamic Range (distortion) The distortion plots in the Typical Performance Characteristics section show distortion as a function of load resistance, frequency, and output amplitude. Distortion places an upper limit on the CLC446’s dynamic range. Realized output distortion is highly dependent upon the external circuit. Some of the common external circuit choices that can improve distortion are: s Short and equal return paths from the load to the supplies s De-coupling capacitors of the correct value s Higher load resistance s A lower ratio of the output voltage swing to power supply voltage Printed Circuit Board Layout High Frequency op amp performance is strongly depen- dent on proper layout, proper resistive termination and adequate power supply decoupling. The most important layout points to follow are: s Use a ground plane s Bypass power supply pins with: s ceramic capacitors of about 0.1 µF placed less than 0.1" (3mm) from the pin s tantalum capacitors of about 6.8 µF for large signal current swings or improved power supply noise rejection; we recommend a minimum of 2.2 µF for any circuit s Minimize trace and lead lengths for components between the inverting and output pins s Remove ground plane underneath the amplifier package and 0.1" (3mm) from all input/output pads s For prototyping, use flush-mount printed circuit board pins; never use high profile DIP sockets Evaluation Board Separate evaluation boards are available for prototyping and measurements. Additional information is available in the evaluation board literature. Low Noise Composite Amp With Input Matching The composite amp shown in Figure 10 eliminates the need for a matching resistor to ground at the input. By connecting two amplifiers in series, the first non- inverting and the second inverting, an overall inverting gain is realized. The feedback resistor (Rf) closes the loop, and generates a set input resistance (Rin) that can be matched to RS. Rf generates less noise than a matching resistor to ground at the input. Figure 10: Composite Amplifier The input resistance and DC voltage gain of the amplifier are: Match the source resistance by setting: Rin = RS. The voltage noise produced by Rf, referred to the source VS, is: The noise of a simple input matching resistor connected to ground can be calculated by setting G to 0 in this equation. Thus, this circuit reduces the thermal noise power produced by the matching resistor by a factor of (1+G). Rectifier Circuit Wide bandwidth rectifier circuits have many applications. Figure 11 shows a 200MHz wideband full-wave rectifier circuit using a CLC446 and a CLC522 amplifier. Schottky or PIN diodes are used for D1 and D2. They produce an active half-wave rectifier whose signals are taken at the feedback diode connection. The CLC522 takes the difference of the two half-wave rectified signals, producing a full-wave rectifier. The CLC522 is used at a gain of 5 to achieve high differential bandwidth. For best NF 10log e i R 4 TR i R R 4 T R R 4TR ni 2 bn s 2 s bi f g 2 f g s = + () ++ ⋅ () +⋅ kk k 4 T 16.0 x 10 J T 290 K , T is in K 21 k = ()⋅ ° ° − CLC446 Applications + - CLC446 Rf Vo Rg2 - + 20 Ω CLC446 Rf2 Rf1 Rg1 Rin Vs Rs + - R R 1G , where G 1 R R R R V V G R RR in ff1 g1 f2 g2 o s in in s = + =+ ⋅ =− ⋅ + e4 TR R R1 G Rf s s in 2 =⋅ ⋅+ () k |
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