I have a project in which I want to measure the time it takes for an input signal voltage to drop from it's normal 2 volts to .2 volts. This drop is going to occur in under 2-3 microseconds. I know none of the arduinos have ADC's fast enough to measure this, but I was wondering if anyone had some external circuit they used to do this.
I thought about just obtaining a fast enough ADC and interfacing it to the Arduino, but I must admit that I am somewhat confused as to how that would work. If the Arduino is not fast enough to measure the initial voltage drop, would it not be fast enough to read the external ADC output? I am fairly experienced in electronics, just have had almost no experience with ADC's. Any help would be greatly appreciated. Thank you.
I think, perhaps something like the approach in the attached picture (in general terms) may help you, although you haven't mention the resolution you want, the pulse repetition frequency (PRF) and how many readings/time you need. You may need to introduce your own modifications according to your needs though.
Please note that the monostable multivibrator is there in case Arduino can't detect the end of the 2-3Us pulse (I don’t know if it can). If it can, then you don't need it. The flip flop is there to block the counting if the PRF is faster than the speed at which Arduino can read the values for one measurement. If the PRF is low, then you don't need that either. You will need really fast components to do that. For the comparator, something like the ultrafast TL3016 (http://www.ti.com/lit/ds/slcs130d/slcs130d.pdf) or faster may work. For the digital components, at least 74ALS or 74S TTL family components (Intelligent Power and Sensing Technologies | onsemi) . If that’s not enough then you may need ECL technology (Emitter-coupled logic - Wikipedia). The resolution will be determined by the oscillator freq. For example, if you use 10MHz then the resolution will be 0.1Usec and you will be able to count about 20 to 30 pulses for your interval of 2-3Usec. Accuracy of the readings is another story though and depends on many factors like the threshold levels you establish for the comparators (VRef1 and VRef2), propagation times of the components you select, oscillator stability, input signal levels stability, etc.
A good Voltage Reference could be the KA431 (Intelligent Power and Sensing Technologies | onsemi) with low tolerance and low temp coef resistors. You will also need a fast memory oscilloscope (perhaps 100MHz or higher) for calibrations, so you can see the input and the counted pulses passing through during your input pulse falling edge to check for accuracy.
At the end Arduino can take the readings and display them where you choose. You can perform statistical calculations also with Arduino, like standard deviation, average value, etc, after taking several readings and also display them if you want. For that just program the equations and use arrays for the values read. This will be useful if you need to take repetitive measurements of that falling edge pulse. If it is a one time measurement, then you don’t need all this and a good scope is just fine.
Hope it helps.
