NPN output and filtering

[sweethomela8]

Hi everyone, quick question on NPN type outputs and how to filter them.

I'm using a detector device which specifies a NPN output of 250mA.

If I want to interface this to a FPGA as an input, does this mean I have to use a pull-up resistor to 3.3V (my IO voltage) which would be (ie. 4.7k....0.702mA)?

Also, if I want filtering on this input to the FPGA, can I use a decoupling cap to GND such as a 0.1uF?

 

[IanP]

If I want to interface this to a FPGA as an input, does this mean I have to use a pull-up resistor to 3.3V (my IO voltage) which would be (ie. 4.7k....0.702mA)?

OK ..

Also, if I want filtering on this input to the FPGA, can I use a decoupling cap to GND such as a 0.1uF?

If you want to eliminate short spikes then putting 0.1µF cap in parallel with NPN doesn't help very much ..
Leave the 0.1µF cap on the FPGA pin and add a series resistor (≈1kΩ) between NPN and the pin, and of course the pullup has to be there too ..

IanP
:|

           +Vcc
            | 
            / 
            \ 
            / 
            |       1K          
   NPN    --+----/\/\/\/--+---- FPGA pin
                          |
                         ---
                 100 nF  ---
                          |
                          |
                          |
          ----------------+---- GND

 

[sweethomela8]

for that filter, would the corner frequency fo = 1/(2*pi*RC) = 1.6 kHz approx?

Also, would I expect to see much noise on that line if its open collector? (lets say the the NPN transitor is 50-100 ft away.)

I'm wondering if I will see any noise since, either the NPN will be low (near 0 V), or floating, and the pullup resistor will pull it high with some rise time based on the RC constant.