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TTL Photodetectors
OPERATION:
EOT's photodetectors with TTL Out are based off our standard detectors with the added feature of a 2 state output whose threshold is adjustable. Thus the Analog Out exhibits performance characteristics on par with our standard products that share the same core part number.
Use caution when aiming Laser on photodiode; reflections may occur off the surface. Appropriate eye safety precautions must be followed.
Mount the detector to an optical stand using the mounting holes on the baseplate of the detector housing. Both English and Metric threads are provided. The holes can be reversed if desired by reinstalling the baseplate, reversed.
Terminate the detector's Analog Out into 50Ω. Use standard RG-58 50Ω coaxial cable as required on the Analog and TTL Outputs . The spec'ed performance will only be realized if the cable is <= 3 feet, and the termination impedance on the TTL Out is <= 500Ω.
Connect the 12V power supply provided.
After being certain that the damage threshold of the detector is not exceeded, aim the laser on the center of the photodiode lens. If an oscilloscope is connected to the Analog Out, align the beam until the voltage is maximized.
The Threshold Adust screw varys the Analog Out voltage at which the TTL Out will change logic states. Its level can be observed by connecting a voltmeter (with input impedance > 100kΩ) to the Threshold testpoint. Performance is optimal when the Threshold is adjusted to half the peak Analog Out, however the useful range extends from about 40mV to the Analog peak. If the Threshold is adjusted much below 40mV, the TTL Out may oscillate.
The duration of the TTL Out can be stretched by moving the toggle switch to the down position. This is useful when connecting to instruments too slow to recognize pulses only 10's of nanoseconds in duration.
TIMING CONSIDERATIONS:
The propagation delay, risetime, falltime, and over/undershoot of the TTL Out are all proportional to both the length of the coaxial cable and the termination impedance. The Risetime and Falltime specs include the propagation delay and can be improved upon by shortening the length of the coaxial cable and/or reducing the termination impendance, although the latter will reduce the Logic High voltage. Do not use termination impedances less than 50Ω.
SENSITIVITY CONSIDERATIONS:
Since the internal photodiode is essentially a current source, the Analog Out voltage can be increased by increasing its termination impedance, at the expense of slowing its response time. In doing this, the TTL Out sensitivity will also increase. If the application only requires signal from the TTL Out, the Analog Out could be left disconnected, leaving an internal termination of 510Ω. Thus the sensitivity of the TTL Out will increase by roughly an order of magnitude.
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