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INA117: Output Series Resistor

Michael Lesiak
Prodigy 10 points
Part Number: INA117

Tool/software:

I plan to use the INA117 with an increased differential input range, inputting up to 26 Vrms, and wiring output on pin 6 to an analog input on DAQ device, via a shielded cable. My question is, what are the pros, cons, and selection criteria to adding a series output resistor for stability and perhaps short circuit protection?

  • 0
    TI__Mastermind 20867 points

    Michael,  Great question.  I hope this information helps.

    1. The stability will depend on the output capacitive load.  This depends on the length of the cable and the capacitance per unit length. 
    2. The data sheet specifies a maximum load capacitance of 1nF for stable operation.  Figure 6-6 illustrates a small step measurement of overshoot vs capacitive load, and that picture doesn't show much overshoot.  That means that the INA117 can drive a pretty heavy capacitive load without issue.  Your cable is probably less than 1nF but you should check this.
    3. You are correct that using a series isolation resistor will further improve the stability of this amplifier (or an amplifier) when driving a capacitive load.  The question is what value? This device was released some time ago, so I checked the SPICE model to see if it covers stability (i.e. models the output impedance).  Based on my review the net-list is simple and I would not use this model to predict stability.  Since we cannot use spice to predict stability, I think a good approach is to use a common rule-of-thumb.  A common isolation resistor range is 10 ohms to 1k ohm.  Since you are driving across a cable to a measurement instrument, I would suggest either a 100 ohm or 1k resistor.  The only disadvantage of using the higher resistor value is if the instrument has a finite input resistance or has a bias current that will flow through the resistor and develop an error voltage.  I think must measurement instrument will not have these issues, so I don't think you would have an issue with 1k ohm.  Also, as noted in item 2 above the INA117 can drive heavy capacitive loads (up to 1nF) without any isolation resistance, so a 100 or 1k resistor will only help.

    I hope this helps out.

    Best regards, Art

  • 0
    Prodigy 10 points

    Art,

    I expect less than 1nF max load capacitance, actually about 75.5pF nominally. I have seen some related designs with a 100 ohm series output resistor used but now I have a better understanding of why it is used and how this is selected. I will use a 100 ohm series isolation resistor.

    Thank you for your detailed response.

    Best regards,

    Mike