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
®
5
4
3
2
1
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
3
1
2
4
5
7
8
11
6
9
12
10

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