Electrical noise is the most common problem associated with the recording of bio-electric signals. It radiates through the air and comes from electrical devices in the lab room or building: lights, power outlets, computers, monitors, and the power supplies. Since the source of power for these devices is 60 Hz alternating current (AC), this electrical noise appears as a distorted sine wave with a repeating period of 16.7 msec.

There are two major sources of electrical noise: pickup and ground loops.

Pickup

Pickup is caused by electrical radiation that produces currents in the electrodes and wires leading to the amplifiers in the recording system. Because the resistance in the electrodes is high, small currents produce large voltages that may be greater than the biopotential being recorded. The major ways to reduce pickup are:

• Faraday Cage: Put a grounded, screened enclosure, known as a Faraday cage, around the preparation and the electrodes. The enclosure separates the source of the radiation from the electrodes. The person operating the equipment might also be a source of noise, and he or she may need to be grounded.

• Shielded Cables: Use shielded cables to carry the signals from the electrodes to the amplifier and the recorder; this puts a protective ground around the wires carrying the bio-electric signal. Electrode wires can be braided and wrapped in tinfoil to minimize noise.

• Differential Recording: Record using both a positive and a negative recording electrode. The noise signals that are equal in magnitude, but opposite in polarity, will cancel each other out and leave a flat baseline.

• Short Cables: Use the shortest cables available to reduce the length of wiring exposed to electrical noise.

• Direct Current Equipment: Use equipment, like preamplifiers and illuminators, that are powered by direct current (DC) transformers.

• Equipment Removal: Unplug or remove unused alternating current (AC) equipment from the area.

Ground Loops

Ground loops are a troublesome source of electrical noise caused by the ground cable itself serving as an antenna for the noise radiating in the room. Using a Faraday cage to shield the preparation and the recording electrodes does not remove the electrical noise caused by ground loops. To avoid ground loops, use the following techniques:

• Ground Hub: Ground all the equipment around the preparation to a common grounding point (hub). This includes all the items that are electrically powered or are made of metal, like illuminators or microscopes. Use simple cables, like banana cords equipped with alligator clips, to connect each device directly to the common grounding point. The common ground point is connected to the ground of the recording device with a single cable. The recording device is connected to the building ground.

• Simple Chain: Ground the devices to the common grounding point using the simplest route that links the first device to the second device, the second device to the third device, and so on. Start the chain at the device that is the farthest from the common grounding point. End the chain by connecting the last device to the common grounding point, which is connected to the ground of the recording device.

• Free-Floating: In addition to using one of the grounding techniques described earlier, plug all devices powered by alternating current (AC), like illuminators, amplifiers, and recording units to power outlets using three-prong adapters.

High Frequency Noise

High frequency noise can also be a problem when recording bio-electric potentials. This type of noise is seen as the thickening of the recorded line. This noise contains many frequencies, and the amplitude of the noise is proportional to the resistance of the electrode. Therefore, intracellular electrodes, with high resistances, pick up a greater amplitude of high frequency noise than extracellular electrodes, with low resistances.

Mechanical Noise

Mechanical noise, like vibrations from the ventilation system in the room, can cause the electrodes to vibrate and produce voltage changes with each vibration cycle. To alleviate this problem, isolate the platform holding the preparation with foam pads or bicycle inner tubes. Also, avoid bumping the table when the recording electrodes are in place.

Grounding

The iWorx units have different locations for grounding depending on the device. The grounding port is green on all devices, and uses a banana jack connection. The opposing end of the grounding cable should be plugged into the ground port on a standard wall outlet.

• On units such as the IX-BIO4, IX-BIO8, IX-ECG6 and IX-ECG12, the grounding port is located on the left side of the recorder, next to the USB port.

• On units such as the IX-RA5S and IX-RA834, the grounding port is located on the back panel, labeled with the ground ⏚ symbol.

• If using a heated bed for the animal, the heated bed must also be grounded. Make sure the heated bed is a shielded, low-noise heater. For devices like the TCS-100 heater controller, the grounding port is on the rear panel. Connect this to the same ground point used by your recording system.

Signal Improvement

1. To improve recordings, move the recorder away from sources of 60 Hz noise. These sources include outlets, computers, monitors, lights, refrigerators, water baths, and other AC powered devices.

2. If the recording still contains a great deal of electrical noise, apply the digital filtering function to the data. Click on the add function (fx) button in the upper margin of the recorded channel. Select Filter from the menu of computed functions.

3. Select the filter type to be applied to the recorded data.

• Notch filter: LabScribe provides a specific option to choose a 50 or 60 Hz notch filter from the Filter submenu. This type of filter is designed to reject a narrow band of frequencies around a center frequency, such as 60 Hz, which is the typical line voltage noise.

• FIR filter: FIR (Finite Impulse Response) filters are flexible and can be configured as low pass, high pass, band pass, or band stop filters. In the Filter Setup Dialog window:

• The Filter Mode is set to Hamming Window by default.

• The Filter Order is initially set to 51, which is the recommended default. LabScribe may automatically adjust the minimum filter order depending on the selected filter cutoff frequencies, but users can begin with a setting of 51.

• The values for the filter cutoffs can be set in several ways. You can type the desired values directly into the boxes to the right of the filter cutoff names. Alternatively, you can click the up or down arrows next to each box to adjust the values incrementally. Another option is to click and drag the margins of the colored area in the filter’s graphic display to visually set the cutoff points.