National Instruments SCXI-1303 Installation Manual

INSTALLATION GUIDE

SCXI-1303 32-C T

ERMINALBLOCK

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.

HANNEL ISOTHERMAL

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

CVI

™,

LabVIEW ™, NI-DAQ

trade names of their respective companies.

321923A-01

™, and

SCXI ™ are trademarks of National Instruments Corporation. Product and company names are trademarks or

❑ SCXI chassis ❑ SCXI-1102/B/C or SCXI-1100 module ❑ One package of four 10 MΩ resistor networks ❑ No. 1 and No. 2 Phillips-head screwdrivers

❑

/10 in. and 1/4 in. 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 symbol within these numbers. The 10 Ω resistor network is labeled 100 (10 × 10 resistor network is labeled 106 (10 × 10 these resistors.

Ω). Figure 1 shows examples of

Ω); the 10 MΩ

10x-1-100

Mfr. code

Pin 1

a. 10 Ω Resistor Network b. 10 MΩ Resistor Network

Figure 1.

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 Ω bias resistor networks (factory shipping configuration) in the SCXI-1303 with the 10 MΩ resistor networks supplied in the kit. With the 10 MΩ resistor networks, it does not matter

SCXI-1303 Installation Guide 2

Pin 1

Resistor Networks

National Instruments Corporation

10x-1-106

Mfr. code

SCXI-1100 Module

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.

For the open thermocouple channel to saturate without disturbing th e 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 Ω bias 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 MΩ pullup 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 Ω bias resistor networks, which will bring the common-mode voltage down to nearly 0 VDC.

National Instruments Corporation 3 SCXI-1303 Installation Guide

Current Leakage

The open thermocouple detection circuitry results in a small current leakage into the thermocouple. With the 10 MΩ bias and pullup resistor networks, the current leakage results in a negligible error. With the 10 Ω bias resistor, the 10 MΩ pullup 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

The SCXI-1303 temperature sensor outputs 1.91 to 0.58 V from 0° to 55° 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 ranges

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 Con vert Thermistor Reading virtual instrument (VI) in the Data Acquisition»Signal Con di tio ning palette. If you are using LabWindows/CVI or NI-DAQ, use the function. The VI takes the output voltage of the temperature sensor, the reference voltage, and the precision resistance and returns the thermistor temperature.

Thermistor_Convert

Alternatively, you can use the following formulas:

T(° C) = T

where T

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.

SCXI-1303 Installation Guide 4

is the temperature in Kelvin

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

ab R

[]

– 273.15

()