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LT1721IGN Datasheet(PDF) 20 Page - Linear Technology |
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LT1721IGN Datasheet(HTML) 20 Page - Linear Technology |
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20 / 28 page  LT1720/LT1721 20 17201fc You don’t need expensive equipment to confirm the actual overall performance of this circuit. All you need is a respect- able waveform generator (capable of >~100kHz), a splitter, a varietyofcablelengthsanda20MHzor60MHzoscilloscope. 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 1.70μs output pulses. A 12 foot cable length difference will result in ~18.4ns delay and 2.07μs output pulses. The difference APPLICATIONS INFORMATION in the two output pulse widths is the per-octave response of your circuit (see Equation (3)). Shorter cable length dif- ferences can be used to get a plot of circuit performance down to 1.5ns (if any), which can then later be used as a lookupreferencewhenyouhavemovedfromquantifyingthe circuit to using the circuit. (Note there is a slight aberration in performance 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 separated by 10ns as shown. The time constant is set by the RC net- work on the output of comparator A. Comparator B and D trip at fixed percentages of the exponential voltage decay across the capacitor. The 4.22kΩ 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 shortened somewhat if the pulse was shorter than 40ns because 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 1kΩ 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 circuit features 20dB of gain, 10MHz full power bandwidth and 100μV/°C baseline uncertainty. Switching delay is less than 15ns and the minimum sampling window width for full power response is 30ns. Figure 14. Log Pulse Stretcher Output Pulse vs Input Pulse tPULSE (ns) 14 12 10 8 6 4 2 0 1 100 1000 10000 17201 F14 10 MEASURED EQUATION 1 Figure 15. RG-58 Cable with Velocity of Propogation = 66%; Delay at Y = (n – 1) • 1.54ns 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) 17201 F15 |
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