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:
360-683-3525
http://www.a-msystems.com
Version 13.0
March 2020

Contents
General Description ............................................................................................................................1
Instrument Features .......................................................................................................................................1
Controls and Connector .......................................................................................................................................2
Operating Instructions ........................................................................................................................4
Typical Set-Up Procedure ....................................................................................................................................4
Problem Solving...................................................................................................................................................6
Theory of Operation ...........................................................................................................................9
Specifications .......................................................................................................................................8
Warranty and Service .........................................................................................................................9
Each Microelectrode Amplifier
is delivered complete with:
Two Head Stage with 5 Foot Cables
Six 4 Inch Electrode Connector Cables
Rack Mount Hardware
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.

A-M Systems 1
131 Business Park Loop, P.O. Box 850 Carlsborg, Wa 98324
Telophone: 800-426-1306 * 360-683-8300 * FAX: [phone]
E-mail: [e-mail] * Website: http://www.a-msystems.com
General Description
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 accommodate 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.
Controls and Connectors
INPUT: This 9-pin connector receives the signal from the
Headstage Probe for further processing.
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.

A-M Systems 2
131 Business Park Loop, P.O. Box 850 Carlsborg, Wa 98324
Telophone: 800-426-1306 * 360-683-8300 * FAX: [phone]
E-mail: [e-mail] * Website: http://www.a-msystems.com
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.
HEADSTAGE PROBE: The
Headstage Probe has three
sockets and comes with
three wires that will connect these sockets to standard
electrode pins. SIGNAL+ (the largest jack.. it accepts a 2mm
pin) is used to connect to the electrode (usually an high
impedance metal electrode). REFERENCE- is used to connect to
the indifferent lead. Either the PROBE GROUND connector or the
front panel GROUND must be connected for proper operation.
Usually the PROBE GROUND 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 GROUND elsewhere. Actual
GROUND 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 X10000 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 in voltage may be harmful to neural
tissue, and care should be exercised in using this control.
Pin Location Signal
A upper left +
B upper right -
H middle ground

A-M Systems 3
131 Business Park Loop, P.O. Box 850 Carlsborg, Wa 98324
Telophone: 800-426-1306 * 360-683-8300 * FAX: [phone]
E-mail: [e-mail] * Website: http://www.a-msystems.com
To accurately measure the impedance, verify at the 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.).

A-M Systems 4
131 Business Park Loop, P.O. Box 850 Carlsborg, Wa 98324
Telophone: 800-426-1306 * 360-683-8300 * FAX: [phone]
E-mail: [e-mail] * Website: http://www.a-msystems.com
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
When the MODE switch is placed in the STIM position, stimulation current (I) is
monitored by measuring the voltage (V) across an internal, fixed resistor (R = 1 kV)
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 ensure 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. 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.
Checking Electrode Impedance
When the MODE switch is placed in the IMP position, the impedance the electrode can
be checked. 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.

A-M Systems 5
131 Business Park Loop, P.O. Box 850 Carlsborg, Wa 98324
Telophone: 800-426-1306 * 360-683-8300 * FAX: [phone]
E-mail: [e-mail] * Website: http://www.a-msystems.com
Theory of Operation
Overview
The Head Stage Probe, containing a high input impedance, 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 kΩ
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

A-M Systems 6
131 Business Park Loop, P.O. Box 850 Carlsborg, Wa 98324
Telophone: 800-426-1306 * 360-683-8300 * FAX: [phone]
E-mail: [e-mail] * Website: http://www.a-msystems.com
Operational Modes
Record Mode Head stage circuitry in Record Mode
In Record Mode, the inputs of the Probe amplifier
are switched by a relay, 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 the Stimulus relay is off so that the Stimulus
leads do not feed noise
into the circuit. The differential output signal passes through the differential amplifier, the
Low Frequency Cut-Off Filter, and the Notch Filter if activated, in that order. Finally the
signal passes through the High Frequency Cut-Off Filter, whose gain selection circuit is
enabled by the gain switch.
Stimulus Mode
In Stimulus Mode, the differential inputs of the Head stage circuitry in Stimulus Mode
Probe amplifier are switched so that they are
across a 1 kΩ resistor that is in series with PROBE-,
the indifferent electrode lead. At the same time,
the stimulus source at the STIMULUS connector is
connected 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 kΩ resistor. The current creates a voltage
across the resistor which is amplified by the Probe amplifier and passes through the
differential amplifier.
Impedance Mode
In Impedance Mode, the differential inputs of Head stage circuitry in Impedance Mode
the Probe amplifier are switched across the
two electrode leads. At the same time an 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 a measure of the electrode
impedance.