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Old 19th February 2004, 01:03 PM   #1
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Default OP-AMP output impedance

Is there any way to effectively decrease the output impedance of an op-amp in voltage follower mode. (THis is small signal stuff, +/- 15v rails). I have an application that requires Zout to be less than .001 ohm.

Any Suggestions?
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Old 19th February 2004, 01:07 PM   #2
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I think that with carefull selection of the opamp, there are some out there that will provide this as-is. What is the freq range over which you want it to be .001? What is the required output current? Just curious, what is the application?

Also, be aware that this will be completely swamped by PCB-trace and wiring impedance, UNLESS you take your feedback point directly from the load.

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Old 19th February 2004, 02:14 PM   #3
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Default Re: OP-AMP output impedance

Quote:
Originally posted by hugeli60
Is there any way to effectively decrease the output impedance of an op-amp in voltage follower mode. (THis is small signal stuff, +/- 15v rails). I have an application that requires Zout to be less than .001 ohm.
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Old 19th February 2004, 05:35 PM   #4
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Any idea how to set up the wiring in order to get this
low impedance not only at the at the output of the OP amp, but
also at that point were you need this low signal impedance.

May be you will have to put the signal wiring also "inside" the
feedback loop of the OP amp...
..and take care about your ground...
... real 1mOhm... let's say: a challenge
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Old 19th February 2004, 05:39 PM   #5
sreten is offline sreten  United Kingdom
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Usually a series resistor in thr output ensures
high frequency stability into capacitive loads.

The real issue here is your applications requirements.

Where do they come from, and is there a better way of doing it ?

Note that feedback derived output impedance varies strongly with frequency.

sreten.
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Old 20th February 2004, 04:05 PM   #6
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Default Fedback Point

Any op-amp will try to have a zero output impedance. However, if it it this scritical, it is essential to do a "Kelvin" connection. Both ends of the feedback path have to be across your load with the shortest possible connection. Also, do not ask the op-amp to behave as if it has such a low output impedance and to give a lot of output voltage. With a ±15V supply, expect that performance perhaps to only ±2V. That is unless you build an output power buffer.

As well, I hope the load impedance still won't be a lot less than a few hundred ohms. If lower, you're in a whole different class of product.
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Old 20th February 2004, 05:56 PM   #7
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OpAmp's output impedance strongly depends on:

1) OpAmp type (i.e. circuit design)
2) closed loop gain
3) frequency

Consult datasheets, you will find answer there.
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