When a single subject is connected via wires to a single MP system, there should generally be one and only one ground connection to that subject*. When multiple subject grounds are used, ground loops can occur: current will flow between the different grounds. This will result in distorted data.
When recording with the EDA100C (GSR100C), EBI100C, or the NICO100C, a ground is already provided through the VIn- or I IN connection (in these three cases as well as their -MRI brethren, an internal connection makes VIn- for EDA amplifiers or I IN for the bioimpedance amplifiers electrically identical to ground). Consequently, no other ground connection to the subject* need be used. In some cases, particularly when power line noise is apparent despite the presence of a single ground, it is beneficial to have more than one ground. In the case of the EDA100C (or GSR100C), additional connections can be made through AC-coupled lead adapters (CBL205).
If using BioNomadix wireless transmitters, the subject requires one ground per transmitter. This only applies to units making an electrical connection with the subject. In addition, the BN-PPGED does not require an extra ground, as the VIN- of the EDA already acts as a ground. However, BN-ECG2, BN-EMG2, BN-EEG2, BN-EOG2 and BN-EGG2 all require separate ground connections.
* “To the subject” meaning specifically an electrical connection to the subject. Many other BIOPAC components, such as the respiratory effort transducer and photoplethysmogram transducer have ground connections to the equipment but are not relevant to this topic because these devices do not make electrical connections to the subject and thus will not produce ground loops through the subject.
To record EDA (GSR) simultaneously with other biopotential signals (for instance, ECG, EEG, EOG, EGG, EMG, or ERS):
- Using 100/100C series amplifiers (e.g., GSR100, GSR100C, EDA100C, etc.), BIOPAC suggests using CBL205 connected to one ground on any of the biopotential amplifiers.
- For example, if an EDA electrode is to be removed during an experiment carried out with an ECG100C and a GSR100C, there should still be a ground connection for the ECG amplifier. To always have appropriate grounding and no ground loops:
1. Connect the Vin- lead of the EDA as ground (when using disposable electrodes there will be nothing connected to the “GND” connection on the front of the GSR100C).
2. Connect an AC-coupled ground to the ECG100C (e.g., LEAD110A connected through CBL205 to the ECG100C’s “GND” connection).
The AC coupled lead will not influence the EDA measurement when all electrodes are connected, but it will continue to function as a ground for the ECG amplifier if the electrode connected to VIn- on the GSR100C is disconnected from the subject.
- Using EDA100D does not have any connection to the system ground. When using this amplifier for monitoring skin conductance, one ground connection from the subject to another amplifier is still required if any biopotential recordings are included. The ground connections for all other D-series amplifiers connect to system ground just like the C-series amplifiers.
For more on this topic see “Special Cases” under “Multiple amplifiers per subject or multiple subjects per system.”
Note—If using any two of the following on the same subject connected directly to the same MP system at the same time: EDA100C (GSR100C), EBI100C, or NICO100C, ground loops will be a problem; you will not be able to make absolute measurements (such as SCL). BIOPAC recommends the use of BioNomadix modules in this case as these transmitters do not share a common ground. Alternatively, one or more of the measures may be made through an additional MP system or with an IPS100C/HLT100C/OUTISO to optically isolate the ground connections. More information may be found under Galvanic Isolation Guidelines.