National Instruments VXI-TB-1303 Bedienungsanleitung

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names of their respective companies.

VXI-TB-1303 32-CHANNEL ISOTHERMAL

TERMINAL BLOCK

This guide describes how to install and use the VXI-TB-1303 terminal

block with a VXI-SC submodule.

Introduction

The VXI-TB-1303 terminal block is a shielded board with screw

terminals that connect to a VXI-SC submodule. The VXI-TB-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 80 screw terminals for easy connection.

Thirty-two pairs of screw terminals connect to the 32 differential inputs

of the VXI-SC submodule. Four terminals labeled GND connect to the

submodule’s chassis ground pins. The AIREF, AOREF, GUARD,

OUTPUT, OUT0+, OUT0-, OUT1+, OUT1-, OUT2+, OUT2-, OUT3+,

and OUT3- terminals are reserved for use with future VXI-SC

submodules.

The VXI-TB-1303 terminal block has a pull-up resistor connected

between CH+ and +5 V and a ground-reference resistor connected

between CH- and chassis ground. This pull-up resistor helps you detect

open thermocouples by detecting saturation of the VXI-SC submodule

amplifier output. The ground-reference resistor references floating

thermocouples to ground.

What You Need to Get Started

To set up and use your VXI-TB-1303, you will need the following:

❑ VXI-TB-1303 terminal block

❑ VXI-TB-1303 32-Channel Isothermal Terminal Block Installation

Guide

❑ One package of four 10 Ωresistor networks

❑ VXI-TB-1000 terminal board carrier and documentation

❑ VXI-SC-1102 or other VXI-SC submodule(s) and documentation

❑ Long-nose pliers

Signal Connection and Installation

Refer to the VXI-TB-1000 Terminal Board Carrier Installation Guide for

instructions on connecting your signals and installing the VXI-TB-1303

terminal block.

Warning: Do not connect hazardous voltage levels (

±±

42 VAC peak or VDC) to this

product.

Figure 1 shows the VXI-TB-1303 terminal block parts locator diagram.

Solder Side

Component Side

1 Assembly Number 3 Product Name

2 Pin 1 of RP2 (see note below) 4 Serial Number

Figure 1. VXI-TB-1303 Parts Locator Diagram

Note: Notice that pin 1 is in the same location on each resistor network socket,

and is represented by a square, in Figure 1.

Temperature Sensor and Switch Configuration

To enable you to use thermocouples with VXI-SC submodules, the

VXI-TB-1303 terminal block has a thermistor temperature sensor for

cold-junction compensation.

You can connect the temperature sensor to a VXI-SC submodule in either

of two ways:

• Multiplexed Temperature Sensor (MTEMP) mode—Set the

VXI-TB-1303 terminal block switch S1 to the MTEMP position.

This is the factory-default setting.

• Direct Temperature Sensor (DTEMP) mode—Set the VXI-TB-1303

terminal block switch S1 to the DTEMP position. This mode

connects the temperature sensor to a separate DAQ channel via

your VXI-SC submodule. Refer to your VXI-SC submodule

documentation to configure your VXI-SC submodule for DTEMP

mode.

Table 1 shows the terminal block switch settings.

Table 1. Switch S1 Settings

Switch S1 Position Description

MTEMP

DTEMP

MTEMP mode selected; factory-default

setting; preferred mode and parking

position

MTEMP

DTEMP

DTEMP mode selected; connect to a

separate DAQ channel

Note: On the VXI-SC-1102 submodule, the MTEMP and DTEMP modes are

equivalent.

Configuring the Resistor Networks

The 10 MΩground-reference networks are recommended for use with

the VXI-SC-1102 and are factory installed.

The VXI-TB-1303 terminal block has a pull-up resistor connected

between CH+ and +5 V and has a ground-reference resistor connected

between CH- and chassis ground. Figure 2 shows how the pull-up and

ground-reference resistors are connected to the CH± inputs.

CH+

CH-

CH+

CH- VXI-SC Module

Rpull-up

(RP2, RP3, RP7, RP5)

(in sockets)

Rground-reference

(RP1, RP4, RP8, RP6)

(in sockets)

Screw Terminals

+5 V

Figure 2. Resistor Connections

Table 2 shows the relationship between the channel input signals and the

resistor networks.

Table 2. Channel Input Signals and Resistor Networks

Channel Pull-up Resistor

Network Ground-Reference

Resistor Network

0–7 RP2 RP1

8–15 RP3 RP4

16–23 RP7 RP8

24–31 RP5 RP6

Table 3 shows which resistor networks to use for your VXI-SC

submodule, signal type, and application.

Table 3. Selecting the Appropriate Resistor Networks

Ground-

Reference

Resistor

Pull-up

Resistor Source

Impedance Signal (Floating or

Ground-Referenced) Open

Thermocoup

le Detection?

Comments

10 M Ω 10 M Ω Low Both Yes Recommended

configuration for the

VXI-SC-1102. Factory-

shipping configuration.

10 Ω10 M Ω Low Floating Yes —

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

A package of 10 Ωground-reference resistor networks is included in the

VXI-TB-1303 kit. These ground-reference resistor networks may be

recommended for future VXI-SC submodules but are not recommended

for the VXI-SC-1102 submodule. You can install them as RP2, RP3,

RP7, and RP5.

Warning: Connecting an external ground-referenced signal with the 10

Ω

ΩΩ

Ω

ground-

reference resistor network in place may cause permanent damage to the

resistor network and the traces on the VXI-TB-1303 printed circuit

board. National Instruments isNOT liable for any damage or injuries

resulting from improper signal connections.

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 1

for the pin 1 location for each resistor network socket. Notice that pin 1

is in the same location on each resistor network socket, and each one is

represented by a square in the drawing.

Each resistor network is labeled with descriptive numbers on the

left front side, and pin 1 is located directly beneath the darkened

symbol within these numbers. The 10 Ω resistor network is

labeled 100 (10 x 100Ω); the 10 MΩresistor network is labeled

106 (10 x 106Ω). Figure 3 shows examples of these resistors.

Mfr. code

10x-1-100

Pin 1

a. 10 ΩResistor Network

Mfr. code

10x-1-106

Pin 1

b. 10 MΩResistor Network

Figure 3. Resistor Networks

Open-Thermocouple Detection

The VXI-TB-1303 circuitry helps you detect an open thermocouple.

To detect whether any thermocouple is open, check whether the

corresponding VXI-SC submodule channel is saturated. The

VXI-TB-1303 has pull-up and ground-reference resistors that saturate

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

will cause saturation to either of the positive or negative rails.

VXI-SC-1102 Submodule

With the 10 MΩground-reference resistor networks, it does not matter

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

open thermocouples will saturate at all sample rates of the submodule.

Errors Due to Open-Thermocouple Detection Circuitry

Open-thermocouple detection circuitry can cause two types of

measurement errors. These errors are caused by common-mode voltage

at the input of the VXI-SC submodule and current leakage into your

signal leads.

Common-Mode Voltage at the Input of the VXI-SC

Submodule

With 10 MΩpull-up and ground-reference resistor networks, 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

VXI-SC-1102 submodule is sufficiently high that the resulting offset

voltage is negligible.

However, if your application demands extremely high accuracy, you

can eliminate this offset error by calibrating your system. You can also

remove the pull-up resistor network, giving up the open-thermocouple

detection feature in the process, or use the 10 Ωground-reference resistor

networks, which will bring the common-mode voltage down to nearly

0 VDC.

Current Leakage

The open-thermocouple detection circuitry causes a small current leakage

into the thermocouple. The magnitude of this current depends on whether

your thermocouple is grounded. The following calculation uses a

20-ft 24 AWG J-type thermocouple as an example. This example

thermocouple has a resistance of 8.78 Ω per lead, based on the equation:

Rlead = 0.439 Ω/ft x 20 ft = 8.78 Ω per lead

If your thermocouple is floating, as shown in Figure 4, a leakage current

of approximately 0.25 µA (5 V /20 MΩ) will flow through both leads of

your thermocouple. The resultant measurement error will be 4.4 µV for

the example thermocouple, based on the equation:

2 leads x 8.78 Ω/lead x 0.25 µA = 4.4 µV

This corresponds to an error of 0.09° C.

2.2 µV 0.25 µA

2.2 µV

10 MΩ

+5 V

+- -

Vthermocouple Vmeas

Rlead

Figure 4. Floating Thermocouple

If your thermocouple is grounded on the negative lead, as shown in

Figure 5, or grounded on the positive lead, as shown in Figure 6, a

leakage current of approximately 0.5 µA (5 V/10 MΩ) will flow through

your thermocouple. The resultant measurement error will be 4.4 µV for

the example thermocouple, based on the equation:

1 lead x 8.78 Ω/lead x 0.5 µA = 4.4 µV

This corresponds to an error of 0.09° C.

4.39 µV 0.5 µA10 MΩ

10 MΩ

+5 V

Vthermocouple Vmeas

Rlead

Figure 5. Thermocouple with Negative Lead Grounded

4.39 µV 0.5 µA10 MΩ

10 MΩ

+5 V

thermocouple

meas

lead

Figure 6. Thermocouple with Positive Lead Grounded

You can make similar calculations when you use a 10 Ω

ground-reference resistor instead of a 10 MΩresistor.

If your application demands very high accuracy, you can eliminate

this error by removing the appropriate pull-up resistor network or by

calibrating the system offset.

Temperature Sensor Output and Accuracy

The VXI-TB-1303 temperature sensor voltage output varies from 1.91 to

0.58 V over the temperature range, as shown in Table 4.

Table 4. Temperature Sensor Voltage Output Accuracy

Temperature Range Voltage Output Accuracy*

0° to 15° C ±1° C

15° to 35° C ±0.65° C

35° to 55° C ±1° C

* 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.

To select and read the temperature sensor, refer to your LabVIEW,

LabWindows/CVI, NI-DAQ, or other software documentation for

programming information.

Alternatively, you can use the following formulas to convert the

cold-junction sensor voltage to cold-junction temperature:

T(°C) = TK- 273.15

where TKis the temperature in kelvin

a = 1.295361 x 10-3

b = 2.343159 x 10-4

c = 1.018703 x 10-7

RT= resistance of the thermistor in Ω

R5,000 V

2.5 V

TTEMPOUT

TEMPOUT

=−











Ω

VTEMPOUT = output voltage of the temperature sensor

T( F) = [T( C)] +

°°9

532

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