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OPA838: Choosing isolation resistor for OPA838 output

Behnam Madadnia
Intellectual 420 points
Part Number: OPA838

Dear all,

I want to choose an accurate vale for an isolation resistor for my OP-AMP (OPA838), based on the datasheet OPA838 is very sensitive to capacitive loads and for make the system robust to instability we need to add an isolation resistor to the OP-AMP output, as we can with and without this resistor, there is an huge difference in the frequency response of the close-loop system:

For choosing the isolation resistor value, datasheet suggested this figure:

My question: is this parameter the close loop gain? if yes, if I have a different gain than, what I can see in the figure, how should I choose the isolation resistor? for example 60dB-> 1000 V/V?

My OPA838 configuration is transimpedance amplifier and the loop gain is 60dB-> 1000 V/V, so I wonder how can I choose this value?

My design:

  • 0
    TI__Mastermind 25975 points

    Hello Behnam, 

      Thank you for sharing the TIA schematic and for looking deeply into the device datasheet. You are correct, that figure is for closed-loop gain. This will vary due to the transimpedance configuration, what is the value of your capacitive load? We can choose this value by simulating it, and checking the phase margin through stability analysis for targeting >30 degrees of phase margin. 

    Thank you,

    Sima

  • 0 in reply to Sima Jalaleddine
    Intellectual 420 points

    Hello Sima,

    Thank you very much for your quick response.

    This capacitive load is related to my PCB trace capacitance and it can vary from 6pF to 25 pF depending on the length of the PCB. Would you please let me know if this statement is correct? 

    Calculate the isolation resistor Riso depending on the load capacitance Cload and the bandwidth of the TIA. With given Cf = 33pF and Rf = 1k the TIA has a pole frequency at about fp = 4.823 MHz. Above fp the gain will decline. With an assumed Cload = 100pF and a RC filter cutoff frequency sufficiently above fp (e.g. 3 x fp) the isolation resistor should be Riso = 110 Ohm (e.g. 100 Ohm). As another example for a Cload is= 20pF, then the resistor values would be 530 Ohm. Would you lease let me know if I'm correct or not?

    Thanks