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The output impedance of the opamp that drives the adc

Yan Fan
Intellectual 360 points

Dear TI, 

I am having a hard time to ask this question since it is with TI precision lab training - adc and there is only TI precision lab training- opmap. I am studying the TI precision labs-adcs, section 6, 6.2 TI precision labs -adc: overview of reference drive topologies. 

On the last page of the PPT, it said " It is not possible to use this kind of filter without the buffer as the 1kohm resistor would limit its ability to respond to the large current transients from the reference input."

My question is why we need a small output impedance to meet the requirement of the large current transients. I understand a small output impedance will increase the driving capability but why we need this small impedance to meet the "fast" transients? Is this related to the output pole of the opamp?

I have an attached a screen-shot of the last page of the PPT as attached. 

Thanks,

B. Fan

  • 0
    TI__Mastermind 22297 points

    B. Fan,

    Thanks for the great question.

    Below is a figure that shows how the large series resistance will cause an error when the large transient current flows.   The current transients are 10 to 100mA in amplitude (depending on the ADC), so the 1k resistor will not allow that level of current to flow. For a 5V reference the largest current that can flow is 5mA in this example. Note that the buffered example (that you show from the slides) has essentially no input current as it is the input of an amplifier buffer.

    I think the amplifier output impedance is another question. Below is a figure that should help you to understand why the open loop output impedance limits the output current of an amplifier. Note that the simplified op amp model has the input stage (think of this as an ideal amplifier with a limited voltage swing), and output stage (which is the output impedance Ro). When the amplifier is driving a heavy load the input stage output the maximum voltage V+, but the current is limited by the output stage (output impedance Ro). Of course this is a simplification, but the concept is true. I suggest looking in the op amp precision labs under stability to learn more about amplifier output impedance. Note that the output impedance changes with frequency so fast transient currents are more difficult to drive than low frequency transients as the impedance is larger.