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LMV321: Basic design for the amplifier and calculation for resistor values

srinithi selvaraj
Prodigy 20 points
Part Number: LMV321
Other Parts Discussed in Thread: LM321,

I have a basic op amp circuit diagram consisting of LMV321 and LM321 as shown below. The reference voltage for both OP Amp ICs are 2.5V. The input voltage is 5V. I want to re-design the circuit with 3.3V keeping same reference voltage of 2.5V. So, I basically want to know the design configuration and how the resistor values in the circuit are calculated. I have searched the datasheets of both op-amps, and I am left confused. Could you kindly provide a detailed calculation so that it will be helpful for the 3.3V design. The circuit diagram is shown below.

Regards,

Srinithi

  • 0
    TI__Genius 11660 points

    Hi Srinithi, 

    Welcome to E2E! 

    Could you clarify a few items: 

    • What is the input stage before the LMV321 that is cut off in on the left side? 
    • What is directly connected to the output of the LM321? 
    • Which part of the circuit are you trying to re-design for 3.3V? The input into LMV321 (circled in green), the 2.5V reference (blue) or the supply voltage (yellow)? 

    Thank you!
    Best Regards,
    Ashley

  • 0 in reply to Ashley Kang
    Prodigy 20 points

    Hi Ashley,

    Greetings.

    Many thanks for the response.

    Please find the responses for the following queries.

    • What is the input stage before the LMV321 that is cut off in on the left side? 

    The input stage before LMV321 is 1.2KHz/2.2KHz PWM signal from STM32G4 MCU.

    • What is directly connected to the output of the LM321? 

    The output of op-amp LM321 is directly connected to input of TI's analog switch 74HC4066

    • Which part of the circuit are you trying to re-design for 3.3V? The input into LMV321 (circled in green), the 2.5V reference (blue) or the supply voltage (yellow)? 

    I am trying to re-design for the supply voltage (highlighted in yellow)

    Regards,

    Srinithi

  • 0 in reply to srinithi selvaraj
    TI__Genius 11660 points

    Hi Srinithi, 

    Thank you for the clarification on the schematic and re-design. 

    What are the desired input and output voltages that you are trying to achieve with the design? 

    Regards,
    Ashley

  • 0 in reply to Ashley Kang
    Prodigy 20 points

    Hi Ashley,

    Many thanks once again for the response.

    The desired input is the HART signal of 4-20 mA having FSK signaling of 1.2KHz low frequency (for 4mA) and 2.2KHz high frequency (for 20mA) and voltage reference of 2.5V. The desired output voltage is 2.5V.

    Best Regards,
    Srinithi

  • +1 in reply to srinithi selvaraj
    TI__Genius 11660 points

    Hi Srinithi, 

    One limitation to consider is the input and output limitations of the op amp when decreasing the supply voltage from 5V to 3.3V. 

    Please see the video here on Input/Output limitations for op amps: http://ti.com/video/6279999026001

    The LM321 has a limited upper headroom from the positive supply, so if you are using a lower supply voltage, the headroom for input and output on the upper end decreases. If you exceed this region, the op amp will behave non-linearly. If you need a wider range, then a rail-to-rail op amp is a better choice.

    Please let me know if you have further questions.
    Thank you!

    Best Regards,
    Ashley

  • 0 in reply to Ashley Kang
    Prodigy 20 points

    Thank you Ashley for the valuable information.

    Regards,
    Srinithi