Frequency calibration problem of AD 5933 impedance measurement board

Dear Sir,

I hope this message finds you well.
I am suing AD 5933 evaluation board for impedance measurement, The data sheet says it can measure the impedance up to 10M ohm. I am currently measuring the impedance of a microfluidic device, which is within the range of 10M ohms. I have encountered an issue where, for instance, if the impedance measures 8M ohms and I use a 3M ohm resistor for feedback during calibration, I do not obtain the correct impedance value. However, using an 8M ohm resistor for calibration provides an accurate impedance measurement.

I have a few questions:

  1. Is it possible to use a single resistor value for feedback calibration to measure impedance ranging from 1 ohm to 10M ohms, thereby overcoming the challenge of frequent calibration? If so, could you please suggest some solutions to address the issue of frequent calibration?
  2. I recently read a paper where the author utilized an HMC 245A switch to address the issue of frequent calibration in impedance measurement. Is this method effective? The HMC 245A switch is described as a non-reflective SP3T (Single Pole, Triple Throw) RF switch.
  3. Could you recommend a method, switch, or IC that would allow me to measure the impedance of my device from 1 ohm to 10M ohms without needing to change the feedback resistor repeatedly?

Your assistance in this matter would be greatly appreciated.

Hi Kamran,

Welcome to the DigiKey tech forum.

I’m not too familiar with this. I’m asking to see if anyone else can help assist you with this.

Hello @Kamran,

The AD5933YRSZ is a fascinating piece of technology. For the signal source, it combines a Direct Digital Synthesis (DDS) sinusoid generator, DAC, and a variable gain amplifier. For the receiver it features an amplifier (gain set by resistors), a low pass filter, and an ADC. Together these devices perform a frequency sweep of the Device Under Test (DUT) returning complex (phase magnitude) data.

As I understand your situation, you are operating the evaluation board at the extreme limits with measurements in the 10 MΩ range.

Thoughts:

  1. Any given measurement involves frequency sweep as well as gain parameters. These may be different for 8 MΩ when compared to 3 MΩ. Would it be possible to manually adjust a parameter such as gain to obtain a compromised measurement? My thinking is that the gain settings is different between the two calibrations. Consequently, without recalibration the receiver may enter saturation.

  2. Before commenting on the suitability of switched such as the HMC245 RF switch we would need to know more about your application. Specifically, what is the frequency of your DUT. My instincts tell me that the RF switch will have leakage that could impact your measurements. I’d like to recommend reed relays with glass ampules but even this may be problematic as contact capacitance and long wire length could interfere with the measurement especially when we consider a 10 MΩ measurement in the 100 kHz range.

  3. This is a challenging application question. I’m curious what others think. Shout out to @abhijeet.sisodia, @rick_1976, @David_1528, and @PaulHutch.

Best Wishes,

APDahlen

2 Likes

Dear Sir,

Thank you for your reply.

I have a microfluidic device with integrated interdigitated electrodes at the bottom of channel designed to capture bacteria. When bacteria adhere to these electrodes within the microfluidic device, the impedance increases from 3M ohms to 8-9M ohms.

In my experiments, I do not need to sweep the frequency; I need to measure impedance at constant frequencies at two different points in time, with a 5-minute interval between measurements. The frequencies I must use for impedance measurements are 100Hz and 1kHz, and it is crucial to maintain these frequencies constant.

Suppose I need to measure an impedance of 8M ohms (as measured using an impedance analyzer). The issue arises when the gain adjusted using an 8M ohm feedback resistor produces results consistent with the impedance analyzer, whereas the gain adjusted using a 3M ohm feedback resistor yields different impedance values, leading to discrepancies between the results.

To address this, I need to repeatedly calibrate the system whenever there is a slight difference between the feedback resistor and the impedance of the microfluidic device. I am looking for a solution to avoid the need for frequent recalibration.

No. That span represents a dynamic range of roughly 24 bits, while the ADC in the AD5933 only offers 12 bits of resolution.

It’s difficult to comment without reading the document in question.

Consider a purpose-built device such as B&K’s 891 LCR meter, or another similar device having specifications that would accommodate the need. Unless the development of measurement apparatus is itself the goal of one’s efforts, time spent doing so usually represents an unnecessary distraction if a commercial solution is available.

1 Like

Dear Sir,

Thank you very much for your reply. I currently have an LCR meter and impedance analyzer in my lab. However, my professor has instructed me to exclusively use the AD 5933 or any other evaluation board and to bypass bulky laboratory instruments such as the LCR meter and impedance analyzer.

I am seeking a solution where 5 different resistors can be connected to the AD 5933 using an IC/circuit/switch like the HMC 245A. The AD 5933 should be able to detect any one of the 5 different resistors and calibrate itself automatically.

I need an automated calibration solution for the AD 5933. If you have any ideas, please recommend them to me. Your assistance would be greatly appreciated.
I am going to attach the picture where the someone use HMC 245A switch to automate the calibration process of AD 5933
Screenshot

If you suggest me something to solve this issue, It will ease my research problem

Hello @Kamran,

You may not like it now, but I suspect you will thank your professor once you are working in the field. You will appreciate the learning challenge and the personal growth that comes with an insistence on system design.

May I recommend:

  1. Research the advantages and disadvantages of analog switches, RF switches, and reed relays. Characterize each technology in terms of leakage current, power consumption, crosstalk, speed of operation, and ease of integration.

  2. Purchase representative components and verify that your characterization matches the real world. Use the “bulky” equipment as truth supporting your experiments.

  3. Document everything including this conversation and bring your lab notes to the professor.

  4. Set milestones with your professor with the understanding that you are working on a wicked problem because there are many unknowns. Be ready to adjust as necessary as new information is discovered. These problems may or may not be known by your professor.

As a side note, I remember working on a very difficult problem. Without doubt, I had bitten off more than I could chew. I remember talking to an FPGA professional who had an entire team at his disposal. “That’s a challenging problem,” was all he and his team could answer. He said this with a smile - perhaps he knew the solution, perhaps he didn’t. While I’m no expert, I prevailed after a tremendous amount of time and frustration.

That’s the point you are facing as you sharpen your skills: steel to steel.

Please drop us a line in the future and let us know your solution.

Cordially,

Aaron