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THVD8000: What is the differential input capacitance of bus port A-B?

John11513
Expert 5720 points

Part Number: THVD8000

Tool/software:

Hi experts,

I want to know the roughly differential input capacitance of A-B bus port of THVD8000. This cap will define the minimum coupling capacitance which coupling the bus carrier signal to/from the device.

Thank you!

John

  • 0
    TI__Guru 54042 points

    Hi John,

    We don't specify the input capacitance on this device.

    There is 0 reason to need that value to calculate the coupling capacitors because the input capacitance is probably somewhere in the tens of pF range where the smallest coupling  capacitors you can use (which would be at max modulation frequency 5MHz) would be 6.4nF so the input capacitance is << than any coupling capacitance you would use and isn't required to calculate coupling capacitors. Not only that - but the inputs allow a DC signal - so the input capacitance would be capacitance to ground - which is different than the coupling network. 

    For DC systems the capacitor value for the coupling network needs to satisfy the following equation 5 >= 1/(2*pi*f_mod*C)

    For AC systems the capacitor value equation is extremely similar --> 5 = 1/(2*pi*f_mod*C) 

    So for both systems (AC or DC) you can use C = 1/(2*pi*f_mod*5) as the design requirement for this capacitor is to block DC or low frequency AC and we do that by setting the coupling capacitors to be 5 ohms at modulation frequency. 

    This part is used in multiple systems and no one has ever needed the input capacitance because the coupling capacitance is always >> than the input capacitance is (so it can be ignored at a system level because of the discrete capacitance that must be added for system) and the only system parameter that is used to calculate the coupling capacitors is the modulation frequency of the THVD8000 and you don't need to include input capacitance of THVD8000 - and that only sets the minimum capacitance. 

    Best,

    Parker Dodson