Cyton impedance checks incorrect at low resistances

[philippitts]

Problem

Several users on the OpenBCI forum report incorrect values from the electrode impedance widget for Cyton when using resistors of 500kOhms or less. Using resistors of greater than 500kOhms appear to be working as expected.

Expected

The Cyton impedance check widget should reflect the values of resistors used in lab validation configurations. If a resistor is connected between the N port of a channel and SRB2, the widget should show an impedance similar to the value of the resistor.

[philippitts]

Below are a series of tests surrounding the problem space. I connected several different resistors between the negative channel 1 pin and each of the following pins: SRB1, SRB2, Channel 1 positive, BIAS 1, BIAS 2, AGND 1, and AGND 2. I performed these tests with a new Cyton using the default settings (including hardware settings) in the GUI. I used GUI version 5.2.2 with Windows 10.

Below are my setups for each of these configurations.

SRB1

SRB2

Channel 1 Positive

BIAS1

BIAS2

AGND1

AGND2

---

The table below shows the values returned in the OpenBCI GUI Cyton Signal widget for each of the different setups. I waited 5 seconds before recording each test for the value to settle.

Resistor Value	Pin	GUI Impedance Reading
1000 kOhm	SRB1	1013 kOhm
1000 kOhm	SRB2	0 kOhm
1000 kOhm	1P	522 kOhm
1000 kOhm	BIAS1	~400 kOhm (dropping)
1000 kOhm	BIAS2	~250 kOhm (dropping)
1000 kOhm	AGND1	973 kOhm
1000 kOhm	AGND2	973 kOhm

Resistor Value	Pin	GUI Impedance Reading
680 kOhm	SRB1	0 kOhm
680 kOhm	SRB2	825 kOhm
680 kOhm	1P	373 kOhm
680 kOhm	BIAS1	~300 kOhm (dropping)
680 kOhm	BIAS2	~150 kOhm (dropping)
680 kOhm	AGND1	715 kOhm
680 kOhm	AGND2	715 kOhm

Resistor Value	Pin	GUI Impedance Reading
470 kOhm	SRB1	2510 kOhm
470 kOhm	SRB2	0 kOhm
470 kOhm	1P	256 kOhm
470 kOhm	BIAS1	30 kOhm (dropping)
470 kOhm	BIAS2	4 kOhm (stable)
470 kOhm	AGND1	479 kOhm
470 kOhm	AGND2	479 kOhm

Resistor Value	Pin	GUI Impedance Reading
51 kOhm	SRB1	2450 kOhm
51 kOhm	SRB2	0 kOhm
51 kOhm	1P	28 kOhm
51 kOhm	BIAS1	~80 kOhm (dropping)
51 kOhm	BIAS2	15 kOhm (dropping)
51 kOhm	AGND1	53 kOhm
51 kOhm	AGND2	55 kOhm

---

General observations:

• The SRB pins were unstable and would either show very high impedance or no impedance (0 kOhm).
• The positive channel pin appeared to show about half the correct impedance.
• The BIAS pins were unstable and would consistently report lower values over time.
• The AGND pins reported imperfect, but generally accurate results.

[wjcroft]

Phil, thanks!

Seems pretty certain that SRB2 should be the pin used, in normal EEG applications. And that SRB2 PREVIOUSLY worked with the original ADS1299 register settings. Which is confirmed on this page,

https://openbci-stream.readthedocs.io/en/latest/notebooks/A2-electrodes_impedance.html

So hopefully this is an easy fix.

Thanks everyone.

[retiutut]

https://openbci.com/forum/index.php?p=/discussion/3790/high-impedance#latest

[philippitts]

Additional testing with different versions of the GUI:

GUI v5.0.0 👍

Resistor Value	Pin	GUI Impedance Reading
51 kOhm	SRB1	197 kOhm
51 kOhm	SRB2	48.9 kOhm
51 kOhm	1P	48.9 kOhm
51 kOhm	BIAS1	~40 kOhm (unstable)
51 kOhm	BIAS2	0 kOhm
51 kOhm	AGND1	0 kOhm
51 kOhm	AGND2	0 kOhm

GUI v5.0.6 👍

Resistor Value	Pin	GUI Impedance Reading
51 kOhm	SRB1	0 kOhm
51 kOhm	SRB2	48.8 kOhm
51 kOhm	1P	49 kOhm
51 kOhm	BIAS1	0 kOhm
51 kOhm	BIAS2	0 kOhm
51 kOhm	AGND1	0 kOhm
51 kOhm	AGND2	0 kOhm

GUI v5.0.8 👍

Resistor Value	Pin	GUI Impedance Reading
51 kOhm	SRB1	0 kOhm
51 kOhm	SRB2	48.8 kOhm
51 kOhm	1P	49 kOhm
51 kOhm	BIAS1	0 kOhm
51 kOhm	BIAS2	0 kOhm
51 kOhm	AGND1	0 kOhm
51 kOhm	AGND2	0 kOhm

GUI v5.0.9 👎

Resistor Value	Pin	GUI Impedance Reading
51 kOhm	SRB1	0 kOhm
51 kOhm	SRB2	511 kOhm
51 kOhm	1P	27 kOhm
51 kOhm	BIAS1	~16 kOhm
51 kOhm	BIAS2	~21 kOhm
51 kOhm	AGND1	~62 kOhm (unstable)
51 kOhm	AGND2	~64 kOhm (unstable)

GUI v5.1.0 👎

Resistor Value	Pin	GUI Impedance Reading
51 kOhm	SRB1	~2400 kOhm
51 kOhm	SRB2	0 kOhm
51 kOhm	1P	57 kOhm
51 kOhm	BIAS1	~30 kOhm
51 kOhm	BIAS2	~2 kOhm
51 kOhm	AGND1	~53 kOhm
51 kOhm	AGND2	~53 kOhm

GUI v5.2.0 👎

Resistor Value	Pin	GUI Impedance Reading
51 kOhm	SRB1	516 kOhm
51 kOhm	SRB2	2450 kOhm
51 kOhm	1P	27.7 kOhm
51 kOhm	BIAS1	~75 kOhm
51 kOhm	BIAS2	~75 kOhm
51 kOhm	AGND1	~70 kOhm
51 kOhm	AGND2	~70 kOhm

The problem appears to have been introduced in v5.0.9