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Old 12th February 2004, 05:49 PM   #1
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Default DC Source Impedance measurement

I have a challenege for you analog guru's out there.

Let's say I have a DC source with some internal impedance (kinda like a battery). This impedance is complex is nature with more than one equiv. C, L and R' inside. Given that this impedance will be on the order of milliohms at any particular frequency:

Does anyone out there know of a simple circuit that could accruately measure the impedance of this source. Keep in mind this is AC impedance. What I am looking for are say curves from 1hz - 10kHz for example.

This is a difficult measurement because the magnitude of the impedance is so small!
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Old 12th February 2004, 06:32 PM   #2
azira is offline azira  United States
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Hmm... a couple of quick 'duh' questions:

1) If your source is DC, why are you interested in your AC impedances?

2) If your source impedance is known to be on the order of milliohms, then what does the value matter. As long as your input impedance is 1-ohm or more, it'll be significantly larger than your source impedance and your attenuation won't really matter.
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Old 12th February 2004, 06:35 PM   #3
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If you have a quality setup like an Audio Precision, this would
not be a problem, as you can drive the DC source with an
AC signal through a big cap and precisely measure the AC
voltages which appear across the DC source. Even milliohms
would not be particularly daunting. You can insert a known
impedance in series with the AP output, or use its internal
25, 50, or 600 ohm source solely.

If you don't have such a refined measurement apparatus, drive
the DC source through a cap and resistor, but with a big fat
power amplifier, and use a less sensitive, but wide band, AC
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Old 12th February 2004, 06:43 PM   #4
sreten is offline sreten  United Kingdom
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Add a reasonable value series resistor and a DC blocking capacitor.

Drive the circuit it with an AC source and measure the AC voltage across the DC source.

This will give its impedance, assuming the R effects a current source.

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Old 12th February 2004, 07:59 PM   #5
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That is the way I do it now. The problem is that I am trying to condense the circuit were a large cap is not needed. I am trying to fit all of this cicuitry into oa very small package. The big cap right now hatkes up all of the space. I am looking for an all op-amp solution.

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Old 12th February 2004, 08:06 PM   #6
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Are you building a super smart auto-correcting zero-impedance power supply?? Admit it!!

Jan Didden
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Old 12th February 2004, 09:01 PM   #7
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No, I am actually measuring AC impedance output imp. of a DC source. For most practical situations this information is meaningless.
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Old 13th February 2004, 03:07 AM   #8
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Default Measuring output impedance

To measure the output impedance of a generator (ac or dc) you can use the load substitution method and Ohms law.

See http://www.mitedu.freeserve.co.uk/Theory/inzoz.htm
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