A-MSystems 1800 User manual

INSTRUCTION MANUAL
FOR
MICROELECTRODE AC AMPLIFIER
MODEL 1800
Serial #__________
Date____________
A-M Systems, Inc.
PO Box 850
Carlsborg, WA 98324
U.S.A.
360-683-8300 800-426-1306
FAX: [phone]
http://www.a-msystems.com
Version 7.0
Febrary 2002

NOTE
This instrument is not intended for clinical measurements using human
subjects. A-M Systems, Inc. does not assume responsibility for injury or
damage due to the misuse of this instrument.
Contents
General Description ............................................................................................................................ 1
Instrument Features ............................................................................................................................................ 1
Controls and Connectors .................................................................................................................................... 2
Operating Instructions ....................................................................................................................... 4
Typical Set-Up Procedure ................................................................................................................................... 4
Problem Solving ................................................................................................................................................. 6
Theory of Operation ........................................................................................................................... 9
Overview ............................................................................................................................................................ 9
Operational Modes ........................................................................................................................................... 10
Component Modules ......................................................................................................................................... 12
Calibration Proceedures .................................................................................................................. 13
Specifications..................................................................................................................................... 18
Warranty and Service ...................................................................................................................... 19
Each Microelectrode Amplifier
is delivered complete with:
Two Head Stage with 5 Foot Cables
Six 4 Inch Electrode Connector Cables
Rack Mount Hardware
Instructions & Maintenance Manual

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1
General Description
Instrument Features
The AC Microelectrode Amplifier Model 1800
is a two-channel, differential amplifier with
headstage probes. The instrument is intended
for extracellular recording and/or stimulating in
conjunction with high impedance metal
microelectrodes. It can be used in a number of
research or teaching applications requiring
extracellular neurophysiological recording from
excitable tissue, such as nerve, muscle (EMG), EEG,
EKG, and ERG recordings. The instrument is not
intended for clinical measurements using
humans.
Each channel of the Model 1800 contains a high-gain, low-noise differential amplifier
stage followed by low frequency, high-frequency, and notch filters. Three operating modes
are available to accomodate recording, stimulating, and verification of electrode
impedance. Record Mode offers three levels of signal gain (x100, x1000, x10 000).
Stimulus Mode allows the current passing through the electrode to be measured during
stimulation. Impedance Mode utilizes an internally calibrated current source to allow in
situ verification of electrode impedance.

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Controls and Connectors
INPUT: This 9-pin connector receives the signal from the
Headstage Probe for further processing.
STIMULUS: This 5-pin connector enables an isolated stimulus
source to be utilized to pass current through the electrode
while the channel is in Stimulate Mode. The pin assignments
are listed in the following table.
Pin Location Signal
Aupper left +
Bupper right -
Hmiddle ground
HEADSTAGE PROBE: The Headstage Probe has three sockets
and comes with three wires that will connect these sockets to
standard electode pins. PROBE+ is used to connect to the
electrode (usually an high impedance metal electrode).
PROBE- is used to connect to the indifferent lead. Either the
PROBE GND connector or the front panel GND must be
connected for proper operation. Usually the PROBE GND
connector is tied to the indifferent lead. If you desire to have
the current constrained to a known path you may want to
place the GND elsewhere. Actual GND placement depends on the application.
MODE: This rotary switch controls the operating mode for its
channel. The switch allows the user to select Record Mode
(REC) Stimulate Mode (STIM), or Impedance Check Mode
(IMP).
GAIN: This rotary switch controls the level of signal gain for its
channel while the channel is in Record Mode. The switch
allows the user to select from X100, X1000, or X10 000 gain.
This switch has no effect in Stimulate Mode and Impedance Check Mode.
CAPACITY COMP: This knob provides control over the amount of
capacitance in parallel with the electrode while the channel is in
Impedance Check Mode. Turning this knob clockwise decreases the level
of capacitance. With the electrode capacitance reduced, the measured
impedance will largely be a measure of the resistive component of the
electrode. Warning: Turning the Capacity Compensation too high will
cause the circuit to oscillate wildly and change frequency, and may also
cause the electrode to behave in a similar manner. The extreme swings

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in voltage may be harmful to neural tissue, and care should be exercised in using this
control. To a ccurate ly mea sure th e im pedanc e ve rif y a t t he OUTPUT connector that the
signal is a pure 1.0 kHz sine wave, and that the sine wave is at its maximum amplitude
(the point just before the signal becomes unstable).
NOTCH: This switch allows the Notch Filter to be included (IN) in the signal processing
path or bypassed (OUT). Warning: Although the Notch Filter provided can significantly
reduce unwanted interference from the power source, it will cause some distortion of the
signal, especially in frequencies below 100 Hz. Therefore, the Notch Filter should only
be used if other noise reduction techniques such as proper grounding and shielding are
inadequate.
LOW CUT-OFF: This rotary switch enables the user to select
the lower boundary frequency at which point the channel’s
input signal begins to be cut-off. Signals below the cut-off
frequency will be attenuated by a factor of 100 (-40 dB) per
decade decrease in the input signal frequency. For
example, if the LOW CUT-OFF switch is set at 100 HZ, then a 10
Hz signal will be attenuated by a factor of 100 while a 1 Hz
signal will be attenuated by a factor of 10,000. The Low Cut-
Off frequency should be selected based on the frequency content of the signal to be
recorded. One of the uses of this filter is to reduce slow variations or DC levels in the
input signal.
HIGH CUT-OFF: This rotary switch enables the user to select the upper boundary frequency
at which point the channel’s input signal begins to be cut-off. Signals above the cut-off
frequency will be attenuated by a factor of 100 (-40 dB) per decade increase in the input
signal frequency. For example, if the HIGH CUT-OFF switch is set at 1 KHZ, then a 10 kHz
signal will be attenuated by a factor of 100 while a 100 kHz signal will be attenuated by a
factor of 10 000. One of the uses of this filter is to reduce high-frequency noise that is
above the frequency content of the signal being recorded.
OUTPUT: This BNC connector provides the output signal from
the amplifier channel.
GND: This connector on the front panel provides access to
the circuit ground for its channel. Either the PROBE GND
connector or the front panel GND must be connected for
proper operation. Usually the PROBE GND connector is tied to
the indifferent lead. If you desire to have the current constrained to a known path you may
want to place the GND elsewhere. Actual GND placement depends on the application. For
low-noise recordings a ground connection should be made in the recording medium (i.e.
bath ground, animal ground, etc.).

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Operating Instructions
Typical Set-Up Procedure
The Head Stage Probe is used to connect the extracellular electrodes to the amplifier. The
4 inch Connector Cables can be used to connect the Probe inputs to electrodes which
have standard gold male pin connectors. The GND connector on the front panel of the
amplifier should be used as the system ground in the particular recording medium (e.g.
bath or animal ground). Note: Either the Head Stage PROBE GND or the GND on the front
panel must be connected to provide a return current path. If neither ground is
connected the Head Stage Amplifier will saturate and no signal will be measured.
Care should be taken to keep the power cables from all instruments as far away as
possible from the recording set-up. Proper grounding and shielding techniques
should be used to insure a minimum of interference.
Recording
The MODE switch should be placed in the REC position. The LOW CUT-OFF Filter, HIGH CUT-
OFF Filter, and NOTCH Filter should be set according to the frequency content of the signal
to be recorded. The GAIN Switch should be set so that the signal at the OUTPUT connector
is less than ±10 V, otherwise higher and/or lower portions of the signal may appear cut-off
or flattened.
Stimulating
The MODE switch should be placed in the STIM position. Stimulation current (I) is
monitored by measuring the voltage (V) across an internal, fixed resistor (R = 1 k) in
series with the electrodes. Since I=V/R and R is known, the voltage is a measure of the
stimulus current. The internal resistor is in series with the indifferent lead (PROBE-) so that
excessive voltages do not appear at the Probe Amplifier inputs. Therefore, to insure that
the current in the indifferent lead is equal to and opposite of that in the active lead, only
isolated stimulus sources should be used. An added benefit of isolated stimulus sources
is that they produce less stimulus artifact than ground referenced sources. The Positive
Conductor of the Stimulus connector (Pin A; upper left; red wire in the stimulation cable) is
connected through the Probe to the active electrode lead, while the Negative Conductor
(Pin B; upper right; blue wire in the stimulation cable) is connected to the indifferent
electrode lead. The shield of the stimulation cable is not internally grounded in the
amplifier and needs to be grounded at the connection to the stimulator. When an isolated
stimulator is used the PROBE- connector must be grounded to function properly.

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Checking Electrode Impedance
At any time during an experiment the impedance of one or both electrodes can be
checked by selecting IMP with the MODE switch. In this mode, an internal oscillator
automatically passes a 0.1 µA, 1 kHz sine-wave current (I) through the electrode, and the
probe amplifier monitors the resultant voltage (V) that develops across the electrode.
Since R=V/I and I is known, the monitored voltage is a measure of the electrode
impedance.

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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
AmplifierNotch
Filter
Low
Cut-OffHigh
Cut-Off
Output
IN
St
i
m
ul
u
s
OUT

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Operational Modes
Record Mode
In Record Mode, the inputs of the Probe amplifier
A1 are switched by relay R1 (in the off position),
connecting them differentially across the two
electrodes in order to amplify with a x10 gain the
neural activity appearing at the electrode-tissue
interface. In this mode relay R2 is off so that the
Stimulus leads do not feed noise into the circuit.
The differential output signal of A1 passes through the differential amplifier A100 (unity
gain), the Low Frequency Cut-Off Filter A101 (x10 gain), and the Notch Filter A102 if
activated (unity gain), in that order. Finally the signal passes through the High Frequency
Cut-Off Filter A103, whose gain selection circuit is enabled by the gain relay RL14,
offering x1, x10, or x100 added gain.
Stimulus Mode
In Stimulus Mode, the differential inputs of the
Probe amplifier A1 are switched by R1 (in
the on position) so that they are across a 1
kresistor that is in series with PROBE-, the
indifferent electrode lead. At the same time,
relay R2 turns on and RL14 turns off. Relay
RL11 connects the stimulus source at the
STIMULUS connector to the electrode leads.
A stimulus current from an isolated source
passes through the PROBE+ lead to the active
electrode, returning through the PROBE- lead and the 1 kresistor. The current creates a
voltage across the resistor which is amplified by the Probe amplifier A1 and passes
through the differential amplifier A100, whose gain is now set to x0.01 by RL11. The signal
then passes through A101 (x10 gain), A102 if activated (unity gain), and finally through
A103, which has a unity gain set by RL14 through disabling the gain selection circuit.
Impedance Mode
In Impedance Mode, the differential inputs of
the Probe amplifier A1 are switched across
the two electrode leads by R1. At the same
time, relay R2 is on, RL14 is off, and RL11 is
switched such that the internal current source
is connected to the electrode leads. The
current source generates a 0.1 mA, 1 kHz
sine wave that passes through the
electrodes, and the voltage that develops is
Differential out
Probe+
Probe -
Head stage circuitry in Record Mode
1

Differential out
Probe+
Probe -
Stimulus +
Stimulus -
Differential out
Differential +
Differential -
0.1 uA p-p Sine Wave
Head stage circuitry in Stimulus Mode
Head stage circuitry in Impedance Mode