An Overly Sensitive One-Shot

Posted on Feb 7, 2014

This circuit is from the data sheet for Linear Technology`s LT1720 (a 4. 5ns comparator). It`s the circuit entitled `Pulse Stretcher` at the very end. Essentially, it`s a one-shot with sensitivity down to 14mV for 5ns to 10ns inputs (i. e. , it doesn`t require a full swing from logical low to high as do most one-shots), and TTL logic level outputs. I

An Overly Sensitive One-Shot
Click here to download the full size of the above Circuit.

t also exhibits a mere 4. 5ns delay from input to output. Here`s an OrCAD schematic ( pdf ) used to simulate the circuit: The diode (D1) is a 1N5711. PSpice simulations indicate that it does indeed have the advertised sensitivity (I was able to get it to fire cleanly with a 5 ns 14mV pulse provided the input had 2ns rise and fall times). This is great. or so one would think. What the data sheet fails to mention is that this one-shot has a large recovery time (roughly 6 time constants), resulting in outputs of less than the desired time constant (1us for the above schematic) for any additional inputs arriving 1 to 7 time constants after the initial input. Discharging the timing capacitor (C7) with a FET switch reduces the recovery time substantially. Here`s an OrCAD schematic illustrating how one might do this ( pdf ): The FET used is a 2N7000. If one could turn this FET on for about 15ns using the falling edge of the circuit`s output, the recovery time is reduced to no more than 20ns. In the above schematic this is done artificially by injecting the desired "on pulse" into the FET`s gate at the appropriate time. Here`s a digital photo of a real version of the circuit: It doesn`t incorporate the FET switch (obviously), but otherwise follows the latter schematic. To improve the circuit`s stability I added a dc bias to the negative input of the lower comparator (U1), and increased the dc bias on the negative input of the upper...

Leave Comment

characters left:

New Circuits



Popular Circuits

Free energy battery charger
Timer circuit
Simplest RF Transmitter circuit diagram
proximity detector schematic
Car Battery Saver Circuit
555 LED flasher
Touch Plate Circuit With High Sensitivity PCB
SRS5250S Application circuit diagram
Class AB output circuit diode coupling