I'm currently doing a project that consists of testing bio-implants to see if they are still good to use.

For this, I have to measure the impedance of the implant from frequency starting at 10Hz to 1MHz.

I'm using an L432KC from STM to control an AD9833 which outputs the 0.6ppV sien wave at the desired frequency.

The implants can have an impedance from a few ohms to around 100 kohm.

Applying this voltage to the implants will generate current and with Ohm's law, I can find the impedance of the implant at each point.

To do the operation, I need to convert the current to voltage so the 12bit ADC of the L432KC can read it.

Here's a "schematic":


I have struggled doing this conversion. I'm trying to use a transimpedance amplifier but I can only use it with low impedance measurements or high impedance. I can't size it for both:

enter image description here

Does anyone have a solution to have the biggest range in terms of resistance?

Am I maybe going about this in the wrong way?

  • \$\begingroup\$ "The implants can have impedance from a few ohms to around 100Kohm." Could you use different circuits for the different impedance ranges? Could you have your circuit have variable range settings? \$\endgroup\$ Commented Mar 14, 2021 at 16:18
  • \$\begingroup\$ @MathKeepsMeBusy The problem is that I don't know the impedance of it before. I just know the range of it \$\endgroup\$
    – Graggon
    Commented Mar 14, 2021 at 19:58

1 Answer 1


The preferred method used by good RLC meters is to inject a voltage limited constant current sweep and then with a 5 decade resistance but only 3 decades of voltage measurement use a switch to change R scaled constant AC current.

But I can look later when time permits to see how to improve your calculations with a scalable voltage amplifier with AGC then the Vagc records the resistance on a log scale with such an IC.

Galvanic corrosion often raises the resistance of electrodes and pressure reduction as well.

  • \$\begingroup\$ Thank you for the answer Tony! When you say 5 decade resistance, is it the big box with the switch ? \$\endgroup\$
    – Graggon
    Commented Mar 14, 2021 at 22:32
  • \$\begingroup\$ No that's your spec for implants that can have an impedance from a few ohms to around 100 kohm. So to measure it a rough way is to use a Log AGC with a current source or current source and measure R directly from voltage with a fixed current across a variable R. \$\endgroup\$ Commented Mar 15, 2021 at 2:14

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