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Minimum Pulse Width


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3 replies to this topic

#1 orestes05

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Posted 22 May 2014 - 08:28 AM

Hi,

 

I have the U3 triggering a single 100 ns pulse on an analog pulse generator.  I am wondering if any LabJack device alone could generate a short pulse width (less than 100 nsec)?

 

The pulse provides digitial TTL input to a laser diode, the diode has separate power source so the pulse just acts as 0V (low) to 5V (high) switch.

 

Thanks you,

Jeremy



#2 LabJack Support

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Posted 22 May 2014 - 09:24 AM

I am not sure what you are asking.  You are using the U3 to generate a 100 ns pulse, or you are wondering how to do it?

 

In theory the U3 can generate a single <100ns pulse using a timer in PWM or FrequencyOut mode along with another timer in stop mode, but I'm not sure if the output will rise fast enough to make a proper pulse.  I need to find a low capacitance scope and then I can measure the time constants of the FIO and EIO/CIO lines.



#3 LabJack Support

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Posted 22 May 2014 - 10:48 AM

An initial check using our scope's 10%-90% rise time measurement reports about 25ns for FIO and 6ns for EIO/CIO, so probably good for you.  If your load adds more capacitance than our scope, these times will be slower.



#4 LabJack Support

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Posted 22 May 2014 - 11:07 AM

To estimate rise/fall time you can use the impedance of the digital outputs and capacitance of the load to calculate the time constant:

 

Tau = R*C

 

1 time constant equates to the signal changing 63% of the way towards the new value, so for a 0 to 3.3 volt step it is the time for the signal to go from 0 to 2.1 volts.

 

The impedance of FIO is ~550 ohms and the impedance of EIO/CIO is ~180 ohms.  Our scope probe has 50pF capacitance, which works out to 28ns for FIO and 9ns for CIO/EIO, which is pretty good agreement with what we actually see on the scope.




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