National Instruments Isolated Current Output Module SCC-CO20 Manuale utente

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.

SCC-CO20 Isolated Current Output Module 2 ni.com

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 (

)

SCC-CO20

–

15 V iso

©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

Load

–

15 V iso

–

20 mA

SUPPLY

SINK

SCC-CO20

Load

–

15 V iso

–

20 mA

SUPPLY

SINK

–

SCC-CO20 Isolated Current Output Module 6 ni.com

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

–

15 V iso

SCC-CO20

–

15 V iso

Load

SUPPLY

SINK

ISO GND

SUPPLY

©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

Load

–

20 mA

SINK

–

SCC-CO20

ISO GND

a. Nonreferenced load and external voltage supply (two circuits)

b. Referenced load and external voltage supply (two circuits)

Load

–

20 mA

SINK

–

SCC-CO20

ISO GND

Load

–

20 mA

SINK

–

SCC-CO20

ISO GND

Load

–

20 mA

SINK

–

SCC-CO20

ISO GND

–

SCC-CO20 Isolated Current Output Module 8 ni.com

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.

Load

–

20 mA

SINK

SCC-CO20

ISO GND

–

©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.

SCC-CO20 Isolated Current Output Module 10 ni.com

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

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