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Owner’s Guide
26
Technical Aspects
The following section describes some of the important technical aspects of the GSR Amp
to give some insight into how it works. You do not need to know the material here to use
a front-end. It is likely to be of special interest to the technically minded, indicating what
a front-end can and cannot do, and its suitability for particular purposes.
Note: You should not use this section as a service manual: user modification of the
equipment voids your rights under warranty.
GSR Amp Operation
The GSR Amp and other ADInstruments front-ends have been designed to integrate
fully into the PowerLab system. Each requires connection to the PowerLab via a special
communications connector called the I
2 C (eye-squared-sea) bus, and a BNC connector.
The GSR Amp is essentially an extension of the PowerLab’s analog input. The GSR Amp
provides:
•
full electrical isolation from power-line (mains) circuitry to guarantee subject
safety
•
measurement of the Féré eff ect, a general measure of autonomic nervous system
activity, directly in SI conductance units (siemens)
•
low, constant-voltage AC excitation (22 mV
rms
@ 75 Hz) allows enhanced safety
and the use of dry electrodes, with no special electrolytes needed
•
measurement of absolute conductivity up to 100 µS (microsiemens).
The internal functions of the GSR Amp are controlled from the PowerLab through the I 2 C
bus, which also supplies power to the GSR Amp. The front-end is also connected to an
analog input channel of the PowerLab via a BNC-to-BNC cable, through which the signal
is sent. The overall operation of the GSR Amp can be better understood by referring to
Figure 8–8.
I 2 C
Output
I 2 C
Input
Signal
Output
Micro
Processor
Controller
12-bit
Offsetting
DAC
1 Hz 2nd
Order Low-
pass fi lter
8 MHz
Crystal
400 Hz
Power
Oscillator
Synchronous
Rectifi er
Isolation
Transformers
Isolation Barrier
S
y
n
c
hr
onous
Re
c
ti
fi e
r
Si
g
n
a
l
Mod
u
la
to
r
Isolated
Power
Supply
75 Hz
Oscillator
Status
Indicator
Subject
Connections
Transimpedance
Amplifi er
_
+
Figure 3–8
Block diagram of
the GSR Amp