Cyton channel disconnection affects other channels ?
Hi,
We use Cyton board to measure impedance of human body and we connect channels 1-4 with 8 electrodes connected to positive and negative terminals. We use the arduino code to measure each terminals' impedance separately. We also have the AGND pin jumpered to pin 4N as we want this electrode to be our ground. Now, if we disconnect one of the electrodes (not the Agnd), for example 1P, then of course 1P impedance will be very very high, but it also affects the measurement of electrode 1N by a bit, and it also affects the impedance of 4P. Do you have an idea why do we see all of this cross talk between the electrodes? Do you have a schematic the circuits of he board?
Thanks in advance.
Comments
Gilad, hi.
I'm not certain, but I thought that in general, AGND should not be used for the subject's skin ground connection. Skin ground connection is via Bias / Ground pin. AGND (along with AVDD, AVSS) are the power supply lines to the ADS1299 chip. Hence are very very low impedance routes into the chip's logic. In general it is not a good idea to connect zero impedance (short circuit) pins to the subject. Bias pin is high impedance. The analog power pins were only brought out to the board headers, because they are used in applications such as active electrodes (ThinkPulse):
https://shop.openbci.com/products/thinkpulse-active-electrode-kit
If you are using both plus and minus pins per channel, there should be no need for jumpering AGND to 4N, or any other channel pin. Board schematic is on this page:
https://docs.openbci.com/Cyton/CytonSpecs/
William
I thought that the Bias pin acts as a current sink according to the BLOG. Is this not true? Does the current come from the P to N for the measurement?
MSL, that link you cite, is discussing the impedance measurement configuration, where a very small 6 nanoamp AC current is injected via the channel pin and Bias return.
Normally Bias is injecting a 180 degrees out of phase mains frequency, to reduce mains interference.
However in terms of Gilad's original question, I still believe that AGND should not be connected to the subject. As AGND is part of the AGND, AVDD, AVSS power supply to the ADS1299. Bias / Ground pin is the 'ground' that is intended to connect to the subject.
William
Yeah, I agree that AGND should not be connected to the body, but be separate and solo for the board. Sorry for the sidetracked question I asked. I think for gilad, it would be good to have BIAS instead of AGND as the ground
Thanks guys
As a follow-up question, is this true for ambulatory recording as well (ECG)? That AGND should not be connected to the subject and BIAS should be used as the Ground instead? When should AGND be used then?
AGND, AVDD, AVSS are all analog POWER SUPPLY lines for the ADS1299, and should not be connected to the subject. Bias is the subject ground line. These analog power lines ARE used for active electrodes, which require power.
https://shop.openbci.com/products/thinkpulse-active-electrode-kit
I'm confused about this. According to the tutorial in the OpenBCI website, for EMG, AGND should be used instead of BIAS.
I'm collecting EOG (which should be the same as EMG) with two channels (ch1 and ch5). Nothing is connected to the other channels. I set up OpenBCI such as SRB2 = Off in ch1 and ch5. Why am I getting some signal on the rest of the channels if nothing is connected?
When channels are left "floating" and disconnected, they can pickup environmental EMF noise, since the amplifier is sensitive to microvolt level signals. Pins can act as "antennas", with no wire connections.
re: AGND
Look at the TI datasheet, pages 5 and 6
https://www.ti.com/lit/ds/symlink/ads1299.pdf
There actually are NO ADS1299 pins labeled AGND. This AGND designation is a feature of the Cyton schematic, see the power supply and ADS1299 sections.
https://docs.openbci.com/assets/images/OBCI_V3_32bit-Schematic-1930fe4303e214795b38b75ba7b35b75.jpg
AGND acts like the overall ground reference for all the power supply filtering (3.3V digital and plus/minus 2.5V analog), ESD (electrostatic discharge protection, the circuit blocks below power supply section), and power supply ground for the ADS1299 itself.
On page 6 of the ADS1299 datasheet you can see that both the digital and analog GROUND pin connections are labeled in the TYPE column as SUPPLY. These pins are NOT labeled as analog inputs or analog outputs. Bias actually acts as both an analog 'input', to center the differential amplifiers, and an analog 'output' (common mode mains cancelation), if it is included in the Bias setting shown here.
https://docs.openbci.com/assets/images/ExG_GUI_2-639468e4bfb10a5090b9b510c0426c2f.png
That diagram from this page:
https://docs.openbci.com/GettingStarted/Biosensing-Setups/ExGSetup/#visualizing-the-data-in-openbci-gui
My impression is that the two docs on EMG and ExG may be referencing AGND as a misinterpretation or attempt to avoid contaminating EEG / ECG Bias usage, with the larger amplitude EMG signals. But the settings panel shown above can disconnect both Bias (common mode detection and cancelation) and SRB2 from channels which have EMG usage. Even though these settings disconnect the Bias cancelation function, Bias / Ground still acts as the differential amplifier centering function.
William