National Instruments SCXI-1303 Manual de usuario

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321923A-01

INSTALLATION GUIDE

SCXI-1303 32-CHANNEL ISOTHERMAL

TERMINAL BLOCK

This guide describes how to install and use the SCXI-1303 terminal block

with SCXI-1102,SCXI-1102B, SCXI-1102C, and SCXI-1100 modules.

Introduction

The SCXI-1303 32-channel isothermal terminal block is a shielded board

with screw terminals that connect to the SCXI-1102/B/C and the

SCXI-1100 modules. The SCXI-1303 has a high-accuracy thermistor,

cold-junction temperature sensor, and an isothermal copper plane to

minimize the temperature gradients across the screw terminals when you

measure with thermocouples.

The terminal block has 78 screw terminals for easy connection. Thirty-two

pairs of screw terminals connect to the 32 differential inputs of the SCXI

modules. One pair of terminals connects to the module’s chassis ground

pins. Three terminals connect to the SCXI module OUTPUT and AOREF

pins and to the SCXIbus guard. All of the other terminals—OUT0+,

OUT0–, OUT1+, OUT1–, OUT2+, OUT2–, OUT3+, OUT3–, and

AIREF—are reserved for future use.

The terminal block has a pullup resistor connected between CH+ and +5 V

and a bias resistor connected between CH– and chassis ground. These

resistors help you detect open thermocouples by detecting saturation of the

module amplifier output.

What You Need to Get Started

To set up and use your SCXI-1303, you will need the following items:

❑SCXI-1303 32-channel isothermal terminal block

❑SCXI-1303 32-Channel Isothermal Terminal Block Installation Guide

SCXI-1303 Installation Guide 2

National Instruments Corporation

❑SCXI chassis

❑SCXI-1102/B/C or SCXI-1100 module

❑One package of four 10 MΩresistor networks

❑No. 1 and No. 2 Phillips-head screwdrivers

❑1/10 in. and 1/4in. flathead screwdrivers

❑Long-nose pliers

Changing Resistor Networks

Use long-nose pliers to remove or replace the resistor networks in the

sockets; be careful not to damage the network package. Make sure pin 1

of each network is in the correct position in the socket. Refer to Figure 4 to

locate pin 1 for each resistor network socket.

Each resistor network is labeled with descriptive numbers on the left front

side, and pin 1 is located directly beneath the darkened symbolwithin these

numbers. The 10 Ω resistor network is labeled 100(10 ×100 Ω); the 10 MΩ

resistor network is labeled 106 (10 ×106Ω). Figure 1 shows examples of

these resistors.

Figure 1. Resistor Networks

Open Thermocouple Detection

The SCXI-1303 circuitry helps you detect an open thermocouple. To detect

whether any thermocouple is open, check whether the corresponding SCXI

module channel is saturated. The SCXI-1303 has pullup and bias resistors

that saturate the channel by applying +5 V at the input of the open channel.

Notice that this will result in saturation to either of the positive or negative

rails.

SCXI-1102/B/C Module

You can replace the 10 Ωbias resistor networks (factory shipping

configuration) in the SCXI-1303 with the 10 MΩresistor networks

supplied in the kit. With the 10 MΩresistor networks, it does not matter

Mfr. code

10x-1-100

Pin 1

Mfr. code

10x-1-106

Pin 1

a. 10 ΩResistor Network b. 10 MΩResistor Network

National Instruments Corporation 3 SCXI-1303 Installation Guide

whether your signal is ground-referenced or floating. The channels with

open thermocouples will saturate at all sample rates of the module. To

replace your resistor networks, refer to the Changing Resistor Networks

section.

SCXI-1100 Module

For the open thermocouple channel to saturate without disturbing the

measurements on any other channel, use an interchannel delay of 200 µs

at a gain of 100 or higher, which corresponds to a sample rate of 5 kHz.

With the 10 Ωbias resistors installed in the SCXI-1303, you can measure

accurately at the module’s maximum sampling rate, but the open

thermocouple channel may not saturate if the interchannel delay is less

than 200 µs or if the sample rate is more than 5 kHz at a gain of 100 or

higher.

If you want fast open-thermocouple detection and you have a short

thermocouple or if high accuracy is not important, you can replace the

pullup resistors with a lower value resistor network. For example, you

could replace the pullup resistor network with a 1 MΩ, 10-pin bused

configuration resistor network (not supplied) and have a sample rate of

20 kHz (interchannel delay of 50 µs typical). With a 10 Ω bias resistor

network, the current leakage would be 5 µA (5 V/1 MΩ), which may result

in a larger offset error because of thermocouple lead resistance. To replace

your resistor networks, refer to the Changing Resistor Networks section.

Errors Due to Open-Thermocouple Detection Circuitry

Open-thermocouple detection circuitry can cause two types of

measurement errors. These errors are the results of common-mode voltage

at the input of the SCXI module and current leakage into your signal leads.

Common-Mode Voltage at the Input of the

SCXI Module

With 10 MΩpullup and bias resistors, a common-mode voltage of

2.5 VDC will develop if the thermocouple is floating. At a gain of 100,

the common-mode rejection of the SCXI-1102/B/C module is sufficiently

high that the resulting offset voltage is negligible.

If your application demands extremely high accuracy, you can eliminate

this offset error by calibrating your system. You can also remove the pullup

resistor, giving up the open-thermocouple detection feature in the process

or use the 10 Ωbias resistor networks, which will bring the common-mode

voltage down to nearly 0 VDC.

SCXI-1303 Installation Guide 4

National Instruments Corporation

Current Leakage

The open thermocouple detection circuitry results in a small current

leakage into the thermocouple. With the 10 MΩbias and pullup resistor

networks, the current leakage results in a negligible error. With the 10 Ω

bias resistor, the 10 MΩpullup resistor connected to 5 VDC causes a

current leakage of approximately 0.5 µA (5 V/10 MΩ) to flow into the

unbroken thermocouple. If the thermocouple is very long, a voltage drop

can develop in the thermocouple because of lead resistance. For example,

if you have a 24 AWG J-type thermocouple that is 20 feet long, a voltage

drop of approximately 8.78 µV (0.878 Ω/double ft ×20 double ft ×0.5 µA)

can develop in the thermocouple, which corresponds to an error of 0.18° C.

If your application demands very high accuracy, you may want to eliminate

this error by removing the appropriate pullup resistor network or by

calibrating the system offset.

Temperature Sensor Output and Accuracy

TheSCXI-1303temperature sensor outputs1.91to0.58 Vfrom0° to55° C

and has an accuracy of ±0.5° C over the 15° to 35° C range and ±0.9° C

over the 0° to 15° and 35° to 55° C ranges1.

National Instruments software can convert a thermistor voltage to the

thermistor temperature for the circuit diagram shown later in this guide. In

LabVIEW, you can use the Convert Thermistor Reading virtual instrument

(VI) in the Data Acquisition»Signal Conditioning palette. If you are

using LabWindows/CVI or NI-DAQ, use the Thermistor_Convert

function. The VI takes the output voltage of the temperature sensor, the

reference voltage, and the precision resistance and returns the thermistor

temperature.

Alternatively, you can use the following formulas:

T(° C) = TK– 273.15

where TKis the temperature in Kelvin

1 Includes the combined effects of the temperature sensor accuracy and the temperature difference between the temperature

sensor and any screw terminal. The temperature sensor accuracy includes tolerances in all component values, the effects

caused by temperature and loading, and self-heating.

TK1

ab R

()

[]

----------------------------------------------------------------=

National Instruments Corporation 5 SCXI-1303 Installation Guide

a = 1.295361 ×10–3

b = 2.343159 ×10–4

c = 1.018703 ×10–7

RT= resistance of the thermistor in ohms

VTEMPOUT = output voltage of the temperature sensor

where T(° F) and T(° C) are the temperature readings in degrees Fahrenheit

and degrees Celsius, respectively.

Notes VTEMPOUT varies from 1.91 V (at 0° C) to 0.58 V (at 55° C). For best resolution,

use the maximum gain for this signal range on the analog input channel.

The SCXI-1102/B/C has a 2 Hz filter on the VTEMPOUT signal.

The SCXI-1100 does not have a filter on the VTEMPOUT signal. Therefore, use an

average of a large number of samples to obtain the most accurate measurement.

Noisy environments require more samples for greater accuracy.

Configuring the Resistor Networks

Note A package of 10 MΩresistor networks is included in the SCXI-1303 kit. If you are

using the SCXI-1102/B/C module, you can install these resistor networks as RP1,

RP2, RP3, and RP4. With this recommended configuration, it does not matter

whether the thermocouples are ground-referenced or floating.

The SCXI-1303 terminal block has a pullup resistor connected between

CH+ and +5 V and a bias resistor connected between CH– and chassis

ground. Figure 2 shows how the pullup and bias resistors are connected to

the CH± inputs.

RT5 000 VTEMPOUT

2.5 VTEMPOUT

–

------------------------------------------





() [T(°C)]9

----------------------- 32+=

SCXI-1303 Installation Guide 6

National Instruments Corporation

Figure 2. Resistor Connections

Table 1 shows the relationship between the channel input signals and the

resistor networks.

Table 2 shows which resistor networks to use for your SCXI module, signal

type, and application.

Table 1. Channel Input Signals and Resistor Networks

Channel Pullup Resistor

Network Bias Resistor Network

0–7 RP5 RP1

8–15 RP6 RP2

16–23 RP7 RP3

24–31 RP8 RP4

CH+

CH-

CH+

CH- SCXI Module

Rpullup

(RP5, RP6, RP7, RP8)

(in sockets)

Rbias

(RP1, RP2, RP3, RP4)

(in sockets)

Screw Terminals

+5 V

National Instruments Corporation 7 SCXI-1303 Installation Guide

Warning Connecting an external ground-referenced signal with the 10 Ωbias resistor

network in place may cause permanent damage to the resistor network and the

traces on the SCXI-1303 printed circuit board. National Instruments is NOT liable

for any damage or injuries resulting from improper signal connections.

Signal Connection

When connecting your signals to the SCXI-1303, follow the labeling on the

SCXI-1303 for the appropriate module, as indicated in Figure 4.

To connect the signal to the terminal block, perform the following steps,

referring to Figures 3 and 4 as necessary:

1. Unscrew the top cover screws and remove the cover.

2. Loosen the strain-relief screws and remove the strain-relief bar.

3. Run the signal wires through the strain-relief opening. You can add

insulation or padding if necessary.

4. Prepare your signal wire by stripping the insulation no more than

7mm.

Table 2. Selecting the Appropriate Resistor Networks

Module Bias Resistor Pullup

Resistor Source

Impedance

Signal

(Floating or

Ground-

referenced)

Open

Thermocouple

Detection? Comments

SCXI-1102

/B/C 10 MΩ10 MΩLow Both Yes Recommended

configuration for the

SCXI-1102/B/C

10 Ω10 MΩLow Floating Yes Factory-shipping

configuration

10 ΩNone High or low Floating No —

None None High or low Ground-

referenced No —

SCXI-1100 10 MΩ10 MΩ— — — Not recommended

10 Ω10 MΩLow Floating Yes Factory-shipping

configuration

10 ΩNone High or low Floating No —

None None High or low Ground-

referenced No —

low source impedance ≤50 Ω

high source impedance >50 Ω

SCXI-1303 Installation Guide 8

National Instruments Corporation

5. Connect the wires to the screw terminals by inserting the stripped end

of the wire fully into the terminal. No bare wire should extend past the

screw terminal. Exposed wire increases the risk of shorting and

causing a failure.

6. Tighten the screw terminal to a torque of 5–7 in.-lb.

7. Connect your shield or earth ground to the earth ground solder lug.

8. Reinstall the strain-relief bar and tighten the strain-relief screws.

9. Reinstall the top cover and tighten the top cover screws.

10. Connect the terminal block to the module front connector as explained

in the Installation section later in this guide.

Figure 3 shows the SCXI-1303 terminal block parts locator diagram.

Figure 3. SCXI-1303 Parts Locator Diagram

1 Strain Relief Bar

2 Strain Relief Screws 3 Earth Ground

Solder Lug 4 Mating Connector

5 Thumbscrew 6 Top Cover Screws

7 Top Cover

Back View Front View

National Instruments Corporation 9 SCXI-1303 Installation Guide

Figure 4 shows the SCXI-1303 signal connections.

Figure 4. SCXI-1303 Signal Connections

Installation

Toconnect theterminal blockto theSCXImodulefrontconnector, perform

the following steps:

1. Connect the module front connector to its mating connector on the

terminal block.

2. Tighten the top and bottom thumbscrews on the back of the terminal

block to hold it securely in place.

Note For accurate cold-junction compensation, place the SCXI chassis away from an

extreme temperature differential.

1 Screw Terminals

2 Bias Resistor Networks 3 Pullup Resistor Networks 4 Product Name, Assembly

Number, Revision Letter, and

Serial Number.

- Pin 1

SCXI-1303 Installation Guide 10

National Instruments Corporation

Cleaning the Terminal Block

Clean the terminal block by brushing off light dust with a soft, nonmetallic

brush. Remove other contaminants with deionized water and a stiff

nonmetallic brush. The unit must be completely dry and free from

contaminants before returning to service.

Specifications

Cold-junction sensor

Accuracy1........................................0.5° from 15° to 35° C

0.9° from 0° to 15°

and 35° to 55° C

Repeatability....................................0.2° from 15° to 35° C

Output..............................................1.91 to 0.58 V from 0° to 55° C

Temperature Sensor Circuit Diagram

The circuit diagram in Figure 5 provides optional details about the

SCXI-1303 temperature sensor.

Figure 5. Temperature Sensor Circuit Diagram

1 Includes the combined effects of the temperature sensor accuracy and the temperature difference between the temperature

sensor and any screw terminal. The temperature sensor accuracy includes tolerances in all component values, the effects

caused by temperature and loading, and self-heating.

4.7 kΩ

5 kΩ

0.1% MTEMP

+DTEMP (Not Used)

5 kΩ

at 25oC 10 µF

16 V 0.1 µF

0.1 µF

-to

+5 V

2.5 V

BERG 2x2

LM 4040

2.5 V

0.1%

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