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OPA4192: Design Inquiry

Abhishek Manoj
Intellectual 925 points

Part Number: OPA4192
Other Parts Discussed in Thread: OPA192,

Hi Team,

I'm planning to the amplifiers OPA4192 and OPA192 in my boards as a TIA stage for the DAC AD3552R.

Please address the below queries.

  1. In the datasheet it is mentioned about RISO and Cload at the output of the amplifier. Does it is mandatory to provide the Riso resistor. Can i directly connect the TIA output to my next stages without this Riso and Cload.

Regards,

Abhishek

  • 0
    TI__Mastermind 29868 points

    Abhishek,

    If I understand your question correctly, you are asking if you can connect the output of one OPA192 stage to another.  In your case there is no capacitive load, correct?  As long as the op amp is not driving a capacitive load, or the capacitive load is small you do not need an isolation resistance.  Below are the two key plots related to this subject.  Notice that the plots  list Riso equal to 0 ohms, 25 ohms, and 50 ohms.  The case with Riso = 0 corresponds to your goal (i.e. no Riso used).  The problem with connecting a capacitor to the output of an amplifier is that it can cause the op amp to oscillate.   This issue is called the stability of the amplifier.  When an amplifier is "unstable" it will have large overshoot, ringing, and possibly a continuous oscillation.  We consider amplifiers to be stable with 24% overshoot and less.  The two graphs below show the percentage overshoot with different capacitive load.  I don't think your application has capacitive load so you don't need a value of Riso to stabilize your op amp.  Here is a document on the subject of stability:  https://www.ti.com/jp/lit/wp/sboa626/sboa626.pdf .

    I hope this help, Best regards, Art

  • 0 in reply to Art Kay
    Intellectual 925 points

    Hi Art Kay,

    Thanks for the reply.

    I'm using OPA4192 and OPA192 in two cases.

    Case 1: TIA stage.

    OPA4192 as TIA.pdf

    1. Please find the attached schematic and provide the feedback.

    2. In TIA application OPAx192 is interfaced with the DAC AD3552R or AD3551R. The amplifier gain is set from the internal feedback resistor of the DAC. While checking with the DAC manufacturer, the capacitor values on the gain resistor is set with respect to the amplifier. Please suggest the capacitor values to be given as tuning capacitor for gain setting resistor.

    Case 2: Unity Gain Buffer.

    OPA4192 as UNITY GAIN BUFFER.pdf

    1. Please find the attached schematic and provide the feedback.

    2. For unity gain configuration, the feedback resistor is provided as 0 ohms. Whether this resistor value is fine or any other suggestion is there

    Regards,

    Abhishek

  • 0 in reply to Abhishek Manoj
    Guru 326500 points

    Both schematics are incomplete. There is not enough information to predict how the opamp interacts with the unknown components connected to it.

    The first schematic shows 4.7 pF and 27 pF capacitors on the outputs. This is mostly harmless (see the graphs in Art's answer).

  • 0 in reply to Clemens Ladisch
    Intellectual 925 points

    Hi Team,

    As i mentioned i want to connect OPA4192 at the output of the DAC AD3552R as a transimpedance amplifier configuration.

    Those capacitor values are kept as default from the DAC evaluation board. But for this OPA4192 can you suggest the recommended capacitor values.

    Also, what are the other information need with respect to the circuit.

    Regards,

    Abhishek

  • 0 in reply to Abhishek Manoj
    TI__Mastermind 29868 points

    Abhishek,

    • Your transimpedance schematic does seem correct.  I do not see any feedback resistor.  Below is the equation that you would use to select CF.  However, you have to have a feedback resistor to include this capacitor.  Below is are two articles on transimpedance amplifiers.  
    • Your "buffer circuit looks ok.  You will need to set the value of the isolation resistor R1680-R1682 if you want to drive a capacitive load (as discussed earlier).  If you do not need to drive a capacitor, you can keep the value at 0 ohms.

    Transimpedance article - theory

    Transimpedance article - measured example

    I hope the above information helps.  Op amp precision labs is a great place to learn about all these op amp concepts.  

    Best regards, Art

  • 0 in reply to Art Kay
    Intellectual 925 points

    Hi Art,

    Thanks for your feedback.

    In my case, the feedback resistors are inside the DAC, i can connect one end of the resistor to the current output IOUT of the DAC.

    In the above shared equation, for Cin, which value should i choose, as in amplifier input i'm not populating any capacitor and my DAC IC AD3552R datasheet also doesn't mention any output capacitance value. 

    Please provide how to calculate the Cin value.

    The current capacitor values shared in the schematics are the values present in my DAC IC evaluation board.

    Regards,
    Abhishek

  • 0 in reply to Abhishek Manoj
    TI__Mastermind 29868 points

    Abhishek,

    Many transimpedance amplifiers connect to a photodiode.  Photodiodes can have large capacitances so installing CF is critical for stability.  In your case the transimpedance is used for converting the DAC output current to a voltage and the input capacitance will be much smaller.  Nevertheless, I do think it is a good idea to include a value for CF.  In your circuit the input capacitance is the parasitic PCB capacitance, the amplifier common mode input capacitance, and any capacitance from the DAC.  The common mode input capacitance is 1pF (see Zic on page 8 of the data sheet).  The PCB capacitance and DAC capacitance are likely also very low.  I think 10pF is would be a good conservative estimate, and the actual capacitance will be lower than that.

    Best regards, Art