HP HP-Bench-Briefs-1980-05-06 free download

File name HP-Bench-Briefs-1980-05-06.pdf

Circuit Test Point Impedance select a probelscope combination While probe capacitance distorts that has an Rin that is a t least 100 pulse shape, the flat portion of Knowing the source impedance at times greater t h a n t h e source the pulse top (maximum the point of measurement is critical. impedance." amplitude) can be used to make If the source impedance is low, rise an accurate amplitude measure- time and amplitude measurements But as frequencies rise, or pulse rise ment since i t contains low fre- are generally no problem. For exam- time becomes very fast, scope input quency information. Conversely, ple, batteries and power supplies capacitance becomes more and more if the pulse width is small com- h a v e source impedances of mil- important, forcing use of an minia- pared to the measurement system liohms. Signal generators are 25, 50 ture passive divider probe to reduce rise time, input capacitance can or 600 ohms. The problem occurs that input capacitance. introduce errors since the source when the source impedance is high. cannot fully charge the input TTL h a s a source impedance of And a t the highest frequencies, if capacitance during its on time. -2.5kR so even a t very low frequen- both amplitude and rise time are This problem becomes worse with cies (single shot), measuring fast important in high source impedance increasing source impedance. transition times is difficult a t best. circuits, an active FET input probe should be used. 3. When source impedance is un- known, the probe with the high- If the ultimate in rise time is needed, est Zin usually yields the greatest Basic Probe Considerations accuracy. However, for frequen- a 500 divider probe may be used. However, you must be careful of DC cies above 10 MHz, high probe If t h e scope is being used a s a capacitance can reduce accuracy monitoring device, the connection loading. A 50R divider probe with a n input Xc of 5000 will attenuate the more than high probe resistance between the signal source and scope can help. is usually a direct 50R cable. How- amplitude of a signa