Problem in measuring impedance at lower frequency from 10 Hz to 1k Hz using AD5933

Quite an elaborate AFE you got there... The suspect would be the circuit R8R9R10C3 that acts as a low-pass filter with a time constant (R8||R9||R10)*C3 = 1.41630901288kOhm*1μF = 1.4 ms, so operating frequencies lower than a few KHz start noticeably leaking through. The suggestion would be to increase the resistors R8, R9, R10 (and, of course, R13 to keep R13/R10 ratio the same) or C3 or all the above. And prepare for long sweep duration.

If your current number of settling cycles works for your measurements at high MCLK, there should be not need to change it at lower MCLK.

Thank you for your reply, I got this circuit from chabowski paper [1],  In this paper page num. 16 it is discussed that (ICA and ICB are for DC bias canceller. the voltage detectors are averaged on R8 and R9 and its correspond to DC bias voltage. 

I think it's not an AFE problem because now I am trying to measure lower frequencies without AFE. I have measured 20kohm resistor with RFB 10kohm, it gives correct measurement from frequency sweep (5k-100k) by using 16Mhz external clock (settling cycle 0x64) and from 1khz to 5khz by using the 4Mhz external clock (settling cycle=0x0F), but when I am connecting 2Mhz external clock it acts as I have shown in video (running infinitely). but if I change the settling cycle from 0x0x0F to 0x05 it gives the result that I have shown in the image (not correct), I have measured 20kohm with 10kohm RFB. 

1: SIMPLE WIDE FREQUENCY RANGE IMPEDANCE METER BASED ON AD5933 INTEGRATED CIRCUIT

     Konrad Chabowski, Tomasz Piasecki, Andrzej Dzierka, Karol Nitsch

     Metrology & Measurement System, Vol. XXII (2015), No. 1, pp. 13–24.

Hi Snorlax , I was using 10kohm pull up resistor initially as I have multiple slxvx (sorry I need to censor the word due to AD "Word Matters") devices on the bus. I am not sure how to calculate the correct pull up resistance when there are multiple slxvx devices on the bus, so I have chosen 10kohm as the safest option. Really thanks a lot for pointing out this problem.

I have changed the pull up resistance back to 2.2kohm and added the 2.5s settling time before frequency sweep (for both calibration and actual measurement). I have also ensured that the modified setStartFrequency() is called correctly with respect to the MCLK used. I can see some improvements in the results as shown below. The frequency sweep from 5kHz to 10kHz is very good, but the gap starts appearing again for 400Hz-5kHz sweep. Besides, I managed to set the settling cycles to 15 (tried with 30, 60 and 255 as well, the results are very consistent) for all my partitions and it did not cause any system hang at all, even when I was scanning down to 10Hz. However, the gap is still very big, and there is no continuation for freq <400Hz. Was wondering if this could be the problem of the breadboard or there is anything I can do to reduce the error?

Besides, please see the attachment below for the newly collected data.

XLSX

I will proceed to separate all the initialisation and sendByte functions, and also use round() for my increment code. Will update soon. Please let me know if there is anything I can do to improve my system.

Really thanks a lot for your help.

DesmondTeo said:

I am not sure how to calculate the correct pull up resistance

Nobody seems to know, the rule of thumb is to start with 10k or 4.7k and go lower if the bus is getting longer and number of devices increases. For some reason your microcontroller seems to like 2.2k installed on the Eval board.
You may ignore the attempts to censor your referring to "slave" devices on the bus - there should be an option to disregard the warning by pushing "Post" button, do you get this "Post" button in the popup dialog?  

DesmondTeo said:

but the gap starts appearing again

To debug this discontinuity issue, can you just calibrate with the lowest Rfb (230 Ω) while sweeping all the way down to 10 Hz while changing MCLK only between to your partitions, but not the Rfb and Rcal? And then sweep the same way with Rcal or some known resistor R > Rcal and see if there still going to be any discontinuities? And then do the same with your R||C network? And post the Exel here? This should help us to see if the discontinuities are caused by switching between Rfb-s and Rcal-s and not by the software calculating Frequency Codes for different MCLKs incorrectly?  (BTW round() could be used in all Code calculations, not just the increment, although not that important)

DesmondTeo said:

XLSX

These are calibration data - all seems to be in order.
The cut-off frequency with C of 0.1 uF is about 64 Hz, which somewhat reduces the effective excitation voltage when you sweep below 100 Hz making measurements slightly suboptimal, but this does not explain the discontinuities and calibration should still lead to correct impedance values.

Another thing is the accuracy of components you are using for calibration and R||C network: the capacitors are known to deviate from their nominal values by up to 10% or more. What are the accuracy of the resistors you are using? It would make sense to try getting some high-precision resistors with 0.1% or better tolerance (avoid the wirewound ones).

DesmondTeo said:

I managed to set the settling cycles to 15

It should be safe to try setting this number at 1 to reduce the duration of your sweep, especially at lower end of your spectrum.

Hi Snorlax ,

Snorlax said:

To debug this discontinuity issue, can you just calibrate with the lowest Rfb (230 Ω) while sweeping all the way down to 10 Hz while changing MCLK only between to your partitions, but not the Rfb and Rcal? And then sweep the same way with Rcal or some known resistor R > Rcal and see if there still going to be any discontinuities?

I have tried with 150ohm, 240ohm and 1kohm||0.1uF from 10Hz to 10kHz with Rfb=230ohm and Rcal=150ohm. I have collected the results twice for each target. Please see the attached excel file for the collected results. For 150ohm, everything seems fine. However, for 240ohm, the error is quite big and the discontinuities are very obvious. For 1kohm||0.1uF, the results are very off for frequency < 600Hz.

XLSX

However, I am not quite sure whether I had performed the frequency sweep correctly as you expected. I basically just combined the partitions together using a for loop so that the data for each partition were collected automatically (previously I would collected the data for each partition first and then combined them together in excel manually). In the beginning of each partition, I have configured the start frequency, num of increments and freq of increment, and also provided a 2.5 settling time. May I know if I have misunderstood what you meant regarding the full sweep? 

Or you would actually want me to perform the frequency sweep with only one start frequency and then adjusted the MCLK only when the frequency reaches the threshold (e.g while sweeping from 400Hz to 10kHz, MCLK is changed from 1MHz to 16.776MHz when frequency reaches 5kHz)?

Snorlax said:

The cut-off frequency with C of 0.1 uF is about 64 Hz, which somewhat reduces the effective excitation voltage when you sweep below 100 Hz making measurements slightly suboptimal, but this does not explain the discontinuities and calibration should still lead to correct impedance values.

I am so sorry. It's a typo in the attached results (Results-20240127). I used 10uF instead of 0.1uF, so the cutoff is actually 0.64Hz. Really sorry about it.

Snorlax said:

Another thing is the accuracy of components you are using for calibration and R||C network: the capacitors are known to deviate from their nominal values by up to 10% or more. What are the accuracy of the resistors you are using? It would make sense to try getting some high-precision resistors with 0.1% or better tolerance (avoid the wirewound ones).

 Noted on this. I have ordered the 0.1% tolerance resistors. Will try it out when I receive them. 

Snorlax said:

It should be safe to try setting this number at 1 to reduce the duration of your sweep, especially at lower end of your spectrum.

 I am not sure why but the results were inconsistent when I set it to 1. When the number of settling cycles is 15, the results are very consistent.

Do you think I should try to calibrate the system manually for each partition (as the results I collected before seem quite consistent with the Rfb and Rcal I chose for each partition) and combine them together in a CSV file? I  will then perform a full frequency sweep on my RC based on the calibrated data from CSV file and switch between different values of Rfb using a MUX as proposed in the schematic before. Or Do you think I should continue to debug the system?

Thank you so much for your help and advice.

DesmondTeo said:

XLSX

Thanks for posting, very helpful and you did it great. Your system seems to function the way it is supposed to, except with the Rfb = 230 and Rcal = 150 there may be a little bit of clipping present and that likely throws the Z = 240 measurements (when there is no clipping) a bit off - down to 231-232 or so. The suspect is the section in your datasheet, where you estimate Rfb based on Zmin = 150:

The formula we discussed earlier calls for Rfb to be less then or equal to the value from the formula, which you calculated at 219.6969697, so it would be safer to pick Rfb of 210 or even 200 to be sure that there is no clipping, the 230 is too risky. It would make sense to repeat this same experiment with lower Rfb and see if the Z = 240 results improve. Something else still could be an issue, later we will probably need to look at the code.

DesmondTeo said:

I used 10uF instead of 0.1uF

DesmondTeo said:

When the number of settling cycles is 15, the results are very consistent.

 Let's keep it this way for now, might need to revisit it once basic functionality is in place. It should not affect the measurements at all, could be something in the code.

Hi Snorlax , I have completed the full measurement (automated each partition with respect to their Rcal and Rfb). First, I collected the calibration data (phases and gain factors with respect to each frequency) based on each partition, and then I stored them as arrays in the code and perform the frequency sweep. Below are the averaged measurements with respect to the partitions and different number of settling cycles
 

Please see the attached below for more results. I have corrected the Rcal value as well. However, I still have no clue why the errors are still so high. Could you give me any suggestion to debug please? My 0.1% tolerance resistors are arriving soon as well. Will try with them and update soon.
XLSX
Btw, as Rfb is chosen based on Zmin to ensure that the ADC won't saturate and Rcal needs to be > Zmin, may I know if there is anyway to calculate the maximum impedance that a certain combination of Rfb and Rcal can measure accurately with respect to specific sweep frequency? Theoretically, the maximum measurable impedance should be infinite right?

Thank you.

DesmondTeo said:

why the errors are still so high

If the software calculates all frequency codes correctly, you are likely running into the DFT systematic errors. To reduce these errors is it necessary ro reduce MCLK below what you are using for your partitions:
10000 - 5000 Hz  MCLK of 16 MHz is barely OK, lowering it down to 8 MHz would be better
5000 - 400 Hz       MCLK at 640 KHz
400 - 200 Hz         MCLK at 320 KHz
200 - 100 Hz         MCLK at 160 KHz
100 - 10 Hz           MCLK at 16 KHz

DesmondTeo said:

I collected the calibration data (phases and gain factors with respect to each frequency) based on each partition, and then I stored them as arrays in the code

Could you please put these calibration data arrays along with the Re(Reg) and Im(Reg) from which you calculated gain and system phase and also the resistor values used as Rfb and Rcal for each and partition(s) in to Excel and post it along with the measurements data?  

DesmondTeo said:

Theoretically, the maximum measurable impedance should be infinite right?

The maximum theoretical measurable impedance is always infinity as is the case when no Z is connected to the circuit. In practice, if nothing is connected to the circuit, the AD5933 output registers show some fluctuating small values resulting from everpresent electronic noise and interference (typically ±5-10 counts), which, with the calibrated gain, can be formally calculated into a noisy "Zmax" (except on occasions when both registers happen to contain exactly 0), but it is is not all that meaningful.

hi aitzaz I have the same problem in measuring impedance at lower frequency using ad5933and I would be grateful if you could share with me the Arduino program and your circuit that you used 

thank you 

Last post by the participant you are addressing was about 5 years ago - many folks tend to move on within this time frame. Perhaps if you could post more details on the problem you are experiencing, somebody still active in this forum could try help you resolve it.
Best of luck with your project!

Hi! Is there anyone active in this forum ? Dealing AD5933 to get low frequency impedance values and facing some problems. 

Post some info regarding what the problems specifically are and some information on your setup causing you these problems. Somebody will likely try helping you.

Post some info regarding what the problems specifically are and some information on your setup causing you these problems. Somebody will likely try helping you.