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AIM3
Low-Level Analog Input Module
The AIM3 Low-level Analog Input Module accepts signals ranging from &lOOmV to
flOV full scale, and offers either 32 channels of single-ended input, or 16 channels of
differential input. The selection of high-level or low-level input is made by setting on-
card switches.
The AIM3 module provides high-speed multiplexing, gain amplification from xl to xl.00
volts/volt, and cold junction reference circuitry for the direct connection of ther-
mocouples and other low-level transducers.
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selection and conditioning circuitry of the AIM1 module, where programmable gain can
be applied prior to A/D conversion.
With optional resistor locations, the AIM3 module can be modified to accept current in-
puts, or to measure thermocouples in the differential mode. In addition, a guard ter-
minal has been provided for the connection of signal shields to lower noise along input
lines.
All signals are connected directly to on-card screw terminals.
The AIM3 module may be placed in slots 3-10 of the baseboard (slots 240 when an
AMMl is used). To install the module, remove the baseboard cover and place the
module in the desired slot with the component side facing the power supply. To
minimize power supply thermal and noise effects, place the AIM3 as close to AIM1 as
possible.
CAUTION: Always turn off the system power before installing or removing modules.
To minimize the possibility of EM1 radiation, never operate the system with the top
cover removed.
User-Configured Components
Switches, optional resistors, and screw terminals are user-configured components on
the AIM3 module (See Table 1 and Figure 1).
Switches SlO3 and S104 select either the single-ended or differential mode for measur-
ing signals. Both switches must be set to the same mode for the module to function.
Single-ended and differential modes are discussed in-greater detail in the reference sec-
tion for the AIM1 module.
Document Number: 500-913-01Rev. B AlM3-1
Switches SlOl. and Sl.02 control the gain factor of the on-card instrumentation amplifier,
and can be set for gains of xl, xl0, and xl00 volts/volt. A setting for external mode is
used in conjunction with an optional resistor or a potentiometer to providean alter-
native to the gain factors offered by the switch setting. Once selected, the resistor-
programmed gain factor applies to all input channels on the AIM3 module.
Resistor locations on JY.57and Jl58 are provided for the installation of optional resistors
between the positive and negative input terminals when signals are measured in the
differential mode. With these resistors in place, the AIM3 module can be modified to
allow for current to voltage conversion or noise filtering.
Resistor locations on Jl55, Jl56, Jl59, and Jl60 permit the installation of a resistor to
ground for each channel when signals are measured in the single-ended mode. With
the appropriate resistors, the AIM3 can be modified to accept current inputs, or provide
a return path for bias currents from "floating source" signals in the differential mode.
Screw terminals banks Jl.53 and J154provide on-card signal connection for all analog in-
puts. Six terminals are available for common ground connection. `Ikvo terminals are ac-
tively driven at common mode voltage as a guard connection for cable shields.
Terminals on the AIM3 accept 16-24 gauge wire stripped 3116of an inch.
Table 1. User-Configured Components on the AIM3 Module
Name Designation Function
Switch 103 slO3 Single-ended/Diffenmtial mode selection
Switch 104 s104 SinglesndedlDifferential mode selection
Switch 101 Slol Local channel gain selection
Switch 102 s102 Local channel gain selection
Resistor User Installed Optional external gain resistor or potentiometer
DIP Headers JB7, JES Optional connection between positive and negative
input terminals (differential mode)
DIP Headers J155,J156 Optional per channel connection
J159,JEO to ground (single-ended mode)
Screw Terminals J153 Input connection for channels O-15
Screw Terminals J154 Input connection for channels 16-31 or minus in dif-
ferential mode
AlM3-2
Connections
The AIM3 module has provisions for a maximum of 32 single-ended input channels or
up to 16 differential input channels. For many applications, single-ended measurements
floated from ground are required; these measurements must be made using the dif-
ferential mode. Note that when the differential mode is used, noise common to both
input lines is reduced due to increased CMRR (Common-Mode Rejection Ratio). To
select either the differential or single-ended mode, you must set switches on the AIM3
module. Table 2 summarizes the input mode settings.
Terminal connections are marked on the board. Typical connections for the single-
ended mode are shown in Figure 2. Differential connections are shown in Figure 3.
CAUTION: To minimize the possibility of EM1 radiation, it is recommended that
shielded cable be used for input signals. Connect the shield to module ground, but
do not connect the opposite end. Maximum input voltage is lt3OV (power on), or
flOV (power off). If any input exceeds iloV, all inputs will be inoperative.
Table 2. Input Mode Switch Settings
Mode Switch Sl03 Switch S104
Single-ended* 32 32
Differential 16 16
*Factory default value
Note: Both switches must be in the same position.
Figure 2. Typical AIM3 Single-ended Connections (Channel 0 shown)
AIM34
Figure 3. Typical AIM3 Differential Connections (Channel 0 shown)
The Guard Feature
On long signal runs, shielded cable will reduce noise pickup. When using shielded
cable with the AIM3, the shield should not be connected to ground, but to the guard
terminal. This terminal is actively driven by an on-card buffer amplifier so that the ter-
minal is maintained at the common mode voltage developed by the signal source. Note
that grounding is useful only in the differential mode.
The shield, if connected in this manner, should not be connected at the transducer, and
never connected to ground at any point. Connecting the shield to ground will short-
circuit the output of the guard amplifier.
For the guard to be effective, the same shielded cable must carry both the positive and
negative leads of the signal source, and no other signal lines.
Grounding
Each input channel has an optional resistor location which permits the installation of a
resistor to ground. In the single-ended mode, locations Jls5 and Jl56 connect input ter-
minals O-15to ground, and locations Jl59 and J160 connect input terminals 16-31 to
ground. In the differential mode, Jl55 and J256 resistors connect the positive input ter-
minals of channels O-15to ground, and Jl56 and J160 resistors connect the negative ter-
minals of these same channels to ground. This provision is useful when modifying the
Series 500 to accept one or more channels of current input, or when measuring
"floating source" signals (such as thermocouples) in the differential mode.
Floating Source Signals
When signals from thermocouples, batteries, transformers, and other "floating sources"
AIM3-5
are measured in the differential mode, the signals must have a path to ground. This
path is provided by installing a resistor of less than 1OkQbetween negative input ter-
minals and ground, using resistor locations Jl59 and J160, for channels O-15and chan-
nels 16-31. These resistors are not required in the single-ended mode.
For more information about floating source signals, consult the reference section for the
AIM1 module (Single-ended and Differential Modes) and the discussion of the ther-
mocouple connection in this section.
Gain Adjustment
Switches SlOl and S102 control the gain factor of the on-card instrument amplifier, and
can be set for xl, x10 and xl00 volts/volt. Table 3 summarizes the gain settings. An op-
tional resistor or potentiometer can be installed by the user to provide a local gain, as
an alternative to the three gain factors available via Switches SlOl and S102. When the
resistor is installed, SlOl and S102 must be set to the xl mode. The chosen gain factor
is then applied to all of the 32 input channels on the AIM3 module.
The installed resistor provides a gain determined by the following formula:
G= 1 + 20,000/R
Where G equals the gain, and R equals the value of the resistor in ohms.
Once the resistor has been installed, SlOl and 5102 should not be set to another gain
factor. Although it will not damage the system, this configuration will result in un-
predictable amplification.
Holes on the AIM3 module are provided for the insertion of a resistor or potentiometer.
Table 3. Gain Switch Settings
Gain Switch S102 Switch SlOl
xl x1* x1*
Xl0 x10 xl
xl00 xl xl00
External xl xl
*Factory Default value
Current to Voltage Conversion
When connecting transducers and instrumentation with current outputs rather than
voltage outputs, resistors can be installed between positive and negative terminals to
convert the current range to an equivalent voltage range. For this purpose, locations
J157 and Jl58 are provided for channels O-31. These resistors should be installed on DIP
headers. When instrumentation provides current output in the single-ended mode,
resistors should be installed between the input terminals and ground, using locations
Jl55, Jl56, J159 and Jl.60. Determine the value of the resistor by applying Ohm's law,
AIM3-6
which describes the relationship of current and resistance to voltage:
E=I*R
Voltage (volts) = Current (amps) * Resistance (ohms)
Set E equal to the upper limit of the voltage range for the A/D converter, and I to the
upper limit of the current range for the signal being measured. R will equal the resistor
to be installed on that channel.
Consider the following example: The AID range is 0 to +lOV, and the anticipated cur-
rent input range is 4 to 20mA. E should be set to 10, and I to .02 (201lOOOA). equals
R
191.02,or 5OOQ. Thus, a 5003 resistor should be installed in the appropriate location.
Input Filtering
In some cases, it may be advantageous to filter the input to minimize noise. A single-
pole filter may be placed at the input, as described in the AIM reference section.
Connecting Thermocouples
A thermocouple is a sensor made by joining two dissimilar metals for the purpose of
temperature measurement. When dissimilar metals are joined in a closed circuit and the
two junctions held at different temperatures, a small electric current will flow around
the circuit. The electromotive force (err@ produced under such conditions is a function
of the temperature difference between the two junctions.
When thermocouples are used in temperature measurement, one junction is kept at a
known reference temperature (often the melting point of ice--0