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TL074: what is the iso resistor value on the amplifier's output

Gary Zeng
Intellectual 1195 points
Part Number: TL074

Tool/software:

Hi team,

My cutsomer wants to generate a 1.5V voltage reference for 1.5V offset input of 10x amplifiers.

They want to add iso resistor behind the amplifier's output. As you can see, there are three caps on the output. On the other 10x amplifiers' side, each input end of amplifier, there will be a 100pF.

10x amplifiers: 

Q1: What is the suitable R193 value for this case? can you show the calculation formula.

We find a circuit stability analysis document. 

It uses zero point frequency to get the resistor.

Q2: how to get above waveform then to get the zero point.

  • 0
    TI__Mastermind 24618 points

    Gary,

    1. For the design method you reference above, the way that you can get the zero frequency is using the cursor in the spice simulation.  What you do is place the cursor to a gain of 20dB above the closed loop gain.  For a buffer, you would set the cursor to the gain of 20dB.
    2. Another method that you can use to find RISO uses an equation.  The equation yields the minimum value for RISO, so it is important to round this value up when choosing a standard resistor value.  Generally, I use this method as it is pretty easy.
    3. It is also important in the dual-feedback configuration to select the feedback capacitor and feedback resistor correctly.  Below image with the math for the dual feedback configuration.
    4. One important thing to note:  it may be counterintuitive, but the value of Riso can be smaller for larger capacitive loads.  Thus, the values given below are for the 20uF load on the dual feedback configuration.  The value of RISO will need to be larger if the capacitive load is smaller.
    5. The Analog Engineer's Calculator can be used to find the values and confirm stability.  Just enter the data sheet information for the amplifier in the "op amp parameters" section, enter the capacitive load, and press "recommend compensation".  This will use the same algorithm in the image below for choosing values, so it gives the minimum resistor.  Round the components up to standard values and press the "graph" button to confirm that the adjusted values still give good phase margin.  There are some indicator lights that will turn red or green for stable or not stable.    

    best regards,

    Art