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Quick Schematic Question

This question is answered
Kyle Aven
Posted by Kyle Aven
on Jan 30 2012 21:45 PM
Intellectual640 points

Hello,

Can someone tell me what the purpose of the two 49.9Ohm resistors in the feedback loop are for? I have seen this topology used for driving capacitive loads, but was wondering why it was included in the ADS1278 datasheet. Does it lower output impedance? If so, do you use a resistor close to that of the output impedance of the amplifier.

Thanks

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  • Collin Wells
    Posted by Collin Wells
    on Jan 31 2012 09:46 AM
    Verified Answer
    Verified by Kyle Aven
    Genius15530 points

    Hello Kyle,

    You are correct, the 1k and 249 ohm resistors set the DC gain of the circuit.  The additional 49.9Ohm and 5.6nF components are present for two reasons.  The first is as you've mentioned to improve the stability of this circuit if it drives a capacitive load.  The second reason is it lowers the bandwidth of the circuit so it also lowers the noise.  The second effect is described in the attached application note:

    http://www.ti.com/lit/an/sbva010/sbva010.pdf

    The application note as well as the ADI material you've already referenced show how to roughly select the circuit values.  I would always recommend simulating the circuit with a step response after you've completed it to ensure a proper response that does not ring or overshoot excessively.

    Regards,
    Collin Wells
    Precision Linear Applications 

    Regards,
    Collin Wells
    Precision Linear Applications

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