Download 3H Project Skin Resistance Interface

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.