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OPA656: The value selection of CD of OPA656

user5994204
Prodigy 185 points
Part Number: OPA656
Other Parts Discussed in Thread: LMH6559, , BUF602

Hi how to choose the value of CD in the real circuit as the following

The data sheet says that CD indicates the diode capacitance. Then, which capacitors are diode capacitors in the following circuit, or how to select the value of CD in the following circuit?

Thanks very much

Fig.1 Circuit connection flow chart.     function generator—SPDT—LMH6559—Capacitance and resistance-OPA656

Fig.2 OPA656 circuit design schematic

Fig.3 OPA656 data sheet ,Wideband, High-Sensitivity, Transimpedance Amplifier

  • 0
    Guru 48395 points

    Well, if you are really driving the test cell with an LMH6559, that low impedance output - 

    1. will have stability problems in the buffer driving a cap load - might want a small series resistor to isolate it from the test cell capacitance
    2. The buffer output impedance will look like an impedance in series with the cap. Being inductive, the simple Zt design might not work - what signal BW are you after here? 

  • 0 in reply to Michael Steffes
    Prodigy 185 points

    Hi Michael Steffes,the following is Tina simulation 

    1.there is a series resistor between the buffer and measuring cell

    2.the measuring cell  is composed of cap and resistor in parallel

    and it really does not work

    please give me some advice to solve those problems

    1.how to choose the value of CD

    2.how to design the buffer circuit

    and there is not real value of cap and resistor in parallel in measuring cell

    Cf：0.057~4.595 pF

    Rf：100-1000Ω-

  • 0 in reply to Michael Steffes
    Prodigy 185 points

    Hi Michael Steffes,the following is Tina simulation 

    1.there is a series resistor between the buffer and measuring cell

    2.the measuring cell  is composed of cap and resistor in parallel

    and it really does not work

    please give me some advice to solve those problems

    1.how to choose the value of CD

    2.how to design the buffer circuit

    and there is not real value of cap and resistor in parallel in measuring cell

    Cf：0.057~4.595 pF

    Rf：100-1000Ω-

  • 0 in reply to user5994204
    Guru 48395 points

    So what are you trying to do? is the signal current that makes it through the measuring cell your info of interest? Looks like it since it is AC couple out of the Buffer. Incidentally, the BUF602 is a 1GHz buffer with that midscale bias stuff built in as a lower noise way to do that. 

    If you have the TINA file, can you attach?

  • 0 in reply to user5994204
    TI__Genius 15555 points

    Hello,

    Cd is the diode capacitance of a trans-impedance amplifier. This capacitance isn't added into the circuit, but exists as a result of using a photo-diode at your input. As Micheal mentioned, we need more information on what you are trying to achieve, and it would help if you could attach the TINA file you have been using.

    Best,

    Hasan Babiker

  • 0 in reply to Hasan Babiker31
    TI__Genius 15555 points

    Hello,

    Any updates on this?

    Best,

    Hasan Babiker

  • 0 in reply to Hasan Babiker31
    TI__Genius 15555 points

    Closing this thread due to inactivity. If further support is needed, please reply or start a new thread.

    Thank you,

    Hasan Babiker