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 optional 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
Switch 101
Resistors
Resistors
Screw Terminals
Designation
SlOl
R102, R106, RllO, R114
User-installed
J163
Function
xl/xl00 gain (channel 0)
Per channel gain resistors
Optional resistors for current to voltage
conversion
Input terminals for channels O-3
Document Number: 500-925-01 Rev. C
AIM5-1
AIM5-2
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.
R123
f-0
J163
/
Figure 2. Typical AIM5 Connections
Gain Adjustment
When shipped from the factory, the AIM5 module will have resistors installed in locations 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 appropriate 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 formula 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 conditions, 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°C.
If the temperature of the reference junction is known, however, it is not necessary that
it be maintained at 0°C. The same tables and curves will be accurate if compensation is
made for the temperature of the reference junction. This is often referred to as “cold
junction compensation”, and is achieved by adding to the voltage output of the thermocouple the voltage which would be produced by measuring the temperature of the
reference junction.
AIM5-4
For example, if the reference junction is at 25”C, and the measuring junction is at 75”C,
the thermocouple will measure a difference of 5O”C, rather than the expected 75°C.
Therefore, the output of the thermocouple will be too small. Adding the voltage
equivalent of a 25°C difference voltage output of the thermocouple will compensate for
the temperature of the reference junction.
Cold junction reference circuitry on the AIM5 measures the temperature of the
reference junction at the screw terminals, When the SELECT CHANNEL command is
loaded with the value 32, the output of the compensation circuitry can be read by the
analog-to-digital converter. Channel 32 (cold junction reference) will read 1OOmV per
degree Celsius so that at O”C, this channel will read OV, and at 5O”C, 5V.
Because this temperature/voltage relationship is linear, the voltage produced by the
compensation circuitry converts easily to temperature in software. To find the appropriate compensation voltage, consult the tables for the particular type of thermocouple being used. Find the voltage produced by that type of thermocouple at the
temperature of the reference junction, and then add the correct voltage to the reading
from the thermocouple itself.
In the previous example, consult the table to determine the compensation voltage produced at 25°C. If the thermocouple was type T (CopperlConstantan), the voltage at
25°C would be 0.99mV. Add this voltage (in software) to the voltage output of the
thermocouple.
The voltage output of thermocouples does not vary linearly with respect to
temperature. When using the AIM5 module, linearization and the conversion of the
voltage into a temperature is carried out in software. This can be done by using a
polynomial describing the specific voltage/temperature relationship for the type of thermocouple in question or by looking up the value in an appropriate table.
The coefficients for these polynomials, and the tables themselves, are readily available
from manufacturers of thermocouples and from books on the use of thermocouples.
The AIM5 is factory configured to provide a gain of xl00 volts/volt on each input channel, suitable for the connection of thermocouples types J, K, R, S, and T. Other thermocouples can be accommodated by replacing resistors Rl-R4. Modifications of this sort
are described in the section dealing with gain adjustment.
When an appropriate global gain is applied via the PGA on the AIM1 module, the A/D
converters can achieve an average resolution of less than 1 degree across the generally
accepted useful temperature range for each thermocouple type. Table 2 indicates the
average temperature represented by each digital step of the A/D converter (a 12-bit converter expect when measuring the full temperature range of a thermocouple with the
AIM5.
The table assumes that a local gain of xl00 is applied by resistors on the AIM5 module,
and a global gain is applied by the AIM1 or AMMl module, as recommended.
AIM5-5
Table 2. Average Temperature Value of Conversion Step (resolution)
Tvue
Gain WGA) t (above 0 OC)
J x2
K x2
R x5
S x5
T x10
.4323 “Cllsb .5993 “Cllsb
.6016 “Cllsb .8000 “Cllsb
.8297 “Cllsb NA
.9378 “Cllsb NA
.2051 “Cllsb .2991 “Cllsb
t (below 0 OC)
Greater resolution can be achieved with any of the following modifications: using the
thermocouple with a smaller temperature range and increasing the applied gain, setting
the A/D converter to a smaller input range (for example, 0 to 5V, when temperatures
below 0°C will not be measured), using a 14-bit AID converter (available on the ADM2
module) instead of a 12-bit converter, and replacing resistors Rl-R4 to provide a different local gain factor. When selecting a gain, remember that the maximum output
range of the AIM5 is *5V. This range can be extended to *lOV by applying a global
gain of x2 at the AIM1 module.
Type B thermocouples can be connected to the AIM5 module provided that a measurement resolution between 1 and 2°C is adequate. Type E thermocouples can also be connected to the AIM5, but should not be used to measure temperatures higher than
66O”C, or the voltage will exceed the input range of the module. Selecting other gain
factors (by installing different resistors) will eliminate these constraints (see Table 3).
Table 3. Useful Range and Associated Voltages
Type Material
B Platinum/b% Rhodium
Range (OC)
38 to 1800
Range (emf)
-0.OOlmV to l3.585mV
Platinum/30% Rhodium
0 to 982 O.OmV to 75.007mV
J”
Chromel-constantan
Iron-constantan -184 to 760 -7.508mV to 42.922mV
K Chromel-alumel -184 to 1232 -5.624mV to 50.99OmV
R Platinum-platinum/l3% 0 to 1593 O.OOmV to 18.759mV
Rhodium
S Platinum-platinum/lo% 0 to 1538 O.OOmV to 16.021mV
Rhodium
T Copper-constantan -184 to 400 -3.923mV to 9.523mV
Maintaining Accuracy
Each of the four inputs on the AIM5 module has a separate isolated input stage, each
of which feed the signal to a common output stage. Each input stage has a separate adjustment potentiometer, making it possible to null the input offset. A single adjustment
takes care of any output offset. To maximize AIM5 accuracy, the following procedure
may be used.
1. Connect a jumper wire between the + and - inputs of channel 0.
2. Set the gain calibration switch (SlOl) to the xl position. Adjust the output offset con-
trol (R119) so that a value of 0 is read back from the module.
3. Set SlOl to the xl00 position.
AIM5-6
4. Adjust the channel 0 input offset potentiometer (R104) for a 0 reading.
5. Short channels l-3 inputs and adjust respective input offset potentiometers for a zero
reading. (R108 = Chan 1, RX2 = Chan 2, R116 = Chan 3).
6. The gain potentiometer for each channel can now be adjusted for xl00 gain (5OmV input = 5v output).
RlOl = Channel 0 Gain
R105 = Channel 1 Gain
R109 = Channel 2 Gain
Rll3 = Channel 3 Gain
Input Attenuation
The AIM5 module may be modified to apply a 1OO:l attenuation factor to the input
signal, thus allowing the module to measure voltages up to k5OOV. To modify the
AIM5, install a 1OkQ resistor in the user installed location for the channel in question.
Then remove the input inductor for that channel (El02 for channel 0, El04 for channel
1, El06 for channel 2, El08 for channel 3) and in its place, install a 990kQ resistor. To
maintain system accuracy, both resistors should have a maximum tolerance of *O.l%.*
When reading data back from a modified channel, you must correct for the attenuation
factor in order to obtain a correct reading. When using commands listed in this manual,
simply multiply the voltage reading obtained by a factor of 100. When using the Soft500
system, a similar correction factor must be applied. These correction factors are exclusive of other factors necessary because of other parameters within the system such as
the global gain applied by the AIM1 module.
*May be obtained from Keithley Data Acquisition and Control. Order part number
Commands
The AIM5 module command is summarized in Table 4. Locations for slotdependent
commands are presented in Table 5.
Table 4. Commands Used with the AIM5 Module
Command
SELECT CHANNEL CMDA (Slot-dependent)
R-263-10k for the lOk0 resistor. Order R-282-90k and R-282-900k and connect in series
to make up 990kQ.
Location
AIM51
Table 5. Locations for Slot-dependent CMDA
Slot
Slot 5
Slot 4
Slot 5
Slot 6
Slot 7
Slot 8
Slot 9
Slot 10
Location
CFF84
CFF86
CFF88
CFFBA
CFWC
CFFBE
CFFBO
CFF92
SELECT CHANNEL
Location: Slot-dependent CMDA
The SELECT CHANNEL command location is used by all analog input modules to
select individual channels for AID conversion. When selecting a channel, the number of
that channel is written into the CMDA location. On the AIM5 module, four channels,
numbered O-3, can be selected with this command.
When using the AIM5 module (and also the AIM3 and AIM7), writing the value 32 to
this location directs the converter to read the output of the cold junction reference circuitry, rather than one of the input channels. The use of this information is described
in the section on thermocouples.
SELECT CHANNEL always either follows or precedes the SELECT SLOT command,
which should be issued with the number of the slot in which the AlM5 module is installed. This command affects the global multiplexer on the AIMl, and is described in
the reference section for that module.
When measuring the same channel repeatedly, the SELECT CHANNEL command need
not be reissued for every measurement. Similarly, when several channels on the same
module are read in succession, the SELECT SLOT command need only be issued once
at the start of the sequence. Both the SELECT SLOT and SELECT CHANNEL must be
issued before starting any AID conversions, or the data will be invalid (see Table 6).
Table 6. Values Written to SELECT CHANNEL
Function
Channel 0
Channel 1
Channel 2
Channel 3
Cold Junction
Binary Hex
00000
00001
00010
00011
100000
HO
Hl
H2
H3
H20
Decimal
0
1
2
3
32
AIM5-8
Use the procedure below along with the information in Figure 3 to calibrate the output
offset control (Rll8) for a reading of 0.000 *l count.
1. Place the AIM5 module in slot 4.
2. Connect the DMM high input to the AIM5 analog output (WlOl). Connect the
DMM low lead to module ground.
3. Connect jumper wires between the + and - terminals of aII four channels.
4. POKE the SELECT CHANNEL location (CFF86) with a value of 0 to select channel
0.
5. Place the calibration switch (SlOl) to the xl position and adjust the output offset
control (Rll.8) for a reading of 0.000 *l count on the DMM.
6. Place SlOl in the xl00 position and adjust the channel 0 input offset control (R104)
for a reading of O.OOO*l count on the DMM.
7. Adjust the offset controls for the remaining three channels in a similar manner.
POKE the SELECT CHANNEL location with the value for the desired channel (1 =
channel 1, 2 = channel 2, 3 = channel 3). Adjust the input offset control for a
reading of 0.000&l count on the DMM. (RlO8 = channel 1, Rll2 = channel 2, R116
= channel 3).
8. Remove the shorting jumper from channel 0 terminal, but leave SlOl in the xl00
position. Leave the inputs of the other three channels shorted.
9. Connect the calibrator high terminal to the + input of channel 0. Connect the
calibrator low terminal to the - input of channel 0.
10. POKE the SELECT CHANNEL location with a value of 0. Set the calibrator output
to a value of 5OmV
11. Adjust the channel 0 gain control for a reading of 5VrtlmV on the DMM.
12. Repeat steps lo-12 for the three remaining channels. Remove the jumper only from
the channel being calibrated. The calibrator must be connected to the channel being
calibrated, and the SELECT CHANNEL location must be POKED with the correct
value for that channel. Adjust the corresponding channel gain control for a reading
of 5V *lmV on the DMM with a 50mV input from the calibrator. Note that the inputs of the three channels not being calibrated must remain shorted.
Temperature calibration of the AIM5 may be performed as follows:
1. Connect DMM high to Wlfll. Connect DMM low to module ground. Apply thermal
grease to the temperature probe tip.
2. POKE the SELECT CHANNEL location (CFF86) with a value of 32.
3. Touch the temperature probe of the d@taI thermometer to the case of the cold junc-
tion reference IC (UlO7). Allow five minutes for the mading to stabilize.
4. Adjust the cold junction offset (Rl21) for a reading of 1OOmV x T”C on the DMh4.
For example, at 2O”C, you would adjust for 2V.
AIM5-9
COLD JUNCTION OFFSET
OUTPUT OFFSET
CHAN 3 INPUT OFFSET
n 11
PL .I
I{
/ ’
/I
/j
PC
5 @
i:; _-
R112
xIoI( t CHAN 2 INPUT OFFSET
RO4
II 1
I I -cm-R-7f.-200K
tr CHAN 2 GAIN
RP-89-IOK
-Em-.-R-309-102.5
CHAN 1 INPUT OFFSET
CHAN 1 GAIN
CHAN 0 INPUT OFFSET
CHAN 0 GAIN
AIM5-10
Figure 3. AIM5 Calibration Adjustments
Theory of Operation
See drawing number 500-176 for the AIM5 schematic diagram.
The central component of the AIM5 module is U101, a four-channel isolated input circuit (AD 2B54). UlOl contains both signal conditioning and multiplexing circuitry, providing independent resistor programmed gains for each input channel. All channels are
electrically isolated from system ground and from other input channels.
Resistors R102, R106, RllO and R114 are installed to provide xl00 gain for each channel,
but can be reconfigured to provide alternate gains. Potentiometers R101, R105, R109 and
Rll3 trim the gain of the amplifiers (internal to UlOl) of channels O-3, respectively;
potentiometers R104, R108, Rll2 and R116 trim the input offset of these amplifiers.
Potentiometer R118 provides offset trim for U101.
The output of UlOl is connected to one input of UlO5, a one-to-two analog switch
(ADG200). The second input of U105 is the output of the cold junction referencegenerator circuitry, comprised of U106 and U107. U105 is driven by U104, a quad
transparent data latch (74LS75) which stores the status of D5. The output of U105 is
routed to the AN OUT signal path leaving the module.
The temperature at the reference junction is measured by U107, a precision semiconduc-
tor temperature sensor with an output of lfi per degree centigrade. Amplifier U106B
converts this current to a voltage output of 1OOmV per degree centigrade, so that at O”C,
the output of U106B is OV, and at 5O”C, the output is 5V. U106B is driven by the +lOV
reference bus buffered by U106A. The circuit is trimmed by potentiometer Rl21.
Channel selection is controlled by U102 and U103. U103 is a quad transparent data latch
(74LS75) that stores the values of bits DO and Dl, set up by the SELECT CHANNEL
command (signal line CMDA). The output of U103 is connected to U102, a one-of-four
binary to decimal decoder (74LSl39), which produces four outputs, YO-Y3, that drive
channel selection in UlOl.
AIM5-11
AIM5 Specifications
Input channels: 4 low level, isolated from each other and ground
Input characteristics:
Gain: x100, user configurable for other gains with optional resistor
Input range:
xl, *lOOmV max
x100, f50mV max
Accuracy:
Gain: f0.2%, adjustable to 1 lsb
Gain linearity: *0.02% max
Input offset: f2OpV max, adjustable to zero
Output offset: kl2mV max, adjustable to zero
Temperature coefficient:
Gain: f .005%/“C
Input offset: *2.5~V/“C
Output offset: *5O~V/“C
Input noise voltage: 1pV p-p, O.OlHz to lOOHz, R, Ikfl
Input bias current: +8nA max
Input resistance: 1OOM
Protection: I3OV RMS max, normal mode
Isolation: 500V peak, channel to channel or channel to ground
Common mode rejection: l30dB, R, c1OOQ f 5 =60Hz
Normal mode rejection: 55db, f 1 = 50Hz
Settling time after channel selection: 2.5msec to 0.01% assuming settled input
Temperature reference junction sensor:
Output: lOOmV/“C
Accuracy: &0.25”C
Temperature coefficient: O.l°C/“C
AIM5-12
Cl050C-237-J
OR-lJ9-IOOK
J
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1
L CS-521-l
0
AIM5 COMPONENT LAYOUT
USER INSTALLED (4PLACES)
IC-42: UlO7
AIM5-WAIM5-14
I
4
m- II
AIM5 SCHEMATIC DIAGRAM
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