Simulation of Photodiode connection to ADA4350

Hello Fellows,

I am attaching a LTSpice simulation circuit. Inverting input of ADA4350 is connected to a recommended equivalent circuit of a photodiode. The stimulus is a 2MHz sinusoidal signal of 2.5uAp value.

To calculate the Rfeedback (R1) and Cfeedback (C1), used the literature - snapshot is attached.

/cfs-file/__key/communityserver-discussions-components-files/385/Sinusoidal.zip

My questions are: 

1. When C2 (900pF) is almost nonexistent, input/output waveforms are not distorted. The gain is still not what is expected according to the formulas from the attached sheet. What I might be misinterpreting in setting up the circuit?

2. Datasheet for ADA4350 states that it can be operated from single supply source of 3.3VDC. When I replace VEE with a ground, The output seems to have been disabled. There is hints of sinusoidal signal at output, but no gain. Again, how should I correct this issue.

Note: The photodiode I am looking for will have nominal terminal capacitance of 900pF and will have up to 2000pF.

Thanks for helping.

Parents
  • 0
    •  Analog Employees 
    •  Super User 
    on Jan 14, 2021 3:17 AM

    Hi Mohi,

    Several points related to your issues with the ADA4350 LTspice file you attached:

    1. LTspice Oscillation: I can't tell what the screenshot instruction you attached for designing the feedback components (R1, and C1) comes from since it's not from the datasheet. However, if you want a Transimpedance gain of 200kohm, ADA4350 datasheet equation 8 shows you how to compute the feedback across it:

    Plugging in:

    RF= 200k, CS = 900pF, fGBW = 175MHz --> CF = 2pF

    Once I changed CF (your C1) to 2pF (vs. 0.2pF you had used), the output oscillations you were seeing disappear.

    Also, the gain is correct: Vout1 / I(R6) = 0.516V / 2.7uA = 191kohm (vs. 200k).

    I have reduced the stimulus frequency to 100kHz (instead of your 2MHz) because I think at that frequency you're already bandlimited, which might make it look like the transimpedance gain (ohm) is off or lower than expected.

    Here is the simulation file:

    ADA4350_Sinewave_Stimulus modified EZ 1_13_21.asc

    2. Single Supply Operation: For single supply operation, I've modified your circuit and tied IN_P to some voltage (1.3V) to make sure I allow bipolar swing at the FET amplifier using single 3.3V supply. I've also reduced the input stimulus to 1uAp to avoid distortion. Gain expression computes correctly:

    Vout1 / I(R6) = (1.51-1.1)V / 2uApp = 205k (vs. 200k).

    Here is the simulation file:

    ADA4350_Sinewave_Stimulus modified Single Supply EZ 1_13_21.asc

    Hopefully, all this helps.

    Regards,

    Hooman

  • Hi Hooman,

    Thank you for your quick reply and corrections to my schematic. I really appreciate your time and efforts.

    I will be using your first modified schematic for further discussion. Following are more of my questions:

    1. The "design-gain x signal-bandwidth" should not exceed the GBW of the OpAmp. The rule does not seem to be applicable for transimpedance amplifier. The signal I need to process is 2MHz. What gain is achievable for this signal? For the transimpedance gain of 200k and the input signal of 100KHz, the design GBW = 200k x 100kHz = 20Ghz - I must be missing a point here.

    2. What does the "The signal bandwidth" [1/(Rf x Cf)] mean mentioned in the paragraph following equation 4? For this design, it is ~2.5MHz but the input signal is limited to ~100kHz. Output starts to reduce above 100KHz of input signal.

    3. If R4 is reduced, the signal start to become DC biased. Is it because of V3?

  • Thanks Hooman.

    Once again, your answer are very through. I will go through both articles to help me understand the root of the issue.

    Mohi

  • 0
    •  Analog Employees 
    •  Super User 
    on Feb 26, 2021 9:01 PM in reply to Mohi

    Hi Mohi,

    Thanks for the compliment :-)

    BTW, I've been told that the the AD4008 actually has a lower 1k list price of $4.95 vs the AD7685’s $6.78, and has better performance overall. So the $30 cost difference is questionable.

    Also, some comments below per the Application eng. responsible for these ADC's for your reference:

    "I will make the general comment though that since you're only sampling at 100SPS, that the RC filter recommended by the data sheet is probably much higher bandwidth than it would really need to be to ensure proper settling. The ADC Driver Tool should illustrate just how much easier it is to settle the inputs when sampling at 100 SPS vs. the 250 kSPS upper limit of the AD7685.

    Outside of the improvement in specs (INL, SNR, etc.) one argument I would give to select AD4008 over AD7685 is that the AD4008 inputs being easier to drive means that for a given analog input bandwidth, you’ll be able to sample the AD4008 faster than the AD7685, which would allow for oversampling and averaging to reduce the overall noise of the measurement – but that only matters if you're really trying to implement a very low noise measurement."

    Regards,

    Hooman

  • Hi Hooman,

    Attaching is a mostly completed circuit (yet to add FB components) - but this time it is from the schematic capture.

    https://drive.google.com/file/d/1EV1c2_NZWZRGQg0iJ73Yo8QllCfITLQJ/view?usp=sharing

    Would you please suggest any modification if necessary. 

    One question about the DVDD: should it be connected to 3.3V to work with microcontroller which is a 3.3V?

    Thanks,

  • 0
    •  Analog Employees 
    •  Super User 
    on Mar 23, 2021 10:29 PM in reply to Mohi

    Hi Mohi,

    I don't see anything obvious, but to be honest I did not spend an inordinate amount of time reviewing your ADA4350 TIA design feeding into the ADC :-).

    I also wanted to point out a TIA device Preliminary datasheet for LTC6563 with ADC driver built-in if you've not seen it before, for your reference, if you wish to go that route:

    https://www.analog.com/media/en/technical-documentation/data-sheets/ltc6563.pdf

    I'm hoping other people on the forum offer suggestions (if any) to your ADA4350 schematic.

    Regards,

    Hooman

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