Resolving fast events, such as pulses from a photodiode or transistor avalanche, requires a detector with enough bandwidth to handle the pulse. The LT1711 high-speed comparator can switch in 4.5 nanoseconds in response to such events, due to its nanosecond rise time and propagation delay, but what if information needs to be relayed to a microprocessor or DSP that is unable to resolve such short-duration events? Figure 1 shows a simple one-shot circuit that stretches any high-excursion-rate output into a 2.5 μsec, or longer, pulse.
Figure 1: A simple one-shot circuit with a high-speed comparator and three external components. Any event that triggers the comparator will cause the output to go high and stay high for a minimum of 2.5 μsec.
The key to the circuit is the latch function of the LT1711. Capacitor C1 (1000 pF) conveys an output's rising edge directly to the latch pin of the comparator. After a 1.5ns typical setup time, the output of the comparator latches high, and stays high until the resistor R1 (20 kΩ) bleeds away the charge on C1 , with a time constant of 20 μs (R1 x C1 ). Once the voltage at the latch pin reaches its threshold voltage, the output unlatches and returns low until the next event occurs. Figure 2 shows the comparator’s output pulse (bottom trace) in response to an 8 ns input pulse (top trace), while Figure 3 shows a zoomed-in version of both the input pulse and the comparator’s output rising edge.
Click to Enlarge Image
Figure 2: Output of the comparator in response to an 8ns pulse at the input. Pulse length is approximately 8 μsec.
Click to Enlarge Image
Figure 3: Zoomed-in view of the input pulse (top) and the comparator output's rising edge (bottom).
Figure 4 breaks down the events that occur at the output of the comparator as well as the voltage at the latch pin.
Figure 4: Diagram of output voltage and latch pin voltage in response to an input event.
Upon a rising edge of the output (A ), the latch pin rises by the same voltage (B ). When the latch pin decreases to the latch threshold voltage (C ), the output switches low (D ). This, in turn, imposes the same negative voltage on the latch pin, forcing it below ground (E ). The 1N5712 Schottky diode prevents too large of a negative excursion. If this negative voltage on the latch pin is not given sufficient time to recover back to zero before the next event, the one-shot pulse width is reduced.
With a 3.3V supply, the measured one-shot interval (stretched pulse width) is approximately 8 μs. Changing the values of R1 and C1 can alter that interval. Over temperature, with the variations of the latch pin threshold and Schottky diode forward voltage, the minimum pulse width can drop to about 2.5 μs. Design this minimum pulse width to meet your minimum requirements, by changing R1 and C1 .
About the Author
Cheng-Wei Pei is is an applications engineer with the Signal Conditioning group at Linear Technology Corp, www.linear.com.