National Instruments Module SCXI-1125 Manual

CALIBRATION PROCEDURE

SCXI™-1125

For NI-DAQ™mx

This document contains information and instructions for calibrating the

National Instruments SCXI-1125 signal conditioning module.

Contents

Conventions ............................................................................................ 1

Software .................................................................................................. 2

Documentation ........................................................................................ 2

Calibration Interval ................................................................................. 3

Password ................................................................................................. 3

Test Equipment ....................................................................................... 3

Test Conditions ....................................................................................... 3

Calibration Process ................................................................................. 4

Initial Setup...................................................................................... 4

Verification Procedure..................................................................... 5

Adjustment Procedure...................................................................... 10

Verifying Adjusted Values .............................................................. 12

Test Limits .............................................................................................. 13

Gain and Offset................................................................................ 13

SCXI-1125 Front and Rear Panel Diagrams........................................... 15

World Wide Support and Services.......................................................... 18

Conventions

The following conventions are used in this document:

»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.

Note This icon denotes a note, which alerts you to important information.

SCXI-1125 Calibration Procedure 2 ni.com

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.

italic Italic text denotes variables, emphasis, a cross-reference, or an introduction

to a key concept. Italic text 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.

monospace italic Italic text in this font denotes text that is a placeholder for a word or value

that you must supply.

Software

The SCXI-1125 calibration procedure requires the NI-DAQmx driver.

NI recommends using the latest driver that supports both your development

environment and the SCXI-1125. NI-DAQmx includes high-level function

calls to simplify the task of writing software to calibrate devices.

NI-DAQmx supports many programming languages, including LabVIEW,

LabWindows™/CVI™, C/C++, C#, and Visual Basic .NET.

Documentation

The following documents are the primary references for writing your

calibration utility:

• The NI-DAQmx Help includes information about creating applications

that use NI-DAQmx.

• The NI-DAQmx C Reference Help includes information about the

functions in NI-DAQmx.

• The DAQ Getting Started guides include information about installing

and configuring NI-DAQ devices.

You can access these documents by selecting Start»All Programs»

National Instruments»NI-DAQmx after installing NI-DAQmx.

For more information about the SCXI-1125, refer to the SCXI-1125 User

Manual. For information about installing and configuring SCXI modules,

refer to the SCXI Quick Start Guide.

©National Instruments Corporation 3 SCXI-1125 Calibration Procedure

Calibration Interval

Calibrate the SCXI-1125 at a regular interval as defined by the

measurement accuracy requirements of your application. NI recommends

that you perform a complete calibration at least once every year. You can

shorten this interval based on the accuracy requirements of your

application.

Password

The default password for password-protected operations is NI.

Test Equipment

NI recommends that you use the equipment in Table 1 to calibrate the

SCXI-1125. If these instruments are not available, use the requirements

listed in Table 1 to select a suitable substitute.

Test Conditions

Follow these guidelines to optimize the connections and the environment

during calibration:

• Keep connections to the SCXI-1125 as short as possible. Long cables

and wires can act as antennas, picking up extra noise and thermal

offsets that affect measurements.

• Use shielded copper wire for all cable connections to the SCXI-1125.

Use twisted-pair wire to eliminate noise and thermal offsets.

• Maintain the ambient temperature between 18 °C and 28 °C.

• Keep relative humidity below 80%.

• Allow a warm-up time of at least 15 minutes for the SCXI-1125 and

30 minutes for the DAQ device to ensure that the measurement

circuitry is at a stable operating temperature.

Table 1. Test Equipment

Equipment Recommended Model Requirements

Calibrator Fluke 5700A 50 ppm

DAQ Device NI 6281 16-bit minimum

DMM NI 4070 or Agilent 34401A 6 1/2 digit, 15 ppm

Terminal Block SCXI-1320 —

SCXI-1125 Calibration Procedure 4 ni.com

Calibration Process

The calibration process has the following steps:

1. Initial Setup—Configure the SCXI-1125 for calibration.

2. Verification Procedure—Verify the existing operation of the

SCXI-1125. This step determines whether the SCXI-1125 is operating

within its test limits.

3. Adjustment Procedure—Perform an external calibration that adjusts

the SCXI-1125 calibration constants with respect to a known voltage

source.

4. Verifying Adjusted Values—Perform another verification to ensure

that the SCXI-1125 is operating within its test limits after adjustments.

Initial Setup

Complete the following steps to configure the SCXI-1125 for calibration:

1. Make sure that all the appropriate driver and application software is

installed.

2. Make sure all components involved in the calibration procedure are

powered off.

©National Instruments Corporation 5 SCXI-1125 Calibration Procedure

3. Assemble the SCXI-1125, SCXI chassis, terminal blocks, and DAQ

device as shown in Figure . You must cable the SCXI-1125 directly to

the DAQ device.

Figure 1. Calibration Connections

4. Power on the SCXI chassis and the external computer.

5. You must configure the hardware properly with Measurement &

Automation Explorer (MAX). Refer to the SCXI Quick Start Guide for

details about configuring the SCXI chassis.

Note When you configure a device in MAX, the device is assigned a device identifier. In

this calibration document, the functions used for demonstrating the calibration process use

the device identifiers Dev1 for the DAQ device and SC1Mod1 for the SCXI-1125.

1 SCXI-1320 Terminal Block

2 SCXI-1125 Module

3 SCXI Chassis

4 SCXI-1349 Cable Adapter

5 Shielded 68-Pin Cable

6 NB1 Cable (50-Pin Ribbon Cable)

7 CB-50 Connector Block

8 Cable to DMM

9 Cable to Calibrator

10 Calibrator

11 DMM

12 DAQ Device

ADDRESS

SCXI 1100

246 8 10 12 14 16 18 20 20 24 26 28 30 32 34 36 38 40 42 44 46 48 50

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49

SCXI-1125 Calibration Procedure 6 ni.com

Verification Procedure

The verification procedure determines how well the SCXI-1125 is meeting

its test limits. You can use this information to select the appropriate

calibration interval for your application.

The SCXI-1125 stores gain and offset calibration constants for each gain

setting per analog input channel. A complete verification of the inputs

involves measuring the accuracy at all possible gain settings on all channels

of the SCXI-1125.

To view the calibration VIs in LabVIEW, select Measurement I/O»

DAQmx - Data Acquisition»DAQmx Advanced»DAQmx Calibration

on the Functions palette. You can also use the Search button, shown in

Figure , to look for a specific VI.

Figure 2. DAQmx Calibration Palette and the Search Button

©National Instruments Corporation 7 SCXI-1125 Calibration Procedure

Complete the following steps to verify the accuracy of each SCXI-1125

analog input channel:

1. Use the DAQmx Self Calibrate VI to verify the DAQ device. This VI

measures the onboard reference voltage of the DAQ device and adjusts

the self-calibration constants to account for any errors caused by

short-term fluctuations in the operating environment.

Note Throughout the verification procedure, refer to the function call parameters for the

LabVIEW input values.

2. Connect the calibrator output to pins AI- and AI+ for all channels.

Refer to the SCXI-1320 High-Voltage Terminal Block Installation

Guide and Specifications for connection instructions. If you are not

using the recommended SCXI-1320, refer to Table 4 for the front

signal pin assignments of the SCXI-1125. If the calibration signal

source is floating, connect AI- to the SCXI chassis ground using the

ground lug available on the SCXI terminal block.

3. Set the calibrator voltage to the desired test point indicated in Table 2.

4. Create a task using DAQmxCreateTask.

LabVIEW Block Diagram NI-DAQmx Function Call

Call DAQmxSelfCal with the following

parameter:

deviceName: "Dev1"

LabVIEW Block Diagram NI-DAQmx Function Call

LabVIEW does not require this step. Call DAQmxCreateTask with the following

parameters:

taskName: "AIVerificationTask"

taskHandle: &taskHandle

SCXI-1125 Calibration Procedure 8 ni.com

5. Add a voltage channel using the DAQmx Create Virtual Channel VI.

6. Set the SCXI-1125 to a gain setting that corresponds to the current test

point in Table 2 using the DAQmx Channel Property Node. You can

find the AI.Gain property by selecting Analog Input»General

Properties»Advanced»Gain.

LabVIEW Block Diagram NI-DAQmx Function Call

Call DAQmxCreateAIVoltageChan with the

following parameters:

taskHandle: taskHandle

physicalChannel: "SC1Mod1/aiX"†

nameToAssignToChannel: "myVoltageChannel"

terminalConfig: DAQmx_Val_Cfg_Default

minVal: -5 V

maxVal: 5 V

units: DAQmx_Val_Volts

customScaleName: ""

† Where Xrefers to the channel you are verifying.

LabVIEW Block Diagram NI-DAQmx Function Call

Call DAQmxSetAIGain with the following

parameters:

taskHandle: taskHandle

channel: ""

data: (gain value for particular test

point) float64

©National Instruments Corporation 9 SCXI-1125 Calibration Procedure

7. Configure timing for the voltage acquisition using the DAQmx

Timing VI.

8. Commit the verification changes using the DAQmx Control Task VI.

9. For each new test point, use the Time Delay Express VI to account for

the 5 second settling time.

LabVIEW Block Diagram NI-DAQmx Function Call

Call DAQmxCfgSampClkTiming with the following

parameters:

taskHandle: taskHandle

source: ""

rate: 1000.0

activeEdge: DAQmx_Val_Rising

sampleMode: DAQmx_Val_FiniteSamps

sampsPerChanToAcquire: 1000

LabVIEW Block Diagram NI-DAQmx Function Call

Call DAQmxTaskControl with the following

parameters:

taskHandle: taskHandle

action: DAQmx_Val_Task_Commit

LabVIEW Block Diagram Function Call

Call a sleep or wait function to suspend

execution for the necessary settling time.

Delay Time: 5

SCXI-1125 Calibration Procedure 10 ni.com

10. Acquire 1000 points of voltage data using the Analog 1D DBL 1Chan

NSamp instance of the DAQmx Read VI.

11. Compute the mean of the 1000 voltage values that you acquired.

12. Compare the resulting average to the upper and lower limits listed in

Table 2. If the result is between the upper and lower limits, the

SCXI-1125 passes the test.

13. Clear the acquisition using the DAQmx Clear Task VI.

14. Repeat steps 4 through 13 for the remaining channels.

15. Repeat steps 3 through 14 for all test points in Table 2.

You have finished verifying the analog input accuracy of the SCXI-1125.

Adjustment Procedure

The adjustment procedure adjusts the gain and offset calibration constants

on the SCXI-1125. Complete the Verification Procedure prior to the

adjustment procedure to determine the pre-calibration accuracy and

whether adjustments are necessary.

LabVIEW Block Diagram NI-DAQmx Function Call

Call DAQmxReadAnalogF64 with the following

parameters:

taskHandle: taskHandle

numSampsPerChan: –1

timeout: 10.0

fillMode: DAQmx_Val_GroupByChannel

readArray: data

arraySizeInSamples: 1000

sampsPerChanRead: &read

reserved: NULL

LabVIEW Block Diagram NI-DAQmx Function Call

Call DAQmxClearTask with

the following parameter:

taskHandle: taskHandle

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