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ONET8551T: Input Impedance

Evgeny Manzhosov
Prodigy 40 points
Part Number: ONET8551T

Hello! 

I would like to use ONET8551t in optical receiver for research. 

In order to see if the package I want to create works, I want to create a small PCB with Voltage to Current converter (based on BJT). Thus I can send some voltage signal (PRBS from AWG) and see that my PCB and wirebonds allow me to operate at desired frequency. 

I see in data sheet there is no value for input impedance of the TIA stage. So I cant match the V2I converter for maximum power transfer. 

What will be the best way to deal with that? 

  • 0
    TI__Expert 7875 points
    Hi,

    If you are using some type of current mirror structure to send a current out of the BJT collector into the device then I wouldn't concern yourself with achieving the maximum power transfer. These devices are designed to operate with a high input impedance from a diode, which would be very similar to a transistor.

    Regards,
  • 0 in reply to Jacob Freet
    Prodigy 40 points

    So, making sure that output impedance of V2C converter based on BJT is enough in this case? 

    Another question, how critical is to make wire bonds about 250um in length on VCC (dc power) connections? What will happen if they, lets say, 1mm? 

  • 0 in reply to Evgeny Manzhosov
    TI__Expert 7875 points
    Hi,

    Long wire bonds will add additional parasitic inductance to the device input. Parts like the ONET8551T are very sensitive to input parasitic capacitance and inductance and could oscillate if the values become too large. I would advise to stay within the datasheet suggested limits for inductance and capacitance.

    Regards,
  • 0 in reply to Jacob Freet
    Prodigy 40 points

    Thank you Jacob! 

    Last question (I hope :) ) :

    I want to bias the PD with negative voltage (as on figure 4, but V_BIAS<0). Will it work? The current will flow in the opposite direction than with positive bias.

  • 0 in reply to Evgeny Manzhosov
    TI__Expert 7875 points
    Hi,

    Apologies for the late reply. The device can only handle current flowing positively into the input. The maximum current rating into the input pin is -0.7mA to 4mA.

    Regards,
  • 0 in reply to Jacob Freet
    Prodigy 40 points

    Hi Jacob! 

    I have a photo diode that is built as PNP device, i.e. signal is coming from the N (cathode). Which means that current is flowing into the cathode. 

    Initially I thought of biasing the photo diode with Bias Tee to negative voltage and using the ONT8551 TIA, but as you pointed out the TIA is not supporting negative current. 

    Is there a workaround for this? How Should I connect my photo diode to ONET8551 TIA? 

  • 0 in reply to Evgeny Manzhosov
    TI__Expert 7875 points
    Hi,

    You will need to have a diode that creates a positive current into the device. You could use a different diode or potentially invert the current using a current mirror structure if you have the option for more components.

    Regards,