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KM103AM Datenblatt(PDF) 4 Page - Cadeka Microcircuits LLC. |
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KM103AM Datenblatt(HTML) 4 Page - Cadeka Microcircuits LLC. |
4 / 6 page DATA SHEET KH103 4 REV. 1A January 2004 KH103 Operation The KH103 is based on a unique design which uses current feedback instead of the usual voltage feedback. This design provides dynamic performance far beyond that previously available, yet it is used basically the same as the familiar voltage-feedback op amp (see the gain equations above). Layout Considerations To obtain optimum performance from any circuit operating at high frequencies, good PC layout is essential. Fortunately, the stable, well-behaved response of the KH103 makes operation at high frequencies less sensitive to layout than is the case with other wideband op amps, even though the KH103 has a much wider bandwidth. In general, a good layout is one which minimizes the unwanted coupling of a signal between nodes in a circuit. A continuous ground plane from the signal input to output on the circuit side of the board is helpful. Traces should be kept short to minimize inductance. If long traces are needed, use microstrip transmission lines which are terminated in their characteristic impedance. At some high-impedance nodes, or in sensitive areas such as near pin 5 of the KH103, stray capacitance should be kept small by keeping nodes small and removing ground plans directly around the node. The ±VCC connections to the KH103 are internally bypassed to ground with 0.1 µF capacitors to provide good high-frequency decoupling. It is recommended that 1 µF or larger tantalum capacitors be provided for low- frequency decoupling. The 0.01 µF capacitors shown at pins 18 and 20 in figures 1 and 2 should be kept within 0.1” of those pins. A wide strip of ground plane should be provided for a signal return path between the load-resis- tors ground and these capacitors. Figure 1: Recommended Non-Inverting Gain Circuit Figure 2: Recommended Inverting Gain Circuit Since the layout of the PC board forms such an important part of the circuit, much time can be saved if prototype amplifier boards are tested early in the design stage. Settling Time, Offset, and Drift After an output transition has occurred. the output settles very rapidly to the final value and no change occurs for several microseconds. Thereafter, thermal gradients inside the KH103 will cause the output to begin to drift. When this cannot be tolerated, or when the initial offset voltage and drift is unacceptable, use of a composite amplifier is advised. A composite amplifier can also be referred to as a feed- forward amplifier. Most feed-forward techniques such as those used In the vast majority of wideband op amps involve the use of a wideband AC-coupled channel in parallel with a low-bandwidth, high-gain DC-coupled amplifier. For the composite amplifier suggested for use with the KH103, the KH103 replaces the wideband AC- coupled amplifier and a low-cost monolithic op amp is used to supply high open-loop gain at low frequencies. Since the KH103 is strictly DC coupled throughout, crossover distortion of less than 0.01dB and 1° results. For composite operation in the non-inverting mode, the circuit in Figure 1 should be modified by the addition of the circuit shown in Figure 3. For Inverting operation, modify the circuit in Figure 2 by the addition of the circuit in Figure 4. Keep all resistors which connect to the KH103 within 0.2” of the KH103 pins. The other side of these resistors should likewise be as close to U1 as possible. For good overall results, U1 should be similar to the LF356; this gives 5mV/°C input offset drift and the crossover frequency occurs at about 2MHz. Since U1 has a feedback network composed of Ra + Rb and a 15k Ω resistor, which is in parallel with R g and the internal 1.5k Ω feedback resistor of the KH103, R b must be adjusted to match the feedback ratios of the two net- works. This in done by driving the composite amplifier |
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