- Blog entry posted in 'Uncategorised', July 23, 2009.
it would be nice to be able to measure impedance of an AC circuit as easily as measuring DC resistance, wouldn't it? there are many times in the audio business, when i would like to find out what the impedance of something is at a particular frequency, and i usually have to resort to some indirect method. an impedance bridge is a piece of test equipment that measures impedance, but such a beast is expensive, and is an indirect method, and a bit more complicated than putting a pair of test leads across a device and reading a number. i would like to be able to put a pair of test leads across a device and know it's impedance as a direct measurement. of course there would be one added step, dialing in the frequency i want to measure at, but i can live with that one extra step. so how to go about it?
a DC DMM ohmmeter circuit is a simple device. you have a digital voltmeter across a pair of probes, and a DC current source set to a known current. if the current were for instance 1mA, then the voltmeter would directly read 1V/kΩ, so a reading of 0.6V would be a measurement of 600Ω. simple.... i like simple, so how to do it for AC? could it be that simple? i think so. i have a function generator with a sine wave output, and a true RMS voltmeter, so how do i get a known AC current, when my FG has a voltage output? a series resistor introduces error, especially once the impedance being measured is more than a few percent of the resistor's value, so a series resistor is not the answer here.
it pays to browse the Application Notes of IC manufacturers like Analog Devices, TI, and National Semiconductor. i've learned a huge amount over the years from app notes, more in fact than one could learn in school. if you have a creative mind, you can see something almost insignificant in an app note and solve a huge problem with it (like using LEDs as opto sensors. somebody saw a little blurb about it in one of TI's app notes back in the 70's, did some experiments with it, and now it's common practice). so, while thinking about how to make an AC current source, i remembered seeing a "bipolar" current source. of course, this current source was shown with a DC input, because nobody in their right mind would ever need an AC "constant current" source (AC constant current source IS a bit of an oxymoron, isn't it?). so i did a search and found a whole app note by NS, about the Howland current source. it turned out to be EXACTLY what i was looking for, but from the text, not hardly a mention of using it for AC. so i loaded up LTSpice, drew the schematic and ran the sim using an AC source instead of DC. perfect, with 1V AC going in and a 1kΩ scaling resistor i got 1mA out, no matter what the load resistance was. stage 2 was actually building it expecting some unexpected error that would disqualify the circuit as unusable. no such error ocurred, it worked exactly like the sim (which if you have any experience building what you have run on a sim, is the exception rather than the rule). so now i have a working current source, and the scale factor is easy to set. with 1Vrms in, and a 1k scaling resistor i get 1mA out. with a 100Ω resistor i get 10mA out. i can literally take a DVM and my device, dial in a frequency, and do a direct measurement of impedance. so, shown below is an excerpt from NatSemi's AN-1515, and the circuit i used for my current source. i used a TI TL2072 op amp instead, and replaced the input voltage source with my function generator (the -2072 is a vastly improved -072).3885738858
NJOE, February 04, 2010
Hello, Any chance of getting the circuit I cannot see it? I am new here perhaps I am missing something. Cheers
unclejed613, February 04, 2010
[QUOTE=NJOE;bt211]Hello, Any chance of getting the circuit I cannot see it? I am new here perhaps I am missing something. Cheers[/QUOTE] yeah no problem, i don't know what happened to the pictures
antknee, February 08, 2010
Hi, That would be handy. I gather that the expensive impedance analyser measures frequency and uses that to calculate impedance. So you want to have your device to have a user input of frequency and then calc impedance. I can see this would be cheaper and easier. I'm not sure how your device would go about doing the calcs for multiple components tho. Say you wanted to measure across a whole board, wouldn't your device need to do dozens of complex number calcs? It occurs to me that different parts of the circuit might be at different frequencies, (I'm not sure if that happens though). Regards, Antknee
unclejed613, February 08, 2010
no calculations needed at all, except multiplying a resultant voltage by the scaling factor of Rset. it really is that simple. the "math" is handled by the components themselves. take for instance a series resonant circuit. with Rset at 1 ohm, the scaling factor is 1volt per ohm (with 1Vrms applied, the current is 1Arms). as an example, the cap will be 10uf and the coil 2.5mH (resonant at 1khz). so we apply 1Arms at 1khz to the series resonant circuit, and get a resulting voltage across the terminals of 0.2V. the total impedance of the series resonant circuit is 0.2 ohms. if you then measure across either the cap or the coil (with the output of the current source still applied across the resonant circuit) you will find that there's about 16Vrms across the coil or the cap. that's because each of the components has a reactance of 16 ohms at 1khz. so where does the other 15.8V go to? because the phase angles of the individual components are 180 degrees out of phase, there is a cancellation effect, and the voltage across the terminals of the resonant circuit indicates the actual impedance of 0.2 ohms.
antknee, February 08, 2010
So what is the catch? There is always a catch! You will know what it is I'm sure, is the market for these devices small? Or perhaps there is already one on the market? If the market for impedance analysers is non competitive this may explain a thing or two. I've always wondered why there isn't a handheld oscilloscope that doesn't require a mortgage, I'm not sure how expensive industrial impedance analysers are but probably not cheap!
schmitt trigger, February 24, 2010
I agree with you 100%, the App Notes of the major Semi vendors of yesterday (National, Motorola, RCA, TI, General Electric, just to name a few) were a motherlode of valuable electronics information. I also learned a lot by reading them Now to your circuit. An "AC ohmeter" requires to display both the scalar value of the impedance and its angle. Otherwise, it won't give you the whole picture. For instance, a 100 ohm resistor, a 15.9mH inductor, and a 1.59uF cap will all show 100 ohms at 1 Khz. But if all you want is a "magnitude" value, your circuit looks fine
unclejed613, April 14, 2010
[QUOTE=schmitt trigger;bt221]I agree with you 100%, the App Notes of the major Semi vendors of yesterday (National, Motorola, RCA, TI, General Electric, just to name a few) were a motherlode of valuable electronics information. I also learned a lot by reading them Now to your circuit. An "AC ohmeter" requires to display both the scalar value of the impedance and its angle. Otherwise, it won't give you the whole picture. For instance, a 100 ohm resistor, a 15.9mH inductor, and a 1.59uF cap will all show 100 ohms at 1 Khz. But if all you want is a "magnitude" value, your circuit looks fine[/QUOTE] sorry for the delay, but i need to find a better way of knowing when somebody has posted a message here... my original intent was to use an oscilloscope as the measuring instrument (and for the most part, still is), but a DMM to get a magnitude measurement would often be handy. the advantage of the oscope would be that , yes, i could see the phase angle as well as the magnitude (by triggering the scope from the signal source). the original purpose was to measure the output impedance of an amplifier directly, and for many amplifiers, this would require the use of the scope, since multimeters don't normally read in tenths of a millivolt, but a scope is usually much more capable of such a measurement. many amplifiers (even some poor quality amps) have output impedances in the tens and hundreds of microohms due to the action of negative feedback. it is this particular behavior of negative feedback that i am interested in. if i can directly read output impedance, i can experiment with various methods of altering the output impedance of an amplifier. it is the output impedance that determines how an amplifier and speaker interact, and is a possible reason why two amplifiers with otherwise identical specs (distortion, noise, frequency response, etc...) may sound different with the same speakers. in the same vein, the impedance analyzer would also allow me to get an accurate impedance vs frequency curve of the speakers in question. as for measuring the phase angle across a single capacitor or inductor (assuming the ESR of the cap, or the series resistance of the inductor are less than a couple percent of the impedance being measured) i already know the phase angle.... 90 degrees. with appreciable series resistance or a complex network (or a speaker with a crossover), the phase angle will be much different, and the scope would of course be the best tool in that case
deweyusa, April 26, 2010
There's a pretty cool tool I found called the VNWA Network Analyzer that injects an AC sweep signal and measures the angle of its reflection...I THINK that's what you're describing. It can be found [URL="http://groups.yahoo.com/group/VNWA/"]here[/URL] There's also a similar unit made by N2PK. It's pretty impessive, and very reasonably priced compared to commercial units, and with stunning accuracy, sometimes beating high-end HP VNAs. Check it out [URL="http://n2pk.com/#TP1"]here[/URL].
unclejed613, April 26, 2010
those are RF analyzers, and are also DSP based. there are DSP based audio analyzers that measure impedance, although i don't think they're designed to measure much below 1/10 ohm. they're also many thousands of dollars and can be found here: [url=http://ap.com/products]AP High Performance Audio Analyzer & Audio Test Instruments : Products[/url] what i'm describing here is not RF, nor DSP based (it could be easily done with DSP, and i still might), nor expensive. it's a simple inexpensive tool that can read impedance and reactance in the audio band, and can read it in the milliohm range (which was my original goal).
