National Instruments Isolated Current Output Module SCC-CO20 User manual

USER GUIDE
SCC-CO20 Isolated Current
Output Module
The SCC-CO20 is an isolated current output module that outputs up to
20 mA. The output current level is proportional to the voltage applied to
the module by an analog output channel of an E Series DAQ device.
Conventions
The following conventions are used in this guide:
<> Angle brackets that contain numbers separated by an ellipsis represent
a range of values associated with a bit or signal name—for example,
P0.<3..0>.
»The »symbol leads you through nested menu items and dialog box options
to a final action. The sequence File»Page Setup»Options directs you to
pull down the File menu, select the Page Setup item, and select Options
from the last dialog box.
This icon denotes a note, which alerts you to important information.
This icon denotes a caution, which advises you of precautions to take to
avoid injury, data loss, or a system crash. When this symbol is marked on
the product, refer to the Read Me First: Safety and Radio-Frequency
Interference document, shipped with the product, for precautions to take.
When symbol is marked on a product, it denotes a warning advising you to
take precautions to avoid electrical shock.
When symbol is marked on a product, it denotes a component that may be
hot. Touching this component may result in bodily injury.
bold Bold text denotes items that you must select or click in the software, such
as menu items and dialog box options. Bold text also denotes parameter
names.

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italic Italic text denotes variables, emphasis, a cross reference, or an introduction
to a key concept. This font also denotes text that is a placeholder for a word
or value that you must supply.
monospace Text in this font denotes text or characters that you should enter from the
keyboard, sections of code, programming examples, and syntax examples.
This font is also used for the proper names of disk drives, paths, directories,
programs, subprograms, subroutines, device names, functions, operations,
variables, filenames, and extensions.
SC-2345 SC-2345 refers to both the SC-2345 connector block and the SC-2345 with
configurable connectors.
SCC SCC refers to any SCC Series signal conditioning module.
What You Need to Get Started
To set up and use the SCC-CO20, you need the following items:
❑SC-2345/2350 with one of the following:
–SCC-PWR01
– SCC-PWR02 and PS01 power supply
– SCC-PWR03 (requires a 7 to 42 VDC power supply, not included)
❑One or two SCC-CO20 modules
❑SC-2345/2350 User Manual, available at ni.com
❑SCC-CO20 Isolated Current Output Module User Guide
❑SCC Quick Start Guide, available at ni.com
❑Read Me First: Safety and Radio-Frequency Interference
❑SC-2345 Quick Reference Label
❑68-pin E Series DAQ device, documentation, and 68-pin cable of less
than 1.5 m length
❑1/8 in. flathead screwdriver
❑Numbers 1 and 2 Phillps screwdrivers

©National Instruments Corporation 3 SCC-CO20 Isolated Current Output Module
❑Wire insulation strippers
❑NI-DAQ (current version) for Windows 2000/NT/XP/Me
Note Configuring the SCC system using Measurement & Automation Explorer (MAX) is
not supported on the Macintosh operating system.
Device Specific Information
Note For general SCC module installation and signal connection information, and
information about the SC-2350 carrier, refer to the SCC Quick Start Guide, available
for download at ni.com/manuals.
Installing the SCC-CO20
Caution Refer to the Read Me First: Safety and Radio-Frequency Interference document
before removing equipment covers or connecting/disconnecting any signal wires.
Plug the SCC-CO20 into either SC-2345 analog output socket J(X+17),
where Xis 0 or 1. The SC-2345 routes the voltage output of E Series DAQ
device analog output channel AO (X) to the SCC-CO20.

SCC-CO20 Isolated Current Output Module 4 ni.com
Connecting the SCC-CO20 to a Load
Note The signal names have changed. Refer to ni.com/info and enter rdtntg
to confirm the signal names.
The screw terminals are labeled by pin number <1..3>. Pin 1 is the ground,
pin 2 is the current sink, and pin 3 is the isolated loop supply. Figure 1
shows how the ground, sink, and supply connect to the E Series DAQ
device.
Figure 1. SCC-CO20 Signal Connections to E Series DAQ Device
AO GND
E Series
DAQ Device
AO (
X )
SCC-CO20
1
2
+
–
15 V iso
3
I

©National Instruments Corporation 5 SCC-CO20 Isolated Current Output Module
Using the Isolated Loop Supply of the SCC-CO20
You can connect the SCC-CO20 to a load using either the isolated internal
loop supply or an external supply. The internal supply outputs a voltage
of 15 V with respect to the isolated ground reference. This allows for a
maximum load resistance of 625 Ωat 20 mA. Figure 2 shows how to
connect the SCC-CO20, using the internal loop supply, to a nonreferenced
(floating) load and to a load with a high common-mode voltage.
Figure 2. Connecting the SCC-CO20 Using the Internal Loop Supply
Note If your load is grounded and you are using the internal supply, connect the load
ground to the current sink terminal at pin 2 on the SCC-CO20. Do not connect the load
ground to the isolated ground terminal because you could thereby defeat the isolation of
the module.
Driving Higher-Resistance Loads with the SCC-CO20
In order to calculate the supply voltage your application requires, multiply
the load resistance by the current you need to sink, then add 2.5 V for the
voltage drop across the protection diode and the current-sink MOSFET.
If you need a higher voltage than the 15 V provided by the internal loop
supply, you can either add a second SCC-CO20 module or provide an
external loop supply.
High
CMV
SCC-CO20
1
2
Load
+
–
15 V iso
3
0 – 20 mA
SUPPLY
SINK
I
SCC-CO20
1
2
Load
+
–
15 V iso
3
0 – 20 mA
SUPPLY
SINK
I
+
–

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Adding a Second SCC-CO20 Module
You can add another SCC-CO20 loop supply to create a supply of 30 V,
as shown in Figure 3. Do not use more than two SCC-CO20 modules on
a single circuit, because the resulting voltage can cause overheating and
damage the modules and other equipment.
Figure 3. Connecting Two SCC-CO20 Modules to a Higher-Resistance Load
SCC-CO20
1
2
+
–
15 V iso
3
SCC-CO20
1
2
+
–
15 V iso
3
Load
SUPPLY
SINK
ISO GND
SUPPLY
I
I

©National Instruments Corporation 7 SCC-CO20 Isolated Current Output Module
Using an External Loop Supply
In order for the current sink to operate properly with an external loop
supply, the voltage must be within the range of 3 to 42 V. If the voltage is
too high, overheating can occur and damage the module. Figure 4 shows
four configurations using an external loop supply.
Figure 4. Connecting the SCC-CO20 Using an External Loop Supply
High
CMV
High
CMV
1
2
Load
0 – 20 mA
SINK
+
–
SCC-CO20
ISO GND
I
a. Nonreferenced load and external voltage supply (two circuits)
b. Referenced load and external voltage supply (two circuits)
1
2
Load
0 – 20 mA
SINK
+
–
SCC-CO20
ISO GND
I
1
2
Load
0 – 20 mA
SINK
+
–
SCC-CO20
ISO GND
I
1
2
Load
0 – 20 mA
SINK
+
–
SCC-CO20
ISO GND
I
+
–
+
–

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When you use an external loop supply, the order of the supply and load does
not matter as long as you do not create a second loop through which current
flows. If both the supply and the load are grounded, connect the signals as
shown in Figure 5.
Figure 5. Connecting the SCC-CO20 to a Grounded External Loop Supply
and a Grounded Load
For information about how to configure the SCC-CO20 module using
NI-DAQmx, refer to the SCC Quick Start Guide.
Calibrating Gain and Offset Errors
The SCC-CO20 is calibrated at the factory before shipment. If you want
to adjust the gain and offset of the SCC-CO20 in your system using an
E Series DAQ device, you need a current meter (ammeter) with an
accuracy of at least ±2µA.
To adjust the gain and offset of the SCC-CO20, complete the following
steps:
1. Disconnect the load from the SCC-CO20 you want to calibrate.
2. Set an output range of 0–10 V for AO (X), the E Series DAQ device
channel connected to the SCC-CO20.
3. Connect the negative lead of the ammeter to the current sink (pin 2)
and the positive lead to the current supply (pin 3) of the SCC-CO20.
4. Using your software, have AO (X) output 10 mVDC.
5. Adjust the offset potentiometer on the top of the SCC-CO20 until the
ammeter reads 20 µA. The offset is now calibrated.
1
2
Load
0 – 20 mA
SINK
SCC-CO20
ISO GND
+
–
I

©National Instruments Corporation 9 SCC-CO20 Isolated Current Output Module
6. Using your software, have AO (X) output 9 VDC.
7. Adjust the gain potentiometer on the top of the SCC-CO20 until the
ammeter reads 18 mA. The gain is now calibrated.
Note Normally you would adjust to 0 µA in order to calibrate the offset, but because
the SCC-CO20 cannot source current, it is impossible to know when you have adjusted
past 0 µA.
Using the SCC-CO20
The SCC-CO20 is a 0 to 20 mA programmable current sink. The current
output is controlled by AO (X), a 0 to 10 V unipolar DAC output channel
of an E Series DAQ device. The relationship between the AO (X) voltage
level and the current output of the SCC-CO20 is linear and is described by
the following equation:
VAO (X) /500 =ICO20
where
VAO (X)is the AO (X) voltage level.
ICO20 is the SCC-CO20 current output in amperes.
The current sink is an N-channel power MOSFET sink to an isolated
ground reference. You can use the SCC-CO20 with industry-standard
0 to 20 or 4 to 20 mA current loops.
Note The current sink does not source current. If the input voltage is 0 V or less,
the module sinks 0 mA.
Isolated Ground Reference
Each SCC-CO20 module is referenced to its own isolated ground. Each
isolated ground can have up to 300 Vrms of common-mode voltage between
itself and any other channel ground or chassis ground. Refer to the
Connecting the SCC-CO20 to a Load section for different ways to use
the isolated ground in signal connections.

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Specifications
These ratings are typical at 25 °C unless otherwise stated.
Electrical
Number of output channels.....................1
Output referencing..................................Nonreferenced (floating)
Input range..............................................0 to 10 V
Output range ...........................................0 to 20 mA
Voltage compliance ................................12.5 V
Maximum load resistance
5 mA ................................................2,500 Ω
10 mA ..............................................1,250 Ω
15 mA ..............................................830 Ω
20 mA ..............................................625 Ω
Absolute accuracy...................................0.1% of full-scale output range
Temperature drift ....................................0.5 µA/°C typ
1µA/°C max
Power Requirement
Analog power .........................................175 mW max
+15 V ...............................................5.8 mA max
–15 V ...............................................5.8 mA max
Digital power (+5 V) ..............................645 mW
+5 V .................................................129 mA max