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Download 3H Project Skin Resistance Interface
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3H Project Skin Resistance Interface The interface is split into two isolated circuits i.e. OUTPUT TO BODY and INPUT FROM BODY. Although the circuits are symmetrical it is recommended that annotation of the input and output connections are observed because of the impedance matching resistor on the INPUT FROM SIG GEN bnc. Input Section The application of a signal at the INPUT FROM SIG GEN bnc produces a differential signal at the OUTPUT TO BODY BNC’s. The 50 ohm resistor R1 provides the correct termination for a 50 ohm output impedance signal generator. If a higher output impedance signal generator is used this resistor should be changed to the appropriate value. Transformer T1 provides isolation and a differential output. T1 also limits the current capability that can be applied to the body. C1,C2,C3 & C4 provide attenuation of any low frequency components especially at mains frequency (50Hz) these capacitors are Y rated 440V AC capacitors and in the unlikely event of mains voltages being present on the input the reactance of these capacitors prevent that voltage appearing at the output. The zener diodes Q10, Q12 and diodes Q3 & Q4 clamp the output voltage to a safe level (typically 5V p-p) should the input be overdriven. Clipping of the output will start at an input voltage of approx 2.5V p-p. A recommended input of 2V p-p max is suggested. The transformer T1 and associated capacitors constitute a resonant circuit at approximately 100 KHz and the resistor R2 provides a constant load in order to destroy the Q of this resonant circuit and thus prevent peaks in the output voltage. The differential output is available between the centre pins of the OUTPUT TO BODY bnc’s. The outer connection of these bnc’s has no connection to any other part of the circuit and is not connected to any common or ground. Output Section Performs the same purpose as the input section BUT in reverse and does not have a termination resistor at the OUTPUT TO PSD bnc. The clamping diodes are included in the circuit in case the output section is inadvertently used as the input section. This is not an advised practice however. The differential input is applied between the centre pins of the INPUT FROM BODY bnc’s. The outer connection of these bnc’s has no connection to any other part of the circuit and is not connected to any common or ground.