TeensyLCR – A Teensy based DIY LCR Meter

[WeTec]

The story
I have a couple of industrial grade PCBs lying around that are used for audio measurements and was wondering what I could do with them. The boards have audio ADCs and DACs with I2S interface and BNC connectors for in- and outputs. Just for curiosity i dig into the theory of LCR meters. Basic LCR meters using frequencies in the audio range. So why not building an LCR meter with audio ADCs? I kept digging, reading stuff and found this very interesting document:
TIDA-060029 LCR meter analog front end reference design https://www.ti.com/tool/TIDA-060029
This demonstrates a LCR meter analog frontend using an auto balancing impedance bridge. Just a couple of OP amps and analog switches for ranging. Exactly what’s on the boards I have. Well, not exactly but more on that later.
However, it seems absolutely doable and I wanted to give it a go. This was my goal so far:
    • build a working LCR meter with an I2S audio codec
    • using the auto balancing impedance bridge principle
    • using a Teensy as the micro to communicate with the audio codec
I chose the Teensy because there is lots of support for audio stuff.

The first attempt
I started with one of the boards with a CS4272 audio codec, a Teensy 4.1 on a breadboard and some wires between them. Just putting a sine wave out and used the two inputs for measuring voltage and current was not a problem but I was not satisfied with the results. This was mainly because of the missing transimpedance amplifier (a crucial part of the auto balancing bridge). Modifying the board  with some cuts and extra wires was not possible. This was the reason to design a new board from scratch to have something to play with.

The first prototype
I found existing designs that are using the same principle but most of them are using a single channel ADC and a relative complex phase detector. I wanted to use both inputs of the CS4272 to measure voltage and current at the same time and calculating the phase difference in firmware.
Great help was the TI reference design linked above and the documents for the Evaluation Board For CS4272 https://statics.cirrus.com/pubs/rdDatasheet/CDB4272-2.pdf.
I used EasyEDA for the prototype design. Schematics are attached.

As you can see, I used four BNC connectors for true Kelvin measurement. For the prototype I used the sample service from TI and Analog Devices to get the OP amps, analog switches, the LDO and switching regulators. I salvaged some other parts from the boards mentioned above. I placed a couple of pin headers to connect keypad and display later. There is an EEPROM to store calibration data and other settings. There is a I2C temperature sensor next to the codec to be able to compensate temperature drifts (not implemented yet).
After some excessive coding sessions with Arduino and the Teensy Audio Library I was able to measure voltage and current and did the phase calculation with help of the Audio Library. The phase calculation works exactly the same way as in the TI reference design document. No fancy FFT stuff, just complex math. And it works very well! It works surprisingly well!

The device
I decided to go further and put the board with a display and a custom keyboard into a 19” case that I have laying around. Now I have a fully functional LCR meter. A very useful device:

Well, the case is quit big for just the main PCB, power supply, display and keyboard. I don’t wanted to buy or build a new case and size was not a limiting factor. But hey, plenty of space for future extensions (DC bias?).
Two 3D printed parts where made for the display and the keyboard front panel. I’ve used the keys of an old pocket calculator from the junk box.

The specs
I did measurements with 0.1% resistors from 100 Ohm to 1 MOhm as reference. Results are within 0.5%. So I guess I got a basic accuracy of at least 1% (not for all impedance ranges of course). I think I can improve that further by reworking the calibration routine.

I set the display resolution to 5 digits, which means in fact 100,000 counts. This is quite much for an accuracy of 1%. But this helps me to find out the limits of the device.
Please note: The CS4272 is a 24 bit codec but the Teensy Audio Library uses 16 bit only.

There are four ranges: 100R, 1k, 10k, 100k. The range will be selected automatically according to the impedance of the DUT.

Selectable test frequencies: 100 Hz, 1 kHz, 10 kHz  (48 kHz sample rate).

Selectable test levels: 300 mV, 600 mV, 1 V

The following complex parameter of the DUT can be measured (fixed combinations of them):
    • Rs   // equivalent series resistance (ESR)
    • Rp   // parallel resistance
    • Cs   // series capacitance
    • Cp   // parallel capacitance
    • Ls   // series inductance
    • Lp   // parallel inductance
    • Phi  // phase angle of impedance
    • Xs   // series reactance
    • Z    // impedance
    • Q    // quality factor
    • D    // dissipation factor
    • G    // conductance
    • B    // susceptance

Moving average of the readings can be set from 1 to 256.

Edit: updated schematics

GitHub project page: https://github.com/wschuma/TeensyLCR

[moffy]

A nice lowish frequency VNA.

[WeTec]

I will post more details and measurement results soon. Please be patient

[WeTec]

I did some capacitor measurements yesterday. Here are the results.

Lets start with a 1.5pF ceramic cap:


Reading is 2.1pF. There is no open/short correction implemented yet.

10pF:


Reading is 10.8pF

100pF:


Reading is 102.1pF

Now some polystyrol film caps. 1.5nF 1%:


Reading is 1.5064nF.

Here is a 13.85nF 0.5% cap:


Reading is 13.943pF.

100nF 1%:


Reading is 100.21nF

1uF:


Reading is 1.0087uF

Ok, lets get to the big ones. Now with 100Hz test frequency and Cs-Rs mode. 1000uF:


Reading is 1.0199mF

5600uF:


Reading is 4.7295mF. Hmm, nearly 15% off. Did I found a bad one in my box?

4700uF:


Reading is 5.6286mF. What? A 5.6mF cap reads 4.7mF and vice versa?

[WeTec]

Lets check another one.


Reading is 4.9859mF. Ok. Well, it is not uncommon that this type of caps have a relativ high tolerance value, e.g. +20%/-10%.

Now some big power supply caps. 10,000uF:


Reading is 10.032mF.

The (physically) biggest cap I have. 22,000uF:


Reading is 23.027mF. Quite low ESR by the way.

35,000mF:


Reading is 30.629mF. The value is 13% lower than specified. The upper end of the capacitance range?

However, it seems to work quite well so far...

[jbb]

Nice one! I’ll look through those schematics with interest; I’m interested in building something like this (but given my interest in power electronics, over a frequency range more like 0.1 - 10 MHz).

A nice lowish frequency VNA.

I could be wrong, but I think VNAs are good around their characteristic impedance (eg 50 Ohm), but not so good as you go further away from that. Whereas LCR meters have those switched ranges (good for higher Z) and 4 wire connections (good for lower Z), but don’t go nearly as high in frequency

[moffy]

Nice one! I’ll look through those schematics with interest; I’m interested in building something like this (but given my interest in power electronics, over a frequency range more like 0.1 - 10 MHz).

A nice lowish frequency VNA.

I could be wrong, but I think VNAs are good around their characteristic impedance (eg 50 Ohm), but not so good as you go further away from that. Whereas LCR meters have those switched ranges (good for higher Z) and 4 wire connections (good for lower Z), but don’t go nearly as high in frequency

Very nice results, and jbb you make valid points similar but not the same.

[WeTec]

Thanks both of you.
At the moment I'm using 48 kHz sample rate. It would be possible to set the fs to 192 kHz and I could use 80-90 kHz as highest test frequency. Its not 100kHz standard but good enough to improve ESR measurements. Thats a feature on the ToDo list.

[jbb]

I had a look at the schematics and have a couple of questions:

• Did you consider keeping the V meas and I meas signals differential throughout? Right now you have instrumentation amps (diff - single ended) and then ADV drivers (single ended - diff)
• Do you use the extra gain from the V Programmable Gain Amplifier (PGA)? I imagine the V gain is useful for low impedance
• Do you use the extra gain from the I PGA?
• Did you have any trouble with transimpedance amplifier stability?
• Do you have any trouble with the MUX resistance in the 100 Ohm range?

[WeTec]

Hi jbb.

• To keep the signals differential all the way to the ADC, a fully differential PGA is required, which makes things more complicated. To keep the PGA simple, I chose this way.
• Both PGAs are set by the auto-ranging algorithm. The auto-ranging algorithm always tries to keep the voltage level at the ADC inputs as high as possible without clipping. And yes, the V (voltage) gain is usefull for low impedance and the I (current) gain is usefull for high impedance.
• Same as 2.
• I haven't had any trouble with transimpedance amplifier stability so far. I guess I need to test this further by pushing the device to its limits.
• The MUX on resistance is just added to the range resistors. The range resistors don't have to be super precise. All ranges are calibrated separately. No problems.

[WeTec]

New feature implemeted: I can now save screenshots as bmp file to a connected USB thump drive.



I've used the USB host interface from the Teensy 4.1 board.

[fonak]

Hello WeTec

Very nice impedance meter
I have a question about OPA2376/OPA376 chips, i.e. are they powered by a symmetrical voltage of +/-5V ?
Or maybe there is an error in the diagram and they are different chips.
I am asking about this because OPA2376/OPA376 have a maximum allowable voltage of 5V.

Regards

[WeTec]

Oops. That is definitely a mistake and must be fixed.
Thank you for reporting.

OPA192 / OPA2192 seems to be a good alternative.

[coromonadalix]

i would love to see this one pushed to 100khz measurements      good work   

will the code be shared ?

my curiosity  is the measurements coding ...

[fonak]

Hi

I am also in the process of building an LCR meter based on a similar topology,
but I am considering using OPA2810 (OPA2156) with BUF634A (or BUF634/LME49610) for TIA and the sine generator output buffer, this will increase the bandwidth and power.

Instead of multiplexers, I will test PhotoMOS AQY221N3V/AQY221R2V or similar.

First, I will use the internal ADC/DAC of STM32G474 or STM32H723 processors, if this does not bring good results,
I will test DDS AD9833 as a DAC and ADC converters like AD4001/AD4003.

Regards

[WeTec]

will the code be shared ?

my curiosity  is the measurements coding ...

Definitely. But I need to cleanup the code before going public. 

[WeTec]

I am also in the process of building an LCR meter based on a similar topology,
but I am considering using OPA2810 (OPA2156) with BUF634A (or BUF634/LME49610) for TIA and the sine generator output buffer, this will increase the bandwidth and power.

Sounds interesting. An additional output buffer helps with very low impedances. What is the lowest impedance range in your design?

Instead of multiplexers, I will test PhotoMOS AQY221N3V/AQY221R2V or similar.

10 times lower on resistance than the multiplexer but expencive. 4-8$ per range.

[fonak]

Hello WeTec

My target lower impedance range is around 10 Ohm (1Vpp/100mApp @10 Ohm DUT)

As for PhotoMosfets, you can buy them cheaper on a well-known Chinese portal, i.e. about $4 for 10 pieces.

PS. I tested two types of Photomosfet for an real switch Rdc @ Idc 200mA, and 10mA Led current:

1. Vishay: VO14642AABTR Rdc =102,1 mOhm
2. Cosmo: KAQY212S Rdc = 257,5 mOhm - (Cosmo KAQY212S is a clone Panasonic AQY212S)

Regards

[WeTec]

At the moment (version R1), the input protection (PTC1-4 + D1-4) is basically useless against charged caps. A capacitor charged to 8V can easily raise one of the power supply rail over 6V. This could be deadly for at least the codec and the differential amplifier.
I don't think it is possible to fix that in the current prototype by simply adding Zener or TVS diodes across the rails.

A possible solution for a next version could be this:


R66 & R78 are used to biasing the diodes. The current of a charged capacitor flows thrugh PTC1, D1 and one of the Zener diodes to GND. The Zener diodes limit the voltage before R80 to about 6-7 volts and R80 finally protects U1 from this.

[WeTec]

Progress has been made. I can now enter the test frequency manually in 100Hz steps. Furthermore I've increased the sample rate to 192kHz. This allows me to set test frequencies up to 90kHz.

[jbb]

Nice!

I’m impressed you could get to 90 kHz with 192 kS/s. Do you still have a good signal at that frequency? I would have thought that the analog parts of the Cirrus chip would start to attenuate the signal when you go much over 20 kHz…

[WeTec]

The CS4272 has attenuation at higher frequencies, depending on sample rate. At 90kHz signal it is about 6dB (192kHz fs). This is still ok, because both channels have the same attenuation (0.1dB interchannel gain missmatch). So voltage and current measurement have the same attenuation. This is canceled out at the impedance calculation.

A much bigger problem is the gain bandwith product (GBW) of the PGA opamp. Here we need a GBW of 9 MHz to get the voltage/current signals unattenuated. Actually we need more than that. Typically, the GBW attenuation is specified as -3 dB in the data sheets. So with a GBW of about 50MHz we are save.

The PGA can have different gain settings for voltage and current which leads to different gain errors at higher frequencies. Therefore we have an impedance error depending on frequency.

There are two ways to fix this:
1. Selecting an opamp with higher GBW or
2. calibrating the PGA gain at every frequency that is used.

At the moment I try to find another opamp with good GBW.

[WeTec]

I need to replace all of the opamps anyway because of the low supply voltage of the OPAx376. The GBW is 5.5 MHz. Too low for the PGA.

I will use OPA192/OPA2192 instead, except for the PGA opamps. LT6221 is a candidat.

[jbb]

I've had the PGAs on my mind.  I was sure that there's a way to keep the measurement signals as differential...

After some thought, I remembered that you can use a MUX to switch between gain setting resistors inside the instrumentation amp (see below).  This cuts out two opamps from each signal chain.  Does that look helpful?

[WeTec]

Definitively worth a try. If I would do it again, I would change a few things in the design.

[WeTec]

And thank you for sharing the simulation.

At the moment I do not plan to make a redesign. The current design works well for frequencies up to 20kHz. I'll try to get more reliable results at 90 kHz with replaced opamps.
If it works, great. More than I expected. If not, then it is still a useful device. And I've learned a lot. 

[jbb]

It looks like a really nice addition to the bench. It would be great to have the design files and software - even if they aren’t perfect :-).

I’m certainly tempted to make one of my very own. Considering a couple of mods:
- use a CODEC chip with built-in headphone amplifier (saves 2 opamps) and PGAs (saves 2 opamps and 2 muxes)
- replace the Teensy 4.1 with I.MX RT1024 or similar chip (smaller version of the Teensy 4.x microcontroller)

Edit: you'll laugh... I went through an exercise of trying assorted MUXes to find a good capacitance vs on resistance trade; I eventually found the MAX4052 was about the best fit

[WeTec]

Yeah, the classics are still good enough for this kind of stuff.

I will put everything on Github. Need to setup a Github account first...

[WeTec]

Here are some insights about the phase calculation:

Teensy Audio System Design


A sine wave is send to audioOutI2S1 to get the sine wave signal source.
audioInI2S1 provides the I2S data from the CS4272 codec. Channel 1 is the measured voltage and channel 2 is the measured current.
Additional, there are two AudioSynthWaveform objects that create a unity magnitude square wave with 0° and 90° phase. Both are multiplied by the voltage and current signals respectively.

The phase is calculated by

Code: [Select]

phase = atan(mean2.read() / mean1.read()) - atan(mean4.read() / mean3.read());

// cap phase to +-90°
if (-phase > PI / 2) {
  phase = phase + PI;
}
if (phase > PI / 2) {
  phase = phase - PI;
}

The impedance is calulated by

Code: [Select]

impedance = analyzeRmsV.read() / analyzeRmsI.read();
The code lines are simplified. The actual code is a bit more complex due to analog level calibration and moving average calculation.

Basically the same math explained in the TIDA-060029.

[jbb]

That makes sense. I can see how using the Teensy audio system saves you a great deal of messing around in code.

Might I suggest that instead of atan(mean2.read() / mean1.read()) you try using atan2(mean2.read(), mean1.read()) ? It won’t blow up if mean1.read() happens to be (very close to) zero.

[WeTec]

Thanks jbb for the hint! This helped me to fix a bug where I get the correct phase but with wrong sign. Now the phase calculation looks like this:

Code: [Select]

phase = atan2(mean2.read(), mean1.read()) - atan2(mean4.read(), mean3.read());

// cap phase to +-90°
if (-phase > PI / 2) {
  phase = -PI - phase;
}
if (phase > PI / 2) {
  phase = PI - phase;
}
Further information: https://stackoverflow.com/questions/283406/what-is-the-difference-between-atan-and-atan2-in-c

[jbb]

I’m glad that helped! atan2 is a helpful one.

On further looking, I see you’re generating the excitation signal with ‘sine1’ and the mixer signals with ‘squarewave’ and ‘squarewave_90’… I wonder if this arrangement will be sensitive to 3rd, 5th harmonic tones in the measured signals? I would have expected the scheme to use ‘sine1’ and a new ‘sine_90’ signal.

Edit: you can also consider the mixer results (once low pass filtered) to be:
V1 = mean1 + j mean2
V2 = mean3 + j mean4

Thus:
mag(V1) = sqrt(mean1**2 + mean2**2)
mag(V2) = sqrt(mean3**2 + mean4**2)
Which may be a little less sensitive to noise than the RMS.

[WeTec]

I've replaced all opamps now:

• U1, U4, U12 - OPA192
• U5, U6 - OPA2192
• U7, U8 - AD8032

I do not have any parts with known values at 90 kHz, so I cannot say much about accuracy. I also have no access to a real LCR meter so I cannot do measurements to compare. The only thing I can do is to measure 0.1 % resistors that I have in stock. The results are pretty good I think. About 2-3% of at 90kHz.

Here is a ESR measurement with a Würth capacitor WCAP-PT5H Aluminium-Polymer 180 µF:

Capacitor connected:


inputs shorted:


9 mOhm @ 90 kHz.
Pretty much close to the maximum value of 10 mOhm @ 100 kHz value from the datasheet.

(Ignore the Cs values. They are typically far of at 90 kHz.)

[WeTec]

The source code is public now! Please check the github project page: https://github.com/wschuma/TeensyLCR

[KE5FX]

Nice job.  The ability to see multiple parameters on the display at once is attractive next to my TongHui LCR meter, which requires a lot of button-pushing in everyday use. 

[WeTec]

If someone (including me) wants to do a redesign, I would recommend to do the following changes to the PCB:

Major improvements:

• Improve input protection
• Replace the single ended PGAs by differential ones

Minor improvements:

• Add button cell holder (for RTC)
• Add I2C test points (would be usefull for debugging)
• Add GND test points (makes it easier to connect your oscilloscope probes)

For those who want to rebuild the current design (R1) I have uploaded the gerber files to GitHub.