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From: "Michael R. Kesti"
Subject: Re: Output Impedence
Date: Fri, 25 Oct 2002 10:18:40 -0700
Organization: MK Associates
X-Mailer: Mozilla 4.7 [en] (Win98; I)
>How would I go about getting the output impedence of a computer sound card.
1) Find the manufacturer's specification.
2) Make an indirect measurement. The sound card's output may be modeled
| Vo |
Vo is the ouput's voltage source and Ro its output impedance. We can
determine the value of Vo by measuring the open circuit voltage between
the output and ground. If we load the output with a resisitor, the
| Vo | \ Rl
The voltage (Vl) across the load resistor (Rl) is determined by the voltage
Vl = Vo * -------
Ro + Rl
Because we can measure Vo, Vl, and Rl, we can solve for Ro:
Rl * (Vo - Vl)
Ro = --------------
To proceed, first set up the card to output a continuous sinusoid and
measure the open circuit output voltage (Vo). Then load the output with
about 1000 ohms (Rl) and again measure the output voltage (Vl). Maximum
accuracy will occur when Vl is about half of Vo. If Vl and Vo are almost
equal, then the value of Rl is too high and you should try again with a
smaller value. Conversely, if Vl is small compared to Vo, try again
using a larger value for Rl. Doubling or halving the value will quickly
get you to an appropriate value.
Another approach is to use a potentiometer for Rl and adjust it until
Vl is exactly half of Vo. Rl and Ro are equal under this condition
and May be determined by measuring the resistance across the potentiometer.
Michael Kesti | "And like, one and one don't make
| two, one and one make one."
firstname.lastname@example.org | - The Who, Bargain
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