Datasheet LT1016 (Analog Devices) - 10

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APPLICATIONS INFORMATION
In Figure 7 the probes are properly selected and applied supplies. The inductance created by a long device ground but the LT1016’s output rings and distorts badly. In this lead permits mixing of ground currents, causing undesired case, the probe ground lead is too long. For general pur- effects in the device. The solution here is simple. Keep the pose work most probes come with ground leads about six LT1016’s ground pin connection as short (typically 1/4 inches long. At low frequencies this is fine. At high speed, inch) as possible and run it directly to a low impedance the long ground lead looks inductive, causing the ringing ground. Do not use sockets. shown. High quality probes are always supplied with some Figure 9 addresses the issue of the “low impedance short ground straps to deal with this problem. Some come ground,” referred to previously. In this example, the with very short spring clips which fix directly to the probe output is clean except for chattering around the edges. tip to facilitate a low impedance ground connection. For This photograph was generated by running the LT1016 fast work, the ground connection to the probe should not without a “ground plane.” A ground plane is formed by exceed one inch in length. Keep the probe ground con- using a continuous conductive plane over the surface of nection as short as possible. the circuit board. The only breaks in this plane are for the Figure 8 shows the LT1016’s output (Trace B) oscillating circuit’s necessary current paths. The ground plane serves near 40MHz as it responds to an input (Trace A). Note that two functions. Because it is flat (AC currents travel along the input signal shows artifacts of the oscillation. This the surface of a conductor) and covers the entire area of example is caused by improper grounding of the com- the board, it provides a way to access a low inductance parator. In this case, the LT1016’s GND pin connection is ground from anywhere on the board. Also, it minimizes one inch long. The ground lead of the LT1016 must be as the effects of stray capacitance in the circuit by referring short as possible and connected directly to a low impedance them to ground. This breaks up potential unintended and ground point. Any substantial impedance in the LT1016’s harmful feedback paths. Always use a ground plane with the ground path will generate effects like this. The reason for LT1016 when input signal levels are low or slow moving. this is related to the necessity of bypassing the power 1V/DIV 20ns/DIV 1016 F07
Figure 7. Typical Results Due to Poor Probe Grounding
TRACE A 1V/DIV TRACE B 2V/DIV 2V/DIV 100ns/DIV 1016 F08 100ns/DIV 1016 F09
Figure 8. Excessive LT1016 Ground Path Figure 9. Transition Instabilities Due to No Ground Plane Resistance Causes Oscillation
Rev D 10 For more information www.analog.com 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