Datasheet LT1671 (Analog Devices) - 8

LT1671
U U W U APPLICATIONS INFORMATION
5V 0.01µF* 0V 25Ω –100mV + Q FET PROBE 25Ω LT1671 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 1671 F02 –5V
Figure 2. Response Time Test Circuit
circuit is the lack of feedthrough from the generator to the Bypass capacitors should be as close as possible to the comparator input. This prevents overshoot on the com- LT1671. A good high frequency capacitor such as a 0.1µF parator input, which would give a false fast reading on ceramic is recommended, in parallel with a larger capaci- comparator response time. tor such as a 4.7µF tantalum. To adjust the circuit for exactly 5mV overdrive, V1 is Poor trace routes and high source impedances are also adjusted so that the LT1671 output under test settles to common sources of problems. Be sure to keep trace 1.4V (in the linear region). Then V1 is changed by – 1V to lengths as short as possible, and avoid running any output set overdrive to 5mV. trace adjacent to an input trace to prevent unnecessary coupling. If output traces are longer than a few inches, be
High Speed Design Techniques
sure to terminate them with a resistor to eliminate any A substantial amount of design effort has made the LT1671 reflections that may occur. Resistor values are typically relatively easy to use. It is much less prone to oscillation 250Ω to 400Ω. Also, be sure to keep source impedances than some slower comparators, even with slow input as low as possible, preferably 1kΩ or less. signals. However, as with any high speed comparator,
About Level Shifts
there are a number of problems which may arise because of PC board layout and design. The most common prob- The LT1671’s logic output will interface with many cir- lem involves power supply bypassing. Bypassing is nec- cuits directly. Many applications, however, require some essary to maintain low supply impedance. DC resistance form of level shifting of the output swing. With LT1671- and inductance in supply wires and PC traces can quickly based circuits this is not trivial because it is desirable to build up to unacceptable levels. This allows the supply line maintain very low delay in the level shifting stage. When to move with changing internal current levels of the designing level shifters, keep in mind that the TTL output connected devices. This will almost always result in of the LT1671 is a sink-source pair (Figure 3) with good improper operation. In addition, adjacent devices con- ability to drive capacitance (such as feedforward capaci- nected through an unbypassed supply can interact with tors). Figure 4 shows a noninverting voltage gain stage each other through the finite supply impedances. Bypass with a 15V output. When the LT1671 switches, the base- capacitors furnish a simple solution to this problem by emitter voltages at the 2N2369 reverse, causing it to providing a local reservoir of energy at the device, keeping switch very quickly. The 2N3866 emitter-follower gives a supply impedances low. low impedance output and the Schottky diode aids cur- rent sink capability. 8