Datasheet LT1394 (Analog Devices) - 8

LT1394
U U W U APPLICATIONS INFORMATION
5V 0.01µF* 0V 25Ω –100mV + Q FET PROBE 25Ω LT1394 10k 0.1µF 130Ω – FET PROBE Q * TOTAL LEAD LENGTH INCLUDING DEVICE PIN. PULSE 2N3866 V1** 50Ω 0.01µF SOCKET AND CAPACITOR LEADS SHOULD BE IN LESS THAN 0.5 IN. USE GROUND PLANE 0V ** (VOS + OVERDRIVE)/200 –3V 50Ω 400Ω 750Ω –5V 1394 F02 –5V
Figure 2. Response Time Test Circuit
circuit is the lack of feedthrough from the generator to the ceramic is recommended, in parallel with a larger capaci- comparator input. This prevents overshoot on the com- tor such as a 4.7µF tantalum. parator input, which would give a false fast reading on Poor trace routes and high source impedances are also comparator response time. common sources of problems. Be sure to keep trace To adjust the circuit for exactly 5mV overdrive, V1 is lengths as short as possible, and avoid running any output adjusted so that the LT1394 output under test settles to trace adjacent to an input trace to prevent unnecessary 1.4V (in the linear region). Then V1 is changed by – 1V to coupling. If output traces are longer than a few inches, be set overdrive to 5mV. sure to terminate them with a resistor to eliminate any reflections that may occur. Resistor values are typically
High Speed Design Techniques
250Ω to 400Ω. Also, be sure to keep source impedances A substantial amount of design effort has made the LT1394 as low as possible, preferably 1kΩ or less. relatively easy to use. It is much less prone to oscillation
Crystal Oscillators
than some slower comparators, even with slow input signals. However, as with any high speed comparator, Figure 3’s circuits are crystal oscillators. In the circuit (a) there are a number of pitfalls which may arise because of the resistors at the LT1394’s positive input set a DC bias PC board layout and design. The most common problems point. The 2k-0.068µF path sets up phase shifted feedback involve power supply bypassing. Bypassing is necessary and the circuit looks like a wideband unity-gain follower at to maintain low supply impedance. DC resistance and DC. The crystal’s path provides resonant positive feed- inductance in supply wires and PC traces can quickly build back and stable oscillation occurs. The circuit (b) is up to unacceptable levels. This allows the supply line to similar, but supports oscillation frequencies to 30MHz. move with changing internal current levels of the con- Above 10MHz, AT-cut crystals operate in overtone mode. nected devices. This will almost always result in improper Because of this, oscillation can occur at multiples of the operation. In addition, adjacent devices connected through desired frequency. The damper network rolls off gain at an unbypassed supply can interact with each other through high frequency, ensuring proper operation. the finite supply impedances. Bypass capacitors furnish a simple solution to this problem by providing a local
Switchable Output Crystal Oscillator
reservoir of energy at the device, keeping supply imped- Figure 4 permits crystals to be electronically switched by ances low. logic commands. This circuit is similar to the previous Bypass capacitors should be as close as possible to the examples, except that oscillation is only possible when LT1394. A good high frequency capacitor such as a 0.1µF one of the logic inputs is biased high. 8