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I need to build up a circuit to detect nsec width pulses. Naturally I found this thread:
I built a circuit with reference to Fig. 48 of the OPA615 datasheet: http://www.ti.com/lit/ds/sbos299e/sbos299e.pdf
I only using the positive peak portion of the circuit.
I also compared with an alternative schematic offered by the original poster:
And now are my questions finally:
1) The TI schematic in Fig. 48 looks a little strange to me, that is, the 300 ohm negative feedback resistor does not have another resistor to go to ground. Please see the alternative schematic, the R4. Why is that?
2) The 27pF capacitor in the TI schematic in Fig. 48 seem to be a little small. We found that with our proto board, the output just follows the input. I tried to increase it to 1nF and I see the output voltage starts to hold a bit. If I look at the alternative schematic, the charging capacitor is also 1000pF. So could this be explained?
3) When I increased the 27Pf to 1nF, the response time, both the rising and falling, gets bigger.
4) Anyway, I think my circuit has some problem. So here are the traces at pins #2 & #3 of the OPA615. The questins on these are:
a) the voltage relationship between #2 & #3 seem to be right, #3 (the base) should be about 0.7V above #2 (the emitter), correct?
b) what are the glitches and ringing in the traces? How do I get rid of them?
Thanks a lot,
Pin #2 of OPA615 (channel #1, and channel #2 is the test signal from the sig generator):
Pin #3 of OPA615(channel #1, and channel #2 is the test signal from the sig generator):
In reply to Samuel Kokel:
I'm going to put this peak detector together and use it for 250nsec wide pulses with a peak of 3V. I need to zero out the capacitor between pulses so we are trying a couple of MOSFET
arrangements to see how much the affect performance.
In reply to Clive Standley:
I modeled the peak detector circuit in TINA and put an NPN transistor across the capacitor and drove it with a 2usec +5V pulse.
the input photo detector pulse train was a 3V, 200nsec wide pulse followed 40 usecs latter by a 1 V 200nsec wide pulse
The result was worryingly good with very little voltage droop. I'd like to try it with a 2N7002, but I can find it in TINA
is there a model for it available?
If there is a spice model of 2N7002 from the vendor, I think you should be able to import it in TINA-TI.
In reply to Rohit Bhat:
Thanks I found a model on line and used the macro wizard to upload it. It works nicely to reset my peak hold circuit.
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