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SC5 24 bit digital Skin Conductance amplifier

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The PSYLAB SC5 coupler provides a 24 bit accuracy A-D converter built into the Amplifier, so that SC is digitized as early on as possible. From that point on, the digitized signal is completely immune to any interference. PSYLAB software reads this data digitally, however, to allow connection to other data acquisition systems, the SAM unit also provides two analoge output sockets, each fed by separate 18 bit D-A converters, presenting precision SC data in two parts that may be accurately registered with normal ADC systems.

When SC5 is used with PSYLAB software, convenient automatic evaluation of SCR and SCL is available both in the stimulus - response paradigm (specific response, SR), and in the non-specific response paradigm (NSR). In the former, the phasic response can be analysed for onset level, onset latency, amplitude and peak latency. In the latter, SCR may be counted (and evaluated for total amplitude if required) in given time periods, allowing performance to be compared in baseline and stimulation phases of the experiment.

SC5 measures directly in conductance, using DC coupling with constant voltage electrode excitation. The high resolution of SC5 allows SCRs lower than 0.01 micro-Siemen to be magnified in software and adequately resolved. The PSYLAB SCX range of instruments originate in 1979 and comply with the recommendations of Lykken and Venables. Further useful information, including recommendation of electrode type, conductive paste etc. may be found in pages 4 - 62 of "Techniques in Psychophysiology"; Martin & Venables (published by Wiley, 1980).

Functional Details

The Amplifier is contained in an isolated plastic box, with electrical power conveyed to the circuit by a high quality medical isolating DC - DC converter. This ensures low noise with complete safety and individual isolation of electrode connections to prevent interaction with other amplifiers. Serial SC data is fed across the isolation barrier in the Amplifier and passed along the connecting cable to the SAM unit. Here a micro-controller organizes data collection and passes it on to output devices including direct digital connection to the PSYLAB host computer and the analogue outputs.  SCL is shown on the LCD display.

A 10 Hz filter is applied to the response signal to prevent aliasing. Pre-amplifier sample rate is 40Hz. Two calibrators are supplied in the pre-amplifier which connect directly to the subject connection terminals. The large value (20 micro-Siemen) can be used to check absolute accuracy (SCL), the small value (0.1 micro-Siemen) may be used to check accuracy of response signal (SCR).

When SC5 is first turned on, it automatically calibrates itself, by switching first to 0 micro-Siemen (open circuit), then to 100 micro-Siemen (a 0.1% 10kOhm precision resistance). Having established these two points, it calcuates the A and B constants required to correctly scale the entire range imbetween in an Ax + B type floating point equation.

Operating Instructions

Ensure the SC5 pre-amp is connected.  Turn on the power to the main unit.  The SC5 immediately begins with an automatic calibration which takes a minute or so.  When the calibration is complete the LCD display will begin to show valid SCL readout, which may be checked by pressing the calibrator buttons on the amplifier.


Analogue output

Two analogue output modes are provided, determined by the settings in the parameters screen. To get to this screen, click on the arrow in the top right corner of the LCD display section.

Mode 0. In this mode, SCL is expressed on both D-A converters to 18 bit resolution.

Mode 1. In this second mode, the most significant section of the data 'level' (0 to 100 micro Siemens) is sent to one D-A converter, and the lower significance section (which shows up very small changes) is sent to the other. When this 'response' signal reaches +5V or -5V, the 'level' changes by one step. The response signal is manipulated so that when it reaches +5V or -5V, the 'level' signal changes but the 'response' signal jumps to 0 Volts rather than all the way to the other extreme. This way, the 'response' signal tends to remain in a useful range to be directly viewed using conventional graphing software, as well as containing the high-resolution part of the signal for logging purposes. The dividing point between the 'level' and 'response' part of the data is chosen such that the full scale 'response' signal represents 1.5625 micro Siemens. Mode 2 provides a steadily increasing readout, which may be used to assist calibration of the external A-D converter readings. V1.0 SAM firmware produces inverted response signal which increases when SCR decreases.


Analogue output connection.

Analogue output is obtained on the analogue socket for the particular side of SAM being used by the amplifier. In mode 1 pin 5 is response, and pin 4 is level. In mode 0 both pin 5 and pin 4 carry the full (18 bit DAC) signal. The analogue output section has a separate ground from the main chassis of the SAM. The shell of the output socket carries this ground, which should be connected to the 3rd party data acquisition system ground. If the acquisition system has differential input, use the negative input for the analogue ground and connect the SAM chassis ground to earth. If a multi way connector is provided for analogue output, normally this would be a 9 way male to 9 way female lead. If the female connector needs to be removed, the conductors are: Orange = level, Yellow = response, Shield = analogue ground.


SC5 Specifications:

Measurement Units      micro-Siemen (micro Mho, reciprocal of mega Ohms)
Absolute accuracy       +/- 0.1 micro Siemen
Relative accuracy         5.96e-6 micro Siemen (theoretically)
Frequency response     DC - 10 Hz
Calibrator accuracy     (20) .05%, (0.1) 5%
Subject excitation        0.5V DC
Range                          0 - 100 micro Siemen
Subject Isolation          > 5000V, 100 M Ohm
Analogue output on      9 way cannon socket, 2 x 18 bit DAC.