I have always been a big fan of the nanoZ as a tool for measuring impedance and plating electrodes. It is specified as having a 1k ~ 100MΩ working range with 1kΩ resolution (see NanoZ User Manual). However, in practice, I have had stability issues with the internal signal generation, making measurements impossible to interpret. This could be due to electrode type, but I have also found grounding to be a critical factor; it’s unclear how to resolve this with a USB-connected device… there will always be influences from your PC or laptop.
Impedance Theory
Impedance is the opposition to electrical current in a circuit, and in brain electrodes, it matters because high impedance can reduce signal quality, leading to poor detection of neural activity or inefficient stimulation delivery. The nanoZ measures impedance using a simple circuit:
“This formula generalizes to AC sinusoidal signals where V1, V2, and Zx are complex numbers whose angles represent phase relations in the circuit. When a known voltage V1 is applied, and V2 is measured, it is possible to solve the above equation for Zx, which is exactly how the nanoZ measures impedance. During impedance measurement, test currents flow through the circuit. The nanoZ uses a 4mV peak-to-peak sinusoidal waveform for V1, which yields a maximum test current through Zx of 1.4nA RMS when Zx is approaching zero, and 0.7nA RMS when Zx is 1MOhm.”
Bench Impedance Measurements
To perform impedance measurements on the bench, you’ll need a few things:
- Test circuit (resistor or potentiometer).
- Oscilloscope (e.g., Siglent SDS 1104X-E)
- Waveform Generator (e.g., UNI-T UTG932E)
- Ohmmeter (e.g., BK Precision 5492C)
We will produce V1 with a sine wave of a known peak-to-peak voltage (50mVpp) and frequency (e.g., 1kHz). The test circuit I’ve created will replace Rref of 1MΩ with a potentiometer that should be roughly the maximum impedance you believe your electrode will be.
Here’s why: instead of fixing Rref and trying to measure the output voltage, I will make it variable and adjust it until V2 is precisely half of V1. That is, if V1 = 50mVpp, Rref will be tuned such that V2 = 25mVpp. In this situation, we maximize the resolution of V2 and know that Rref=Zx, a measurement we can make with an ohmmeter.
The other advantage of using your own signal generator is that you can increase the voltage beyond the 4mVpp that the nanoZ outputs, potentially overcoming the noise band.
This electrode was submerged roughly 3mm into normal saline (0.9% NaCl). The oscilloscope was set up to measure signal amplitude and frequency.
Once I obtained V2 = 0.5 * V1, I disconnected everything from the circuit and measured Rref: this is the electrode impedance!
| Electrode | Impedance |
|---|---|
| 1 | 100.108 kΩ |
| 2 | 186.05 kΩ |
| 3 | 185.90 kΩ |
| 4 | 132.02 kΩ |
| 5 | 90.013 kΩ |
This method was first tested with a fixed 10kΩ resistor to verify the procedure.
End Notes
How can I lower impedance?
Resistance is inversely related to surface area. As the surface area increases, the resistance (or impedance) decreases in electrodes because a larger area provides more pathways for current to flow, reducing the overall opposition to the current. Conversely, a smaller surface area results in higher resistance due to fewer pathways for current.
The theory behind gold plating is that small nano-spheres accumulate on the electrode tip, increasing the surface area.
I was curious about the electrode tips of the large-diameter twisted pair electrodes I tested and whether similar strategies could be exploited.
By simply wet sanding these electrodes, the impedance fell from 132.02kΩ to 22.14kΩ. This can make recording and stimulating more effective!
If you’re struggling to home in on your single electrode impedances either due to range or noise issues, this method may help you get over the hump. Also, depending on your application, a little electrode prep can make a huge difference.