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Old 13th January 2008, 11:27 PM   #1
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Default Measuring Output Impedance of Power Supply

Folks,

Found a practical reference online:

http://www.hparchive.com/Application...-Letter-04.pdf

I'll be curious to know if anybody has a different/more accurate method they use.

Enjoy,

-- josť k.
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Old 14th January 2008, 04:51 AM   #2
flg is offline flg  United States
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Well, I don't know about that paper I just gave up on but, where I work we just take a deltaV/deltaI calculation. Say the supply in question outputs 1.2V @ no load. At 50A load it's output V drops to 1.15V. (1.2-1.15)/50=1mohm.
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Old 14th January 2008, 11:58 AM   #3
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That is an impedance calculation at a single (mains line) frequency. I am more interested in measuring the power supply impedance across the entire audio band.

-- josť k.
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Old 14th January 2008, 03:00 PM   #4
oshifis is offline oshifis  Hungary
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The method described in the article is good. An alternative would be to use a current output oscillator or just a large resistor in series (decoupled from the power supply through a capacitor). A constant current through the internal impedance of the PSU would give directly measurable voltage that is proportional to the impedance. This method could be applied in-circuit, that is with the amplifier stages in place. The oscillator could be replaced with a noise generator and the DVM with a FFT analyzer...
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Old 14th January 2008, 03:18 PM   #5
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Quote:
Originally posted by oshifis
The method described in the article is good. An alternative would be to use a current output oscillator or just a large resistor in series (decoupled from the power supply through a capacitor). A constant current through the internal impedance of the PSU would give directly measurable voltage that is proportional to the impedance. This method could be applied in-circuit, that is with the amplifier stages in place. The oscillator could be replaced with a noise generator and the DVM with a FFT analyzer...

That's what I have used also very often. For instance, if you insert a 1A current into the supply output, the measured signal at that output is directly the Zout in ohms. Insert 1A, and if you measure say 53 mV, the Zout = 53mOhms. You can do that at several frequencies if you don't have a signal sweeper/analyzer. Record the Zout at each freq, put it into a table in Exel and presto, you have a nice graph of Zout vs. frequency....

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Old 14th January 2008, 04:20 PM   #6
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A decent dynamic supply impedance monitor could be made pretty simply. Attached is schematic. Monitor supply with scope to see how much it dips under transients created by signal generator.
Attached Images
File Type: jpg supply_impedance.jpg (24.2 KB, 649 views)
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Old 14th January 2008, 10:58 PM   #7
Hi_Q is offline Hi_Q  England
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When I worked in a calibration lab we had an adjustable constant current active load which was set initially to fully on (Static Load) and adjusted for a particular current drain e.g. 1 Amp. We then switched the load on and off (Active Load) with a square wave signal and monitored the amount of ripple produced by the square wave. The higher the supply impedance then the larger the ripple produced. By knowing the static current first applied it is easy to calculate the supply impedance by measuring the square wave amplitude. In our example, a 150mV ripple would indicate an output impedance of 0.15 Ohms. The square wave can be set to any particular frequency of interest but in our particular application it was usually set to either 400 Hz or 1kHz.
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Old 15th January 2008, 05:44 AM   #8
oshifis is offline oshifis  Hungary
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An alternative (and most logical) method would be to use the actual amplifier as the load. Drive it with white noise into a dummy load and tap the PSU with an FFT analyzer. It needs some sort of calibration, though.
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Old 15th January 2008, 07:53 AM   #9
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Quote:
Originally posted by Hi-Q
When I worked in a calibration lab we had an adjustable constant current active load which was set initially to fully on (Static Load) and adjusted for a particular current drain e.g. 1 Amp. We then switched the load on and off (Active Load) with a square wave signal and monitored the amount of ripple produced by the square wave. The higher the supply impedance then the larger the ripple produced. By knowing the static current first applied it is easy to calculate the supply impedance by measuring the square wave amplitude. In our example, a 150mV ripple would indicate an output impedance of 0.15 Ohms. The square wave can be set to any particular frequency of interest but in our particular application it was usually set to either 400 Hz or 1kHz.
That method gives you a feeling for the ringing and stability of the supply, but it doesn't give you a quantitative result against frequency. If you do it with a sine wave, at several frequencies, and graph it (or let Exel graph it), you have something that can be very accurately compared to other supplies or to changes you make. You also see clearly where it rises with frequency so you can concentrate on that issue if you wish.

There are many valid methods; often the crux is to find the method that helps you to design/improve it most effectively.

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Old 15th January 2008, 07:54 AM   #10
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Quote:
Originally posted by oshifis
An alternative (and most logical) method would be to use the actual amplifier as the load. Drive it with white noise into a dummy load and tap the PSU with an FFT analyzer. It needs some sort of calibration, though.

What would that give you?

Jan Didden
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