National Instruments Switch SCXI 1321 User Manual

™

SCXI -1321 OFFSET-NULL AND

SHUNT-CALIBRATION HIGH-VOLTAGE

TERMINAL BLOCK INSTALLATION GUIDE

This guide describes how to install and use the SCXI-1321 offset-null

and shunt-calibration terminal block with the SCXI-1121 module. You can

only use the SCXI-1321 terminal block with SCXI-1121 revision C or later

modules. In addition to the 18 screw terminals, the SCXI-1321 has circuitry

for offset-null adjustment of Wheatstone bridges, and a socketed shunt

resistor for strain-gauge shunt calibration. This terminal block was

primarily designed for Wheatstone bridge transducers such as strain

gauges, although it can easily accommodate thermocouples, RTDs,

thermistors, millivolt sources, volt sources, and current-loop receivers.

Thermocouples have cold-junction compensation (CJC) support.

Conventions

The following conventions are used in this guide:

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.

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.

LabVIEW™, National Instruments™, NI™, ni.com™, NI-DAQ™, and SCXI™ are trademarks of National Instruments

Corporation. Product and company names mentioned herein are trademarks or trade names of their respective

companies. For patents covering National Instruments products, refer to the appropriate location: Help»Patents in your

software, the patents.txtfile on your CD, or ni.com/patents.

January 2003

321924C-01

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Connecting the Signals

Note Refer to the Read Me First: Safety and Radio-Frequency Interference document

before removing equipment covers, or connecting or disconnecting any signal wires.

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

referring to Figure 1 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. Enable or bypass each of the nulling circuits, depending on the signal

you are measuring.

Note Disable the offset-nulling circuitry when you are not using a Wheatstone bridge or

when the excitation channel of the SCXI-1121 is in current mode.

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

insulation or padding if necessary.

5. Prepare the signal wire by stripping the insulation no more than 7 mm

(0.28 in.).

6. 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 a short circuit and a

hardware failure.

7. Tighten the screws to a torque of 0.57 to 0.79 newton-m (5 to 7 lb-in.).

8. Connect safety earth ground to the safety-ground lug. Refer to the

Read Me First: Safety and Radio-Frequency Interference document

for connection information.

9. Reinstall the strain-relief bar, and tighten the strain-relief screws.

10. Reinstall the top cover, and tighten the top-cover screws.

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

in the Installing the Terminal Block section.

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Back View

Front View

7 Top Cover

Strain-Relief Bar

Safety-Ground Lug

Mating Connector

Thumbscrew

Strain-Relief Screws

Top-Cover Screws

Figure 1. SCXI-1321 Parts Locator Diagram

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20 19

18 17

Screw Terminals

Product Information

W1 (CH0 Null Enabled)

W2 (CH1 Null Enabled)

R3 (CH0 Null Resistor)

R4 (CH0 Shunt Resistor)

R5 (CH1 Null Resistor)

R6 (CH1 Shunt Resistor)

R7 (CH2 Null Resistor)

15 W4 CH3 Null Enabled)

16 W3 (CH2 Null Enabled)

17 R15 (CH3 Null Potentiometer)

18 R14 (CH2 Null Potentiometer)

19 R2 (CH1 Null Potentiometer)

20 R1 (CH0 Null Potentiometer)

10 R8 (CH2 Shunt Resistor)

11 R9 (CH3 Null Resistor)

12 R10 (CH3 Shunt Resistor)

13 W5 (CJC Mode)

14 Warning Label

Note: R3 through R10 are socketed.

Figure 2. SCXI-1321 Circuit Parts Locator Diagram

Installing the Terminal Block

To connect the terminal block to the SCXI module front connector,

complete the following steps:

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

block.

2. Make sure that the module top and bottom thumbscrews do not

obstruct the rear panel of the terminal block.

3. Tighten the top and bottom screws on the back of the terminal block to

hold it securely in place.

4. Refer to the Performed or Supported Signal Conditioning section for

information on specific signal conditioning.

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Note To minimize the temperature gradient inside the terminal block, move the SCXI

chassis away from any extreme temperature differential.

Specifications

All specifications are typical at 25 °C unless otherwise specified.

Electrical

Cold-junction sensor

Accuracy¹........................................0.9 °C

Output..............................................10 mV/°C from 0 to 55 °C

Resistors

R_SHUNT.............................................301 kΩ 1%

R_NULL...............................................39 kΩ 5%

R_TRIM...............................................10 kΩ

Nulling potentiometer

Range...............................................0 to 10 kΩ

Step size...........................................infinite (user adjustable)

Mechanical

Resistor sockets

Connecting lead size........................0.023 to 0.026 in.

Connecting lead length....................0.110 to 0.175 in.

Lead spacing....................................0.500 in.

Maximum Working Voltage

Maximum working voltage refers to the signal voltage plus the

common-mode voltage.

Channel-to-earth .....................................300 V, Installation Category II

Channel-to-channel.................................300 V, Installation Category II

The temperature sensor accuracy includes tolerances in all component values caused by temperature and loading,

and self-heating.

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Environmental

Operating temperature............................ 0 to 50 °C

Storage temperature ............................... –20 to 70 °C

Humidity ................................................ 10 to 90% RH, noncondensing

Maximum altitude.................................. 2,000 meters

Pollution Degree (indoor use only)........ 2

Safety

The SCXI-1321 is designed to meet the requirements of the following

standards of safety for electrical equipment for measurement, control and

laboratory use:

•

IEC 61010-1, EN 61010-1

UL 3111-1, UL61010B-1

CAN/CSA C22.2 No. 1010.1

Note For UL and other safety certifications refer to the product label or to ni.com.

Electromagnetic Compatibility

Emissions ............................................... EN 55011 Class A at 10 meters;

FCC Part 15A above 1 GHz

Immunity................................................ EN 61326:1997 + A2:2001,

Table 1

EMC/EMI............................................... CE, C-Tick and FCC Part 15

(Class A) Compliant

Note For EMC compliance, operate this device with shielded cabling.

CE Compliance

The SCXI-1321 meets the essential requirements of applicable European

Directives, as amended for CE Marking, as follows:

Low-Voltage Directive (safety) ............. 73/23/EEC

Electromagnetic Compatibility Directive (EMC)89/336/EEC

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Note Refer to the Declaration of Conformity (DoC) for this product for any additional

regulatory compliance information. To obtain the DoC for this product, click Declarations

of Conformity Information at ni.com/hardref.nsf/.

Performed or Supported Signal Conditioning

This section provides information on types of signal conditioning

performed by the SCXI-1321 or is supported by it.

Offset Nulling

Offset nulling is a hardware nulling procedure used with

Wheatstone-bridge transducers that have an initial offset error. Correcting

this error improves measurement accuracy. The nulling circuitry operates

with quarter-bridge, half-bridge, and full-bridge strain-gauge

configurations. Each channel has its own nulling circuitry and its own

trimming potentiometer as listed in Table 1.

Table 1. Trimmer Potentiometers and Corresponding Channels

Channel Number

Trimmer Potentiometer

R14

R15

To null the static offset voltage of the bridge, complete the following steps:

1. Connect the bridge configuration to the selected channel.

2. Select and read the channel output.

3. While monitoring the output, rotate the trimmer wiper with a flathead

screwdriver until you reach 0 V.

You have nulled the bridge and are ready for a measurement.

The nulling range that is provided with the terminal block is 2.5 mV,

assuming that you have a 120 Ω strain gauge quarter-bridge configuration

and 3.3 V excitation voltage. You can change this range by replacing the

nulling resistor with a resistor of another value. Each channel has an

independent socketed nulling resistor. Therefore, you can mix the ranges to

accommodate each channel requirement. Table 2 lists the nulling resistors

and their corresponding channels.

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Table 2. Nulling Resistors and Corresponding Channels

Channel Number

Nulling Resistor

The factory default value of all the nulling resistors on the terminal block

is 39 kΩ.

Note These resistors are socketed for easy replacement. These sockets best fit a 1/4 W

resistor lead size.

To determine the nulling range, use the following formula while referring

to Figures 3 through 5:

V_exc

--------- – ----------------------------------------------------------

R_nullR_g+ R_d(R_null+ R_g)

V_excR_d(R_null+ R_g)

V_{nulling range}

where

V_excis the excitation voltage (3.3 V or 10 V)

R_dis either a completion resistor or a second strain-gauge nominal

resistance

R_nullis the nulling resistor value

(range of trim potentiometer + nulling resistor)

R_gis the nominal strain-gauge resistance value

For example,

V_{nulling range}= 2.56 mV

V_exc= 3.3 V

R_d= 120 Ω

R_null= 39 kΩ

R_g= 120 Ω

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Assuming a strain-gauge range with a gauge factor of GF = 2 and a

quarter-bridge configuration, this range corresponds to 1,498 µε, as given

by the strain formula for a quarter-bridge strain-gauge configuration:

–4V_r

ε = -------------------------------

GF(1 + 2V_r)

where

strained voltage – static unstrained voltage

V_r= -------------------------------------------------------------------------------------------------------

V_exc

SCXI-1121

SCXI-1321

Transducer

R_L

CH+

R_SCAL

–

301 kΩ

CH–

EX+

CH–

EX+

R₄(gauge)

R_L

R₁

R_Null

CH+

4.5 kΩ

–

R_Trim

10 kΩ

V_EX

39.1 kΩ

R_L

R₂

EX–

R₃(dummy)

R_Trim

(Screw Adjusts Potentiometer)

Note: R₁and R₂are completion resistors.

Figure 3. Quarter-Bridge Nulling Circuit

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SCXI-1121

SCXI-1321

Transducer

R_L

CH+

R_SCAL

–

301 kΩ

CH–

EX+

CH–

EX+

R₄(gauge)

R_L

R₁

R₂

R_Null

CH+

4.5 kΩ

R_Trim

10 kΩ

V_EX

–

39.1 kΩ

R_L

R₃(gauge)

EX–

R_Trim

(Screw Adjusts Potentiometer)

Note: R₁and R₂are completion resistors.

Figure 4. Half-Bridge Nulling Circuit

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SCXI-1121

SCXI-1321

Transducer

R_L

CH+

CH–

CH+

CH–

R_SCAL

301 kΩ

–

R_L

EX+

R₁

(gauge)

R₄

(gauge)

R_L

EX+

EX–

EX+

EX–

CH– CH+

R_Null

–

R_Trim

10 kΩ

R₂

(gauge)

R₃

(gauge)

V_EX

39.1 kΩ

R_Trim

(Screw Adjusts Potentiometer)

Figure 5. Full-Bridge Nulling Circuit

Shunt Calibration

Shunt calibration provides a method of adjusting for gain error to improve

accuracy. The shunt-calibration circuitry configuration places a shunting

resistor in parallel with the resistive element connected between EX+ and

CH+ (element R4) of the Wheatstone bridge gauge, as shown in Figure 5.

Shunt calibration circuits of each channel are independent from each other,

although they are controlled together in software; therefore, when SCAL is

engaged on a channel, all the shunt switches of the channels are closed.

When SCAL is disengaged, all the switches are open. At startup or reset,

all switches are open by default. You can control SCAL with the NI-DAQ

function SCXI_Calibrate_Setup. Set the Cal_Opparameter to 2for

engaged or 0for disengaged.

You can use LabVIEW to take measurements from channels with the shunt

resistors connected by using the SCXI channel string

obx ! scy ! mdz ! shuntw.

where

x is the onboard channel number (0 for single chassis systems)

y is the chassis ID (1 by default)

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z is the module slot of the SCXI-1121

w is the channel of the module that you want to engage the shunt and

take measurements

For example, if you want to measure the voltage at channel 0 with the shunt

resistor enabled, use the SCXI channel string

ob0 ! sc1 ! md1 ! shunt0.

You also can specify a list of channels by using

ob0 ! sc1 ! md1 ! shunt0:w, for example. Refer to the LabVIEW

Measurements Manual for information on using SCXI channel strings.

The shunting resistors R_SCALare socketed so that you can replace them with

a resistor of another value to achieve the required nulling range for your

application. The sockets and corresponding channels are shown in Table 3.

The factory installed R_SCALprovided on the terminal block have a 301 kΩ

1% value.

Table 3. Socket to Channel Relationship

Channel

Shunt Resistor Socket

R10

Assuming a quarter-bridge strain-gauge configuration with a gauge factor

of GF = 2, the equivalent strain change introduced by the R_SCALshunting

resistor is –199 µε.

Refer to the Traditional NI-DAQ User Manual for more information on

strain-gauge bridge configurations and formulas.

Use the following formula to determine the change due to this shunting

resistor:

V_exR_d(R_SCAL+ R_g)

V_change= ----------------------------------------------------------------- – -------

R_gR_SCAL+ R_d(R_SCAL+ R_g)

V_ex

Next, using the appropriate strain-gauge strain formula, and assuming that

you have no static voltage, determine the equivalent strain the R_SCALshould

provide. For example, R_SCAL= 301 kΩ and a quarter-bridge 120 Ω strain

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gauge with a gauge factor of GF = 2, V_exc= 3.3 V, and R = 120 Ω produces

the following:

V_change= 0.3321 mV

Replacing the strained voltage by V_changein the quarter-bridge strain

equation produces an equivalent 199 µε of change. Refer to the NI-DAQ

User Manual for more information on voltage to strain conversion

equations.

Cold-Junction Compensation

CJC is used only with thermocouples and provides improved accuracy of

temperature measurements. The CJC temperature sensor, mounted in the

SCXI-1321, outputs 10 mV/°C and has an accuracy of 0.9 °C over the

0 to 55 °C temperature range. To determine the temperature, use the

following formulas:

T (°C) = 100 × V_TEMPOUT

T (°C) × 9

T (°F) = ------------------------- + 32

where V_TEMPOUTis the temperature sensor output voltage, and T (°F) and

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

Celsius, respectively.

Note Use the average of a large number of samples to obtain the most accurate reading.

Noisy environments require averaging for greater accuracy. You cannot use virtual

channels to take the readings.

You can enable the CJC sensor in one of two ways depending on the input

mode configuration of the SCXI-1121. Jumper W5 switches the

temperature sensor output between MTEMP (multiplexed mode) and

DTEMP (parallel mode) modes. In MTEMP mode, you must scan the

cold-junction temperature independently of the other AI channels on the

SCXI-1121 using the LabVIEW Getting Started Analog Input VI, available

in examples\daq\run_me.llb, with the channel string

ob0 ! sc1 ! md1 ! mtemp. This reads the temperature sensor on the

terminal block connected to the module in slot 1 of SCXI chassis 1.

You then can average several measurements of the cold-junction

temperature and use this average to compensate for the cold junction of the

thermocouple. Using this averaging method compensates for temperature

variations during the measurement period and makes the CJC temperature

more accurate.

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You can read a temperature at the beginning of the test and use that value

with the data that follows. Using this method assumes that there are no

temperature variations during the measurement period. If there are

temperature variations, the measurements can be less accurate. You also

can read the cold-junction temperature once per scan of the thermocouple

channels.

Use the DTEMP mode if you are operating the SCXI-1121 in parallel

mode. DTEMP uses CH4 analog input, therefore CH3 is not available.

RTD and Thermistor Excitation

By properly setting the excitation, you can configure the SCXI-1321 on

a per channel basis for RTD and thermistor measurements. With the

SCXI-1121 excitation set in the current mode, you must disable the nulling

circuit of the channel of interest. You can do this in two steps:

1. Place the enable/disable jumper in position D (disable) as shown in

Table 4.

2. Remove the nulling resistor from its sockets.

Table 4. Jumper Settings of the Nulling Circuits

Jumper

Position

Description

Nulling circuit of Channel 0

is enabled (factory default setting)

Nulling circuit of Channel 0

is disabled

Nulling circuit of Channel 1

is enabled (factory default setting)

Nulling circuit of Channel 1

is disabled

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Table 4. Jumper Settings of the Nulling Circuits (Continued)

Jumper

Position

Description

Nulling circuit of Channel 2

is enabled (factory default setting)

Nulling circuit of Channel 2

is disabled

Nulling circuit of Channel 3

is enabled (factory default setting)

Nulling circuit of Channel 3

is disabled

*321924C-01*

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