Fast thresholding of analog signal

Support Wixel
1

I have a sensor that outputs an analog signal (between 0 and 3.3v), and I need to know when that signal rises above a certain threshold voltage (the signal will be near 0 for a while, and then will suddenly rise). I haven’t decided on a threshold yet, but once I do it won’t change. Let’s just say it’s 1v for now.

Obviously I can use the Wixel’s ADC for this, but the ADC is a little too slow for me. Even on the lowest resolution (7 bits) it takes 20uS for an ADC read. I want to determine when the signal passes the threshold to within 10uS or better. I was thinking of using an analog comparator or something like this to give me a binary output, then reading that (a digital read should be fast enough). Is this necessary, or is there any way I can do this with a Wixel alone and no other hardware? Or better yet, how would you go about doing this?

2

To detect the transition, I would use a simple op amp comparator configured as a Schmidt trigger, connected to a wixel input pin. See a ways down this page swarthmore.edu/NatSci/echeev … mp%29.html). The trigger level is set by the voltage divider resistors R1 & R2.

This circuit would be very fast to respond (less than 1 us, even faster if you use a comparator instead of a standard op amp). However with the Wixel you either have to poll the inputs to keep everything alive or use interrupts to respond to the pin change from the comparator output, e.g. svn.modlabupenn.org/mod-ros-pkg/ … link_led.c. Given interrupt latency and the fact that the wixel has other things to do, it may not be possible to guarantee 10 microsecond response time.