SC5 24 bit Skin Conductance system

SC5 uses a high resolution 24 bit digital converter to ensure full accuracy. Skin conductance has a range up to 100 micro Siemens, and within that range it is necessary to be able to resolve very small responses. The standard criterion for a skin conductance response is 0.02 micro Siemens, and to properly detect such a response it should be resolved to at least 0.002. With 24 bit accuracy SC5 easily achieves this, and can show responses below levels previously considered viable. SC5 uses the preferred constant voltage method (0.5V).
The SC5 24 bit accuracy A-D converter runs at a fixed internal sample rate of 80Hz, however the cycle sample rate for the SC-EKG unit may be set to a higher rate if required, in which case samples will be padded with identical readings. The 24 bit ADC digitizes SC immediately at the point where it is measured, minimising the possibility of interference or error. The circuit is self-calibrating, using known high precision resistances which are automatically switched in circuit by reed relays, which themselves exhibit very low contact resistance and thus minimize error. Calibration occurs when the unit is first powered up. When calibration is complete, the 24 bit code accurately represents SC measured between the red and black input terminals. The green terminal is provided only for cable shield ground, if an extended input cable is required. PSYCHLAB directly conveys the 24 bit value to the host computer.
When SC5 is used with PSYCHLAB software, convenient automatic evaluation of phasic SCR 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 SC range of instruments originate in 1979 to 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. A micro-controller organizes data collection and passes data via the USB to the host computer and also to the analogue outputs.
A 10 Hz filter is applied to the response signal to prevent aliasing. Pre-amplifier sample rate is 80Hz. Accuracy can be checked by connecting known precision resistances (e.g. 50kOhms, 20 microSiemens) between the black and red input terminals.
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 equation using floating point arithmetic in firmware.
Analogue Output.
To allow connection to other data acquisition systems, the unit can also provide two analoge outputs, found on pins 1 & 2 of the 9 way cannon connector. The necessary components for this function will not be included unless requested. Aanalogue outputs are fed by separate 18 bit D-A converters, re-converting the digital signal back to analogue. Precision SC is presented in two parts that may be accurately registered with normal +/-5V input ADC systems. These outputs are controlled in three ways, selected in the host control panel:
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.



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