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LT1720 Datasheet(PDF) 19 Page - Linear Technology |
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LT1720 Datasheet(HTML) 19 Page - Linear Technology |
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19 / 28 page  19 LT1720/LT1721 APPLICATIONS INFORMATION tPULSE (ns) 14 12 10 8 6 4 2 0 1 100 1000 10000 1720/21 F14 10 MEASURED EQUATION 1 Figure 14. Log Pulse Stretcher Output Pulse vs Input Pulse SPLITTER 2V 0V CIRCUIT OF FIGURE 12 n FOOT CABLE 1 FOOT CABLE NANOSECOND INPUT RANGE MICROSECOND OUTPUT RANGE X Y L tOUT (SEE TEXT) 1720/21 F15 Figure 15. RG-58 Cable with Velocity of Propogation = 66%; Delay at Y = (n – 1) • 1.54ns You don’t need expensive equipment to confirm the actual overall performance of this circuit. All you need is a respectable waveform generator (capable of >~100kHz), a splitter, a variety of cable lengths and a 20MHz or 60MHz oscilloscope. Split a single pulse source into different cable lengths and then into the delay detector, feeding the longer cable into the Y input (see Figure 15). A 6 foot cable length difference will create a ~9.2ns delay (using 66% propagation speed RG-58 cable), and should result in easily measured 1.70 µs output pulses. A 12 foot cable length difference will result in ~18.4ns delay and 2.07 µs output pulses. The difference in the two output pulse widths is the per-octave response of your circuit (see equation (3)). Shorter cable length differences can be used to get a plot of circuit performance down to 1.5ns (if any), which can then later be used as a lookup reference when you have moved from quantifying the circuit to using the circuit. (Note there is a slight aberration in perfor- mance below 10ns. See Figure 14.) As a final check, feed the circuit with identical cable lengths and check that it is not producing any output pulses. 10ns Triple Overlap Generator The circuit of Figure 16 utilizes an LT1721 to generate three overlapping outputs whose pulse edges are sepa- rated by 10ns as shown. The time constant is set by the RC network on the output of comparator A. Comparator B and D trip at fixed percentages of the exponential voltage decay across the capacitor. The 4.22k Ω feed-forward to the C comparator’s inverting input keeps the delay differences the same in each direction despite the exponential nature of the RC network’s voltage. There is a 15ns delay to the first edge in both directions, due to the 4.5ns delay of two LT1721 comparators, plus 6ns delay in the RC network. This starting delay is short- ened somewhat if the pulse was shorter than 40ns be- cause the RC network will not have fully settled; however, the 10ns edge separations stay constant. The values shown utilize only the lowest 75% of the supply voltage span, which allows it to work down to 2.7V supply. The delay differences grow a couple nanoseconds from 5V to 2.7V supply due to the fixed VOL/VOH drops which grow as a percentage at low supply voltage. To keep this effect to a minimum, the 1k Ω pull-up on comparator A provides equal loading in either state. Fast Waveform Sampler Figure 17 uses a diode-bridge-type switch for clean, fast waveform sampling. The diode bridge, because of its inherent symmetry, provides lower AC errors than other semiconductor-based switching technologies. This cir- cuit features 20dB of gain, 10MHz full power bandwidth |
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