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OPA857: Input Impedance

Samuel Phiri
Prodigy 30 points
Part Number: OPA857
Other Parts Discussed in Thread: OPA847, OPA855

I want to use an OPA857 as a trans-impedance amplifier for an output signal from a photodiode (SM05PD1B). The photodiode datasheet says V0 = Power • Responsivity (lamda) • Load-resistance. I just want to approximate the photodiode voltage output before amplification. However the OPA857 data sheet only gives a 1.5pF input capacitance requirement.

What I want to know:

1. What is the input impedance of an OPA847 chip?

2. With the SM05PD1B photodiode, how can I confirm that my bias puts the input capacitance around 1.5pF as required by the OPA857?

Thank you,

Sam

  • 0
    TI__Genius 15555 points

    Hello Sam,

    The input capacitance is not a requirement but the parasitic capacitance of the input of the OPA857 datasheet. The "load resistance" in the photodiode datasheet in this case will be the feedback resistor setting you choose on the OPA857. 

    Best,

    Hasan Babiker

  • 0 in reply to Hasan Babiker31
    Guru 48395 points

    Yes, the OPA857 input C becomes part of the design equations for the feedback C. The load the diode sees is actually not the feedback R since it is usually seeing the virtual ground at the inverting node. 

  • 0 in reply to Michael Steffes
    TI__Genius 15555 points

    Hello Michael,

    You are right, to further clarify on my comments:

    -While the feedback resistor is not technically the load seen by the photodiode it is what should be used for the load-resistance variable in the equation in the original post "V0 = Power • Responsivity (lamda) • Load-resistance" to obtain the output voltage of the TIA circuit. The equation in the photodiode datasheet is assuming you are just using a resistor to convert to voltage rather than a TIA circuit.

    -In regards to the input capacitance there seemed to be confusion on the input capacitance spec. The original post referred to the input capacitance as a "requirement" and was asking how to bias their photodiode to add 1.5pF to the OPA857. The input capacitance is actually the parasitic input capacitance of the amplifier, any capacitance added from the photodiode will only add to the overall capacitance. Like Michael mentioned, this total input capacitance will need to be considered in the design of your TIA circuit.

    Best,

    Hasan Babiker

  • 0
    Prodigy 30 points

    Hi Hasan and Michael,

    Thank you both for your replies.

    Under "Detailed Description" section of the OPA857 datasheet "amplifier is optimized to achieve greater than 100-MHz bandwidth on either the 5-kΩ or 20-kΩ transimpedance gain for the lowest possible RMS noise on the output for a targeted low input capacitance of 1.5 pF..."  Also says "...This value is selected because the device is expected to be driven by a photodiode with biasing high enough to include the photodiode capacitance contribution between approximately 0.5 pF and 0.7 pF, leaving between 0.8 pF to 1 pF for the external parasitic."

    If I understand this correctly, there's a way I can bias the photodiode to ensure that its contribution is between approximately 0.5 pF and 0.7 pF, hence my original question.

    Best,

    Sam

  • 0 in reply to Samuel Phiri
    TI__Genius 15555 points

    Hello Sam,

    My bad, I missed the 1.5pF source capacitance test condition. Since input capacitance is rated at 2pF it looks like the parasitic from the device is around 0.5pF. In regards to the capacitance of your photodiode, this will depend on the specs of the photodiode itself. There doesn't seem to be any plots on the SM05PD1B datasheet of bias voltage vs. capacitance, but at 20V it is rated at 24pF.

    Rather than trying to bring the capacitance of the photodiode down to 0.5pF, it might be better to try to design your circuit for that larger input capacitance. In this case you will have to add external capacitance to the feedback of the OPA857. You can use the calculator found in this link: https://e2e.ti.com/blogs_/b/analogwire/archive/2016/05/06/what-you-need-to-know-about-transimpedance-amplifiers-part-1 to design for the best feedback capacitance to maximize your bandwidth. Note that you will have to account for the internal feedback cap already provided within the OPA857. 

    Best,

    Hasan Babiker

  • 0 in reply to Hasan Babiker31
    Guru 48395 points

    The OPA857 has two gain selections internally - do you need that? If not maybe just do a transimpedance design with the OPA855