A-M Systems
131 Business Park Loop, P.O. Box 850 Carlsborg, Wa 98324
Telophone: 800-426-1306 * 360-683-8300 * FAX: 360-683-3525
E-mail: [email protected] * Website: http://www.a-msystems.com
6
Theory of Operation
Overview
The Head Stage Probe, containing an high input inpedance, low-noise, differential
amplifier, is connected to the Model 1800 through a 9-pin Input connector assigned to
each channel. The Mode Switch for each channel controls the input configuration of the
Probe attached to that channel as well as the operation of the subsequent stages of the
amplifier. In Record Mode, a combination of the Probe and the main circuit amplify the
signal which appears across the two electrodes attached to the Probe. In Stimulus Mode,
the signal at the Stimulus connector to be applied to the electrodes and the Probe
monitors the stimulus current by measuring the voltage developed across a 1 k
resistor
in series with the electrode. In Impedance Mode, an internally calibrated current is
passed through the Probe to one electrode, and the Probe monitors electrode impedance
by measuring the voltage that develops across the two electrodes.
The signal from the Probe passes through the differential amplifier which has a unity gain
in Record and Stimulus Modes and a gain of x0.01 in Impedance Mode. The signal then
passes through the Low Cut-Off Filter and is increased by a factor of 10. Next the signal
enters the Notch Filter if it is activated. Finally the signal passes through the High Cut-Off
Filter which has a unity gain in Stimulus and Impedance Modes and an adjustable gain
controlled by the Gain Switch while in Record Mode
Figure 2. Main circuit description and block diagram
*Only for impedance mode
**Only for record mode
X 1 0
+
-
X 1
X .01*
Mode
Rec.
Imp.
Stim.
X10
X1
x1
x10*
x100**
GAIN
Current
Source
Cap.
Comp.
He ad
S tage
Difference
Amplifier
Notch
Filter
Low
Cut-Off
High
Cut-Off
Output
IN
Stimulus
OUT
Summary of Contents for 1800
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