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AIM5
Isolated Low-Level Analog Input Module
The AIM5 Isolated Low-level Analog Input Module is designed to accept inputs from
+5mV to *lOOmV, including signals from thermocouples. Isolated input permits the
connection of signals with high common mode voltages--up to 500V DC or peak AC--
and provides normal mode input protection up to l3OV. Channel-to-chaimel isolation is
500V DC or peak AC. The input bandwidth is below 2Hz, reducing sensitivity to 50/60
cycle AC noise.
There are four input channels on the AIM5 module, each with individually adjustable
gain amplification. The module is shipped from the factory with a preset gain of xl00
volts/volt, appropriate for thermocouple types B,E,J,K,R,S, and T. This gain can be reset
for each channel by changing the value of the gain-setting resistor.
The AIM5 is equipped with cold junction reference circuitry, which allows accurate
measurement of the junction temperature, eliminating the need for ice-bath reference
junctions.
The AIM5 may be placed in slots 3-10 of the baseboard (slots 2-10 if AMMl is used). To
install the module, remove the top cover of the mainframe and insert the module in the
desired slot with the component side facing the power supply. Place the AIM5 as far
away from the power supply as possible to minimize noise or thermal effects.
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. Short the input terminals of unused channels together.
User-Configured Components
Overall gain is factory set to x100. Gain amplification for each channel can be modified
by changing resistors in the locations provided on the AIM5 module card. Installing op-
tional resistors provides current to voltage conversion, allowing connection of current
loop inputs. All inputs are connected to a bank of screw terminals, with positive and
negative (high and low) terminals for each of the four input channels (see Table 1 and
Figure 1).
Table 1. Summary of User-Configured Components
Name Designation Function
Switch 101 SlOl xl/xl00 gain (channel 0)
Resistors R102, R106, RllO, R114 Per channel gain resistors
Resistors User-installed Optional resistors for current to voltage
conversion
Screw Terminals J163 Input terminals for channels O-3
Document Number: 500-925-01 Rev. C AIM5-1
Figure 1. AIM5 Module Configuration
Connections
Connecting terminals for the AIM5 are marked on the module board. Figure 2 shows
typical connections. The use of shielded cable for input connections is recommended to
minimize noise pickup and the possibility of EM1 radiation. Connect the shield to AIM5
ground only.
WARNING: Dangerous user-supplied voltages may be present on the input terminals.
CAUTION: Maximum input voltage is 50mV with xl00 gain.
AIM5-2
R123
f-0
J163
/
Figure 2. Typical AIM5 Connections
Gain Adjustment
When shipped from the factory, the AIM5 module will have resistors installed in loca-
tions R102, R106, RllO, and R114, providing a gain factor of xl00 for each input channel.
The xl00 gain enables the AIM5 module to accommodate thermocouples of types
B,E,J,K,R,S, and T, as well as other low-level signals with similar ranges. These resistors
can be removed or replaced to provide alternate gains.
The following formula describes the relationship of the gain to resistance in this circuit:
G= 1 + 10,000/R,
Where G equals the gain, and R, the total resistance.
To provide accurate trimming for this gain, potentiometers Pl, P3, P5, and P7 have been
installed parallel to resistors Rl-R4. The total resistance for any channel is a function of
both the value of the resistor and the value of the potentiometer installed on that chan-
nel. This relationship is described by the following formula:
RA
R, =
R, + R,
Where RT is the effective parallel resistance, R, is the value of the gain resistor, and RP
is the value of the potentiometer.
Current to Voltage Conversion
With the installation of optional resistors in user-installed locations the AIM5 can be
AIM53
configured to accept floating current loop inputs. The resistors are installed between the
high and low input terminals, converting the current range of the signal to an ap-
propriate voltage range.
Because the AIM5 module can accept input signals with a full scale range of *5mV to
*lOOmV, the resistor installed should yield a voltage range of *lOOmV. A local gain of
x50 can then be applied via resistors to bring the voltage output to a range of f5V, the
maximum output range of the module. If the input range of the A/D converter is set to
*lOV, the PGA on the AIM1 module can be used to apply a supplementary gain of x2.
The selection of user-installed resistor should be based on Ohms law:
E=I*R
Voltage (volts) = current (amps)* Resistance (ohms)
Where E is set to lOOmV, and I to the maximum value of the current input range. If, for
example, the range of the input signal is 4-2OmA, I should be set to 2OmA, and the for-
mula solved as follows:
1OOmV = 20mA* R
R=5
Thus, a 5Q resistor should be installed on that channel.
Note that additional loads in series with the current loop can be connected on either
side of the isolated input.
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 (emf) 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: O'C). Under these condi-
tions, the emf is a function of the temperature at the second junction (the measuring
junction). Tables and curves that describe the relationship of the voltage produced l+
the thermocouple to the measured temperature assume that the temperature of the
reference junction is 0