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12th February 2004, 06:49 PM  #1 
diyAudio Member

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! 
12th February 2004, 07:32 PM  #2 
diyAudio Member
Join Date: Dec 2003
Location: Near Seattle

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 1ohm or more, it'll be significantly larger than your source impedance and your attenuation won't really matter. 
12th February 2004, 07:35 PM  #3 
The one and only

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 voltameter. 
12th February 2004, 07:43 PM  #4 
diyAudio Member
Join Date: Nov 2003
Location: Brighton UK

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. sreten. 
12th February 2004, 08:59 PM  #5 
diyAudio Member

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 opamp solution.
Thanks! 
12th February 2004, 10:01 PM  #7 
diyAudio Member

No, I am actually measuring AC impedance output imp. of a DC source. For most practical situations this information is meaningless.

13th February 2004, 04:07 AM  #8 
diyAudio Member
Join Date: Jan 2004
Location: Perth

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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