Waveform Acquisition Hardware
PI Crampton and students have built eight AC coupled wide band differential amplifiers. These units have a 0 to x 1,000 variable gain (2 variable gains 0-10, 10-1,000) and a frequency response of +/- 3 dB from 0.2 Hz to 110 kHz. The external electrode is plugged into a Lemo 3-pin probe socket. There are two outputs: 1. BNC to the digitizer. 2. Mono phono jack to an external mini loudspeaker/headphones with independent volume control. The unit is water resistant and charged by two 1600 mA 9V rechargeable battery packs, giving approximately 36 hours of autonomous use in the field at medium gain settings. Other features include a polarity reversal switch and on-off switch with green LED power indicator. These units are ideal for low-noise AC-coupled data acquisition and are both portable and water resistant.
Custom built AC-coupled wideband differential amplifier. Casing with dials and internal view showing circuit board and battery bay.
We will also use the Signal Recovery 5113 low-noise preamplifier. This unit has AC and also DC coupling in addition to various sophisticated filtering options, communication to a computer via a Serial cable and LabView drivers.
Differential signals from the SR-5113 must be input with two separate BNC cables. We have designed an input module to convert signals from the audio cable of an electrode to two 6” double-shielded BNC cables. The unit also allows polarity reversal before amplification. Very low noise levels are possible with this amplifier/input module/electrode system.
Custom built bipolar input module for Signal Recovery 5113 amplifier. Internal view (left) showing: input (Lemo 3-pin socket [EGG.2B.303.CLM]), polarity reversal switch, output (2 male BNC plugs). External view (middle) showing Lemo input socket. External view (right) showing two channel BNC outputs and polarity reversal switch on top of unit.
Signals are picked up from custom built silver-chloride plated pure silver wire electrodes encased in 1/8” or 1/4” PVC piping filled with silicon rubber. We have built probes with pole spacing from 4-16 cm at 2 cm intervals (1/8” casing) and from 16-40 cm at 4 cm intervals (1/4” casing). For very large fish we will utilize an electrode comprising adjustable pole separation of 40-120 cm. The longest known Gymnotus is a specimen of G. inaequilabiatus (1 m). The ground electrode (center) is connected to the shield. The two pick-up electrodes are connected to channels 1 and 2 of the cable.
Right: Custom built electrode with Lemo 3-pin plug (FGG.2B.303.CLCD52Z and GMA.2B.050.DN). The cable is Amtech Industries 24 AWG EZID shielded audio cable. Left: Electrode connected to custom-built portable amplifier.
For the digitization of individual EOD waveforms we will use two National Instruments data acquisition systems: The Daqcard 6062E (12-bit) and the Firewire Daq 6052E (16-bit). Both systems can be powered using rechargeable 12V batteries in the field. The 6062E uses the SC-2345 Signal Condition box with 8 SCC-FT01 feed through modules connected in differential mode. Temperature logging at the same time as recording is possible using the SCC-TC02 thermocouple input module and K-type mini thermocouple probes. Several electric fish can be recorded simultaneously using these instruments.
Daq-Pad 6052E (top), SC-2345 (bottom left), 6062E (bottom right).
National Instruments systems offer the advantages of LabView support and DC coupling. For the recording of AC coupled signals from custom built amplifiers we will use an Edirol FA-101 A/D system. This allows 24-bit 96 kHz sampling on up to 10 channels and 192 kHz sampling on up to two channels. We will also use the Edirol UA5 device (96 kHz sampling on two channels).
Edirol UA5 (left) FA-101 (right)
For long recordings of fishes, which will occupy considerable hard-disk space, we will use a Sony TCD D7 or D3 DAT (digital audio tape) recorder. We will always carry at least one of these units in the field. In the event of computer failure the DAT recorder will serve as a completely effective back up digital recording system. With a sample rate of 48 kHz (more than twice the sample rate of the upper frequency content of Gymnotus and other gymnotiform electric fishes) DAT recorders sample at well over the Niquist frequency (aliasing threshold).
Signals will be monitored (when required) using a Thurlby-Thandar SC-110A portable battery powered cathode-ray oscilloscope.