Tektronix AIM7 User manual
AIM7
Thermocouple Input Module
The AIM7 Thermocouple Input Module accepts signals
from thermocouple tyies J, K, S, T, B, E, and Rior generic
voltages up to L-1OOmV full scale. The AIM7 offers 16
channels of differential input, as well as guard and ground
screw terminals.
The AIM7 provides channel selection through on-card
multiplexing. Gain is fixed at 100 volts/volt. On-card cold
junction reference circuitry provides an accurate tempera-
fi~r~r~f~r~n~~fnrfharmnrnr~nlnrnnac~~rnrnnntc
C-L A/_ljbAbLLx.L AVIL
Signals from the ATM7 are routed along the AN OUT signal
path to the system global amplifier and A/D converter
where programmable gain can be applied before A/D
conversion.
All thermocouple inputs are applied to screw terminals
located on an isothermal block to minimize errors caused
by temperature differences between the input connectors
and the reference junction sensor. The cold junction reference sensor itself is mounted in the isothermal block to
accurately measure the temperature of the block. In addition, a driven guard connection has been provided for the
connection of signal shields. Guarding can minimize the
noise picked up by signal input lines.
L.L&UAAL"~"Ly/
IL A&LLUUuAL~1LA.W.
The AIM7 may be placed in slots Z-10 of the system
baseboard. However, to
and noise effects, the ATM7 should be placed in the lowest
numbered available slot. To install the AlM7 in a Model
5OOA, 5OOP, or 556, remove the baseboard top cover and
place the module in the desired slot with the component
side facing the power supply. For the Model 570 or 575,
open the system clamshell cover and place the module in
the system option slot with the components facing upward.
Always turn off the data acquisition system
power supply before installing or removing
modules. Always operate the system with the
top cover in place to minimize the possibility
of EM1 radiation.
minimize power supply thermal
CAUTION
User Configured Components
The signal input screw terminals located on the isothermal
block are the only user-configured components on the
AIM7 module. For the location of these terminals, refer to
Figure 1. Note that the terminals provide connections for
the 16 input channels as well as separate connections for
ground and guard.
Document Number: 500-917-01 Rev. D
AlM7-1
TJaermocouple Input Module
,Q@
-- ------ ---------__
PI74
‘igure 1. AIM7 Module Configurafion
Connecting Terminals
The AlM7 accepts up to 16 differential input channels.
These channels are marked channel 0 through channel 15.
Each channel has two input terminals labeled (+> and (-1 for
positive and negative inputs, respectively. Note that you
must observe proper polarity when connecting thermocouples to the AlM7 module. The positive thermocouple
wire must
be connected to the (+> terminal, and the negative thermocouple wire must be connected to the (-1 terminal. The negative TC wire is usually red. Typical connections for channel 0 are shown in Figure 2. In this example,
the shield is connected to the guard terminal; guarded connections are discussed in the following section.
Color coding of thermocouple wires varies according to
type. Table 1 gives the ANSI color codes for the various
thermocouple types supported by the Module. When
adding extension cable, use only the specific type of cable
recommended for the thermocouple being used.
Table 1. Thermocouple Color Codes
Type Material
B
Platinum 30% rhodium
(+) Wire
Gray
Platinum 6% rhodium
;
K
R
Chromel-constantan
Iron-constantan
Chrome&alumel
Platinum 13% rhodium
Purple
White
Yellow
Black
Pure platinum
S
Platinum 10% rhodium
Black
Pure platinum
r
Copper-con&man
Blue
CAUTION
Maximum input voltage is 215V (power on),
or HOV (power off). Exceeding these values
may damage the module. If any input exceeds
klOV (power on), measurement error is likely
to occur.
t-1 Wire
Red
Red
Red
Red
Red
Red
Red
AlM7-2
Thermocouple Input Module
Guarding
On long signal runs, or in electrically noisy environments,
use shielded cable to reduce noise pickup. If your temperature reading is unstable, noise induced into the thermocouple wire may be the problem.
When using shielded thermocouples with the AlM7, the
shield should be connected to one of the AIM7 guard
terminals rather than moduleground or baseboard ground.
These terminals are actively driven by an on-card buffer
amplifier, so that GUARD is maintained at the common
mode voltage of the input signal.
If the shield is connected in this manner, it should not be
connected to the thermocouple signal wires or to ground at
any point. Doing so will short-circuit the output of the
guard amplifier, possibly damaging the AIh47.
For guarding to be effective, the same shield must carry
both the positive and negative leads of the thermocouple
signal, and no other signal lines. Each input line should
have its own shield, and all shields should be connected to
one of the GUARD terminals. Figure 2 shows typical
guarded connections. Note that the shield is connected to
GUARD, while the thermocouple signal lines are con-
netted to the (+) and (-) terminals of channel 0.
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 are held at different temperatures, a small electric
current will flow through the circuit. The voltage generated under such conditions will depend on the temperature difference between the two junctions as well as the
types of metals being used.
When thermocouples are used for temperature measurement, one junction is kept at a known reference temperature (often the melting point of ice: O’C). Tables and curves
that describe the relationship of thermocouple voltage to
temperatures assume that the temperature of the reference
junction is 0°C.
If the temperature of the reference junction is known
precisely, 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. Such compensation is often referred to as “cold
junction compensation”, and is achieved by adding to the
voltage of the thermocouple the voltage which would be
produced by measuring the temperature of the reference
&
j;nction. *
Ii
+5-
$gue 2. Typical AIM7 Connections (Channel 0, Guarded Connections shown)
THERMOCOUPLE
JUNCTION
Thermocouple Input Module
For example, if the reference junction is at 25”C, and the
measuring junction is at 75”C, then the thermocouple will
measure a difference of 50’Crather than the expected 76°C.
Therefore, the output of the thermocouple will be smaller
than expected. Adding the voltage equivalent of a 25°C
difference willcompensate for the fact that the reference
junction is not actually at 0°C.
Cold junction reference circuitry located on the AIM7
measures the temperature of the reference junction at the
screw terminals. Because of the isothermal block, thermal
offsets between the measurement point and the terminals
are kept to a minimum. When the SELECT CHANNEL
command is performed with a value of 32, the voltage
output of the compensation circuitry can be read by the
A/D converter. Channel 32 (cold junction reference) will
read lOOmV/‘C. At O’C this channel will read OV, and at
5o”c, 5v.
Because the temperature/voltage relationship of the refer-
ence sensor is linear, the voltage produced by the compensation circuitry easily converts to temperature in software.
To find the appropriate conversion 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
correction voltage to the reading from the thermocouple
itself.
In the previous example, consult the table to determine the
compensation voltage at 25°C. If the thermocouple were
typeT (Copper/Cons&&an), the voltage at 25°C would be
0.992mV- Add this voltage (in software) to the actual
voltage output of the thermocouple.
This can be done by using a polynomial equation describ.
ing the specific voltage/temperature relationship for the
thermocouple in question, by looking up the correct value
in a table, or by using piecewise linear approximations.
Keithley’s various software packages perform voltage-to-
temperature conversion and linearization automatically.
The AIM7 is factory configured to provide a gain of 100
volts/volts on each of the 16 input channels. This gain
value is suitable for using thermocouple types B, E, J, K, R,
S, and T.
When an appropriate gain is applied via the PGA on the
A/D module, the A/D converter can achieve an average
resolution of l°C or better across the useful range of the
thermocouple. Table 2 indicates the average temperature
increments represented by each digital step for a 12-bit and
16-bit modules. In either case, these figures will give a good
indication of the resolution to expect when measuring the
full temperature range of the ALMY.
Resolution can be increased with any of the following
modifications:
1. usingathermocouplewithasmallertemperafurerange,
and increasing the PGA gain,
2. setting the A/D converter to a smaller input range (for
example, 0 to 1OV when temperatures below 0°C will
not be measured), or
3. by using a 16-bit A/D module instead of a 12-bit A/D
module. This will increase resolution by a factor of 16.
Unfortunately, the voltage output of thermocouples is
non-linear with respect to temperature. When using the
AIM7 module, linearization and the conversion of the
voltage into temperature must be carried out in software.
AlM7-4
Table 3 gives temperature ranges and output voltages for
the thermocouple types supported by the AIM7 module.
For all those listed, better than 1°C resolution can be
expected as long as a suitable PGA gain is used (see Table
2).
Loading...