Datasheet LT1016 (Analog Devices) - 7

link to page 7 LT1016
APPLICATIONS INFORMATION
The circuit shown in Figure 1 is the best electronic means in its linear region, a feature no other high speed compara- of generating a known fast, clean step to test comparators. tor has. Additionally, output stage switching does not ap- It uses a very fast transistor in a common base configura- preciably change power supply current, further enhancing tion. The transistor is switched “off” with a fast edge from stability. These features make the application of the 50GHz the generator and the collector voltage settles to exactly 0V gain-bandwidth LT1016 considerably easier than other in just a few nanoseconds. The most important feature of fast comparators. Unfortunately, laws of physics dictate this circuit is the lack of feedthrough from the generator that the circuit environment the LT1016 works in must be to the comparator input. This prevents overshoot on the properly prepared. The performance limits of high speed comparator input that would give a false fast reading on circuitry are often determined by parasitics such as stray comparator response time. capacitance, ground impedance and layout. Some of these To adjust this circuit for exactly 5mV overdrive, V1 is considerations are present in digital systems where design- adjusted so that the LT1016 output under test settles to ers are comfortable describing bit patterns and memory 1.4V (in the linear region). Then V1 is changed –5V to set access times in terms of nanoseconds. The LT1016 can overdrive at 5mV. be used in such fast digital systems and Figure 2 shows just how fast the device is. The simple test circuit allows The test circuit shown measures low to high transition us to see that the LT1016’s (Trace B) response to the pulse on the “+” input. For opposite polarity transitions on the generator (Trace A) is as fast as a TTL inverter (Trace C) output, simply reverse the inputs of the LT1016. even when the LT1016 has only millivolts of input signal! Linear circuits operating with this kind of speed make many
High Speed Design Techniques
engineers justifiably wary. Nanosecond domain linear A substantial amount of design effort has made the LT1016 circuits are widely associated with oscillations, mysteri- relatively easy to use. It is much less prone to oscillation ous shifts in circuit characteristics, unintended modes of and other vagaries than some slower comparators, even operation and outright failure to function. with slow input signals. In particular, the LT1016 is stable 5V 0.01µF** 0V –100mV 25Ω + Q 10X SCOPE PROBE (CIN ≈ 10pF) 0.1µF 130Ω 25Ω LT1016 10k – 10X SCOPE PROBE 2N3866 Q L (CIN ≈ 10pF) PULSE IN V1† 0V 10Ω –3V –5V 50Ω 400Ω 750Ω 0.01µF –5V * SEE TEXT FOR CIRCUIT EXPLANATION ** TOTAL LEAD LENGTH INCLUDING DEVICE PIN. SOCKET AND CAPACITOR LEADS SHOULD BE 1016 F01 LESS THAN 0.5 IN. USE GROUND PLANE † (VOS + OVERDRIVE) • 1000
Figure 1. Response Time Test Circuit
Rev D For more information www.analog.com 7 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Applications Information Typical Applications Appendix A Package Description Revision History Applications Information Related Parts