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LMH32401: LMH32401

Part Number: LMH32401
Other Parts Discussed in Thread: OPA855, OPA3S2859-EP

    Hello, I'm looking for a product, but I can't find a specific model with related functions on the market. Now I will briefly describe it:
    As shown in the red circle in the figure below, it can realize the switching of the front-end transimpedance gain of LMH32401; similar to a 1:2/4 mux switch, it can realize the connection between one end (input end) and one of the other end (output end). The signal bandwidth is from DC to AC (it could be 200MHz~500MHz, no strict restrictions). Since the channel realizes the transmission of photocurrent, parameters such as parasitic capacitance and leakage current of the input and output pins need to be considered, and the expected value is <4pF and nA level. Looking for your help here, that you can recommend a specific chip model that can achieve the above-mentioned related functions, or other alternative methods.

  • Hi Chao,

    and what is wrong with the LMH32401?

    it can realize the connection between one end (input end) and one of the other end (output end)

    I have no idea what you mean. Can you please elaborate? Can you show a drawing or sketch?

    Kai

  • Hi  Kai.  I am so sorry for my previous statement was not clear, now I will introduce my problem.
    I am currently using OPA855 to design  a TIA which can achieve  gain switching, as shown in the figure below. And my requirement is to find a switching chip like S1, which could achieve 1:4 mux (1:2 or something else).  


    When the switch is closed, the path behaves like a wire which could transmit the photocurrent signals, and the bandwidth is from DC to AC (it would be 200MHz~500MHz). Due to the limitation of using, there are certainly requirements for the switching chip , such as pin's parasitic capacitance and leakage current should be as small as possible to meet the conditions for the TIA.
    Also i have considered using jumper to switch the resistance. However, in this way the parasitic inductance brought by the wire would adversely affect high-frequency signals; Therefore, I wanted to find a switching chip that would be able to achieve gain switching and without too much effect on high-frequency signals. Of course, if you have other suggestions, thank you for sharing with me.
    Thank you.
    Chao

  • Hi Chao,

    in any case, I would limit the number of switches in the feedback loop to the absolute minimum. And I would move the switches from the -input of OPAmp to its output. This will minimize the problematic stray capacitance at the -input of OPAmp which would otherwise decrease the phase margin.

    Kai

  • Hello Chao,

      Good question and good note on leakage current and parasitic that unfortunately come along with switches. I agree with Kai; it is best to avoid adding additional parasitic to high speed amplifier designs. We ran into the same type of application with more details on this thread.

      As suggested in the earlier thread, I believe the OPA3S2859-EP will also work for your application. It has a low input referred noise of 2.2nv/sqrtHz, three gain switches optimized for feedback resistors in the range of 1kOhm to 100kOhms, and is suitable for Cpd of <4pF. 

    Thank you,

    Sima  

  • Thank you very much,  Kai .   Your suggestion is of great guiding significance to me!

  • Hi Sima, Thanks a lot for your reply and suggestion.
    Just like the LMH32401 and OPA3S2859-EP, it controls the signal path by analog switch. If I use the analog switch and configure it outside of the OPA, do you have a recommended product of the analog switch (the signal bandwidths is from DC to 400MHz)?


    In addition, if I use a physical switch (such as a Half-Pitch Dip Switch in the picture), which compared with an analog switch, it will not generate leakage current, but it will bring a larger parasitic inductance. Are there other advantages and disadvantages?

  • Hello Chao,

      That is correct, the OPA3S2859-EP does contain analog switches between resistor configurations. However, the effects are mitigated in integration compared to choosing discrete switch, and the performance is documented in the datasheet. It is possible to build a discrete TIA with analog switches, but would require fine tuning due to the switch capacitance. Higher inductance at higher frequency will cause a break in the feedback loop which will cause instability. 

      Also, each feedback resistor will need to have a feedback capacitance to compensate the input capacitance. In this case it is very low, but will depend on additional parasitic capacitance seen at the input. 

      Image from this link: e2e.ti.com/.../precision-op-amp-series-switched-gain-transimpedance-amplifier

      Another method would be to use two TIAs and add a switch to the output: 

    Thank you,
    Sima