# Sound Card LCR Meter

• posted
[SHAMELESS PLUG] Possibly relevant to "On old electrolytic caps" thread, but also for general interest, I've just released Daqarta v7.60 which includes a macro "mini-app" to use your sound card as an LCR meter. (Many thanks to Robert A. Macy for inspiration and advice on this!)

The basic idea is that the sound card outputs a sine wave (about 1 kHz default) that drives a voltage divider made of a known reference resistor (say, 1k) in series with the DUT. The sound card stereo inputs monitor the voltage at the top of the resistor, and the top of the DUT. The (complex) voltage across the resistor allows the determination of the current, which also flows through the DUT, so the complex impedance of the DUT can be found via Ohm's law.

To calibrate, you only need to know the value of the resistor (read if from a good DMM), then there are 3 buttons you click on one by one:

1) Resistor shorted, no DUT

2) Resistor present, no DUT

3) DUT shorted.

That allows the macro to determine the sound card input impedance, which is otherwise in parallel with the DUT, and compensate for that in subsequent operation. With a 1k reference, and a typical sound card impedance of 15k, this allows reading DUT resistance above 1 Meg.

The big (resizeable) meter shows R (ESR) and X by default, labelled with proper units (Ohms, K, M, pF, nF, uF, uH, mH, H). You can change one line in the macro to include DF (or Q for inductors) as well. One button saves a reading to a log file; another saves notes/comments.

Regarding the original question about typical ESR values, I was puzzled for a long time about the fact that ESR was always inversely proportional to C, such that a 47 pF cap might show an ESR over 500k.

Turns out that the basic impedance measurement doesn't distinguish between series and parallel connections of the real and complex impedance components, so you have to help it out by toggling a "Series" button to "Parallel" based on your judgement. Typically, this turns out to be pretty simple: Use the default Series for electrolytics, and use Parallel for 1 uF and below. That 47 pF cap reading then shows a parallell (leakage) resistance well over a Megohm.

Full write-up from the Help system is at , including a screen shot, photo of a simple test fixture, and circuit diagrams.

Best regards,

Bob Masta DAQARTA v7.60 Data AcQuisition And Real-Time Analysis

Scope, Spectrum, Spectrogram, Sound Level Meter Frequency Counter, Pitch Track, Pitch-to-MIDI FREE Signal Generator, DaqMusiq generator Science with your sound card!

• posted

Bob, congratulations on this one. ...thanks for mention.

• posted

A 47 pF ceramic cap will typically have a shunt resistance in the gohms and a series resistance well under 1 ohm. Characterization of either will be about impossible at 1 KHz, where Xc is over 3 megohms.

I use a TDR oscilloscope, or a fast pulse generator and a scope, to measure the ESR of stuff like this.

Here's a 50 ohm square wave generator dumped into a polymer aluminum cap.

The superfast spike is ESL, and the roughly 1/2 div step is ESR. The gross slopes are C. Maybe a sound card could do something like that, square pulse mode, and even display the waveform across the DUT. What's the usable bandwidth on a sound card, in and out?

Even at low bandwidth, you might interpolate the line slopes and compute the step size.

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John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    ```
• posted

Thanks for that, but where does one get this kind of info? I could find nothing on the Web regarding R values of low-value caps, either series or shunt. (And I've learned from personal experience that "there ain't no such thing as a resistance over 100 Meg" with real-world parts, in real-world circuits, on real-world boards... or at least, not for long, despite alcohol rinse and teflon stand-offs, etc! )

The highest sound card sample rate I've heard of is 192 kHz, which would imply a bandwidth under 96 kHz. The only one I've actually tested at 192 kHz was in a laptop (not a place where I expect high performance), and the upper cutoff was closer to 48 kHz.

Yes, I think if big-cap ESR is the main interest, then a pulse-based approach is the way to go. I was attracted to Macy's slick network analyzer approach because it gives wide-range LCR measurements, even on el-cheapo systems. Seemed like it might have wider appeal, especially for beginners. (I would have loved to have had a decent LCR meter back when I was starting out, though lack of same did lead to some educational experiments with AC bridges and headphone null-detectors!)

The pulse-based scheme is still on the "To Do" list, however.

Best regards,

Bob Masta DAQARTA v7.60 Data AcQuisition And Real-Time Analysis

Scope, Spectrum, Spectrogram, Sound Level Meter Frequency Counter, Pitch Track, Pitch-to-MIDI FREE Signal Generator, DaqMusiq generator Science with your sound card!

• posted

The 192kS/s rate works well out to 89kHz.

I ALWAYS recommend steady state vs pulsed. Because in steady state all the components have arrived at their quiescent operating points. But in a pulsed system, each pulse, well you get the drift. And, you'll be limited by any inherent voltage vs characteristics in your components. Also, pulsed is more of an 'amplitude' measurement, not a phase, or frequency measurement and the dynamic range will ALWAYS be less.

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I have some sample 0805 1 Tohm resistors.

Here's a little PCB, deliberately cruddy with rosin flux and fingerprints, pinning my meter on the 1e14 ohm range:

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