MAX30009 tetrapolar accuracy under high contact-impedance and high-current drive

Hello,

I am evaluating the performance of the MAX30009 in tetrapolar bioimpedance measurements under bench (resistor) conditions. I have a few technical observations and would appreciate clarification / guidance:

1. Four-wire (tetrapolar) measurement affected by high contact impedance:
- Test setup: R_test = 35 Ω (precision resistor), series contact impedances of R_c = 365 Ω and R_c = ~2 kΩ for the “outer” electrodes.
- Observation: when increasing contact impedance, the 4-wire measured magnitude deviates significantly, even though R_test stays constant.
- Question: Could you help me understand why the 4-wire measurement may still be strongly affected by high contact impedance?
Are there configuration steps, filtering options, or measurement-path requirements that I may be missing?
Under what conditions is the 4-wire input expected to reject the contact-impedance contribution correctly?

2. Current-drive stability at higher stimulus current:
- Observation: stable, expected current output only at 32 µA and 64 µA. At higher requested currents (e.g. 128 µA) actual measured current is lower than requested and varies with frequency.
- Question: Are there known limitations in MAX30009 for stable current delivery above certain current levels (e.g. >64 µA)? Does the datasheet or application notes specify conditions under which the current source may become inaccurate or unstable? Are there known operating limits, output-compliance conditions, or load constraints that could explain why the current source does not remain stable at these levels?
My goal is to understand whether these behaviors are expected under certain conditions, or whether I may be configuring or loading the device incorrectly.

3. Frequency-dependent current output deviation:
- Observation: Even when current output seems stable at a given current setting, the actual delivered current varies significantly as a function of frequency (for higher requested currents). This causes variation in measured impedance magnitude/phase across frequency.
- Question: Is frequency-dependent current-drive variation a known behavior of MAX30009? Under what conditions should one expect frequency-independent (flat) current output? Are there recommended practices (e.g. wiring, guarding, calibration) to ensure stable current across frequency?

I’m using a bench resistor (not biological tissue) to isolate contact-impedance and drive-related effects. Any guidance on expected behavior, datasheet interpretation, or recommended test-configuration would be much appreciated.

Thanks in advance!