High-speed analog-to-digital converters can be tricky—and sometimes costly—to work with. In his Circuit Cellar 259 article, “Playing with High-Speed ADCs,” columnist Robert Lacoste provided handy tips on effectively implementing them to your advantage. He used an Analog Devices AD9265-125 as an example.
In response to the article, a reader wrote:
“Hello, Robert. Thanks for all the articles in CC. Your High Speed ADC covered some of the concepts I have been thinking about with creating a laser range finder using Pulsed time of flight. The problem is the frequencies are in the 20 GHz for 8-mm resolution. The ADC can be very low resolution, but the sampling has to be in the 40-GHz range. The sensors need to be dark 50% of the time to get even 10-GHz response times from the crystals. So, I was wondering how do the small handheld range finders work? They are accurate to 8 mm and cost $150+. Is there a easier way to do Time of Flight? Do you know how they do this? The internet is kind of a endless trail with nothing that leads to IC’s or circuits.”
Robert provided an insightful answer:
“The short and honest answer would be ‘I don’t know,’ as I’ve never worked or inspected the internals of a laser range finder. However I’m sure that they don’t use 20Gsps+ ADC’s… I would guess that they use more some kind of time interval to voltage converters, followed by a slow but high resolution ADC. The idea is basically to charge a capacitor between the transmit and receive pulses, and to measure to voltage on the capacitor. This technique is for example used in all Agilent’s period/frequency meters: these devices measure single-shot events with a resolution down to 10ps or so, and don’t include anything faster than a 100MHz clock. Look for ‘dual Vernier interpolation’ techniques. On the laser range side, I dug a little on the net and found this link which seems to confirm this assumption: http://herkules.oulu.fi/isbn9514269667/html/c305.html. There is at least one company who develops custom chips adequate for this job: http://www.acam.de/index.php?id=149. See there: www.acam.de/products/time-to-digital-converters/applications/laser-distance-meters/.”
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