National Instruments 9237 Manual

NI 9237 Calibration Procedure
This document contains information about calibrating National
Instruments 9237 modules using NI-DAQmx. This calibration procedure is
intended for metrology labs.
This document does not discuss programming techniques or compiler
configuration. The NI-DAQmx driver contains online help files that have
compiler-specific instructions and detailed function explanations. You can
install these help files when you install NI-DAQmx on the calibration
computer.
Contents
Conventions ............................................................................................ 2
Software .................................................................................................. 2
Documentation ........................................................................................ 3
Calibration Interval ................................................................................. 3
Test Equipment ....................................................................................... 4
Test Conditions ....................................................................................... 4
Calibration Procedure ............................................................................. 5
Calibration Process Overview ......................................................... 5
Initial Setup...................................................................................... 5
Verification Procedure ..................................................................... 6
Specifications .......................................................................................... 12
Test Limits .............................................................................................. 12
Using the Table................................................................................ 12
Excitation Setting Value........................................................... 12
Calibrator Output ...................................................................... 12
Range ........................................................................................ 12
Test Point Value ....................................................................... 13
1-Year Absolute Error .............................................................. 13
Where to Go for Support......................................................................... 14

NI 9237 Calibration Procedure 2 ni.com
Conventions
The following conventions appear in this manual:
»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.
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 and hardware labels.
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 Monospace text 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
Install NI-DAQmx 8.1 or later on the calibration computer. NI-DAQmx
includes high-level function calls to simplify the task of writing software to
calibrate devices. You must have the proper device driver installed on the
calibration system before calibrating the device.
Note NI recommends that you install the NI-DAQmx driver software before physically
installing the NI 9237. NI-DAQmx, available at ni.com/downloads, configures and
controls the NI 9237.
NI-DAQmx supports a number of programming languages, including
LabVIEW, LabWindows™/CVI™, Microsoft Visual C++ 6.0, Microsoft
Visual Basic 6.0, Microsoft .NET, and Borland C++.
You can access the NI-DAQmx header file, NIDAQmx.h, like any standard
library. You can find examples of how to use the NI-DAQmx driver in the

©National Instruments Corporation 3 NI 9237 Calibration Procedure
Program Files\National Instruments\NI-DAQ\Examples
directory.
Documentation
You might find the following documentation helpful as you write the
calibration procedure:
•NI-DAQmx Help—This help file contains general information about
measurement concepts, key NI-DAQmx concepts, and common
applications that apply to all programming environments. To access
this help file, select Start»All Programs»National Instruments»
NI-DAQ»NI-DAQmx Help.
•NI-DAQmx C Reference Help—This help file contains C reference and
general information about measurement concepts. To access this help
file, select Start»All Programs»National Instruments»NI-DAQ»
NI-DAQmx C Reference Help.
•DAQ Getting Started Guide for NI-DAQ 8.0 or later—This guide
describes how to install NI-DAQmx for Windows software and
NI-DAQmx-supported DAQ devices, and how to confirm that your
device is operating properly. To access this guide, select Start»
All Programs»National Instruments»NI-DAQ»DAQ Getting
Started Guide.
Note The documents above are installed with NI-DAQmx. You can also download the
latest versions from the NI Web site at ni.com/manuals.
•NI 9237 Operating Instructions—This document describes how to use
the NI 9237 and includes specifications and terminal assignments for
the NI 9237. You can download the latest version of this document
from the NI Web site at ni.com/manuals.
Calibration Interval
The NI 9237 should be calibrated at a regular interval as defined by
the measurement accuracy requirements of your application. National
Instruments recommends that you routinely perform a complete calibration
at least once every year. You can shorten this interval based on the accuracy
demands of your application or requirements of your processes. Adjusting
the calibration constants must be done by a factory calibration at NI.

NI 9237 Calibration Procedure 4 ni.com
Test Equipment
National Instruments recommends that you use the following equipment
for calibrating the NI 9237.
Test Conditions
Follow these guidelines to optimize the connections and the environment:
• Keep connections to the device as short as possible. Long cables and
wires act as antennae, picking up extra noise that can affect
measurements.
• Use shielded copper wire for all cable connections to the device.
Use twisted-pair wire to eliminate noise and thermal offsets.
• Maintain an ambient temperature of 23 ±10 °C. The device
temperature will be greater than the ambient temperature.
• Keep relative humidity below 80%.
• Allow a warm-up time of at least 10 minutes to ensure that the
measurement circuitry is at a stable operating temperature.
Table 1. Recommended Equipment
Equipment Recommended Model Requirements
Calibrator Fluke 5700A If this instrument is unavailable, use a
high-precision voltage source with an
accuracy of at least 10 ppm and an
output impedance of less than or equal
to 50 Ω.
DMM NI 4070 DMM If this instrument is unavailable, use a
multiranging 6 ½ digit DMM with an
accuracy of at least 40 ppm.
Chassis NI cDAQ-9172 —
Connection accessories RJ-50, RJ-50 cable Use an RJ-50 10-position connector
plug and an RJ-50 cable.
Bridge completion
accessories
NI 9949, 350 Ωresistors Use one NI 9949 screw terminal
accessory and two 350 Ωresistors with
at least 1% tolerance.

©National Instruments Corporation 5 NI 9237 Calibration Procedure
Calibration Procedure
This section provides instructions for verifying the performance of the
NI 9237.
Calibration Process Overview
The calibration process consists of the following steps:
1. Initial Setup—Configure the device in NI-DAQmx.
2. Verification Procedure—Verify the existing operation of the device.
This step confirms whether the device was operating within its
specified range prior to calibration.
3. Adjustment—Submit the device to NI for a factory calibration to adjust
the calibration constants.
4. Verification Procedure—Perform another verification to ensure that
the device operates within its specifications after adjustment.
The first two steps are explained in the following sections.
Initial Setup
You must configure the device in Measurement & Automation Explorer
(MAX) to communicate with NI-DAQmx.
Complete the following steps to configure a device in MAX.
1. Install the NI-DAQmx driver software.
2. Make sure that no power is connected to the module terminals. If the
system is in a nonhazardous location, the chassis power can be on
when you install the module.
3. Insert the module into an available slot in the cDAQ-9172 chassis.
4. Launch MAX.
5. Right-click the device name and select Self-Test to ensure that the
device is working properly.
Note When a device is configured with MAX, it is assigned a device name. Each function
call uses this device name to determine which DAQ device to calibrate. This document uses
dev1 to refer to the device name. In the following procedures, use the device name as it
appears in MAX.

NI 9237 Calibration Procedure 6 ni.com
Verification Procedure
Verification determines how well the device is meeting its specifications.
By completing this procedure, you can see how the device has drifted over
time, which helps you determine the appropriate calibration interval for
your application. Table 4 in the Test Limits section shows all acceptable
settings for the device type. Throughout the verification process, use
Table 4 to determine if the device is operating within its specified range.
You must perform verification on all analog input channels.
Complete the following steps to test the performance of the device.
1. Make the following connections on the NI 9949 terminal block:
• Insert one leg of each 350 Ωresistor into AI–.
• Insert the second leg of one 350 Ωresistor into EX+.
• Insert the second leg of the other 350 Ωresistor into EX–.
• Connect RS+ to EX+.
• Connect RS– to EX–.
• Connect the positive input of the DMM to RS+.
• Connect the negative input of the DMM to RS–.
• Connect the positive output of the calibrator to AI+.
• Connect the negative output of the calibrator to AI–.
• Leave the SC, T+, and T– terminals unconnected.
• If the calibrator has a separate guard connection not connected to
LO, connect that terminal to AI–.
Refer to Table 2 for the pin assignments and signal names of the NI 9237.
Refer to Figure 1 for an illustration of the NI 9949 connections.

©National Instruments Corporation 7 NI 9237 Calibration Procedure
Figure 1. NI 9949 Connections
Table 2. Pin Assignments and Signal Names
Module RJ-50 Pin Signal Name Signal Description
1SC Shunt calibration
2AI+ Positive analog input
3AI– Negative analog input
4RS+ Positive remote sense input
5RS– Negative remote sense input
6EX+ Positive excitation output*
7EX– Negative excitation output*
8T+ TEDS DATA
9T– TEDS Return*
10 SC Shunt calibration
* These signals are shared by all four RJ-50 connectors on the NI 9237.
–+
–
+
EX+
RS+
RS–
EX–
DMM
AI–
Calibrator
Guard
AI+

NI 9237 Calibration Procedure 8 ni.com
2. Set the calibrator voltage to the Calibrator Output value indicated in
Table 4. NI recommends that you verify all values, although you can
save time by verifying only the values used in your application.
3. If you use C function calls, create a task using DAQmxCreateTask, as
shown in the following table. If you use LabVIEW, skip this step. The
task is created in step 4 in LabVIEW.
LabVIEW Block Diagram NI-DAQmx Function Call
LabVIEW does not require this step.
Call DAQmxCreateTask with
the following parameters:
taskName:
AIVerificationTask
taskHandle: &taskHandle

©National Instruments Corporation 9 NI 9237 Calibration Procedure
4. Create and configure an AI Custom Voltage with Excitation channel
using the DAQmx Create Virtual Channel VI, as shown in the
following table. Use Table 4 to determine the minimum and maximum
values for the device.
Note Throughout the procedure, refer to the NI-DAQmx function parameters for the
LabVIEW input values.
LabVIEW Block Diagram NI-DAQmx Function Call
Call
DAQmxCreateAIVoltageChan
WithExcit with the following
parameters:
taskHandle: taskHandle
physicalChannel: dev1/ai0
nameToAssignToChannel:
myVoltageChannel
terminalConfig:
DAQmx_Val_Cfg_Default
minVal: –0.025
maxVal: 0.025
units: DAQmx_Val_Volts
bridgeConfig:
DAQmx_Val_FullBridge
voltageExcitSource:
DAQmx_Val_Internal
voltageExcitVal:*2.5, 3.3,
5.0, or 10.0
useExcitforScaling: TRUE
customScaleName: NULL
*Use the excitation value that corresponds to the calibrator voltage value you set in step 2. Refer to Table 4 for corresponding
excitation setting and calibrator output values.

NI 9237 Calibration Procedure 10 ni.com
5. Configure the timing properties for the voltage acquisition using the
DAQmx Timing VI, as shown in the following table.
6. Start the acquisition using the DAQmx Start Task VI, as shown in the
following table.
LabVIEW Block Diagram NI-DAQmx Function Call
Call DAQmxCfgSampClkTiming
with the following parameters:
taskHandle: taskHandle
source: NULL
rate: 50000
activeEdge:
DAQmx_Val_Rising
sampleMode:
DAQmx_Val_FiniteSamps
sampsPerChan: 10000
LabVIEW Block Diagram NI-DAQmx Function Call
Call DAQmxStartTask with the
following parameter:
taskHandle: taskHandle

©National Instruments Corporation 11 NI 9237 Calibration Procedure
7. Acquire and average 10,000 points of voltage data using the DAQmx
Read VI, as shown in the following table.
8. Take a reading of the excitation value from the DMM. You must take
this reading for each excitation value that you enter in step 4.
9. Calculate the upper and lower limits using the following equations:
UpperLimit = (CalibratorOutputSetting / DMMReading) +
AbsoluteErrorfromTable4
LowerLimit = (CalibratorOutputSetting / DMMReading) –
AbsoluteErrorfromTable4
10. Compare the result from step 7 to the upper limit and lower limit
values you calculated in step 9. If the result is between these values, the
device passes the test.
11. Clear the acquisition using the DAQmx Clear Task VI, as shown in the
following table.
12. Repeat steps 2 through 11 for all voltage values, all excitation values,
and all channels.
13. Disconnect the calibrator and the DMM from the device.
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: 10000
sampsPerChanRead: &read
reserved: NULL
LabVIEW Block Diagram NI-DAQmx Function Call
Call DAQmxClearTask with the
following parameter:
taskHandle: taskHandle

NI 9237 Calibration Procedure 12 ni.com
Specifications
The values in the following table are based on calibrated scaling
coefficients, which are stored in the onboard EEPROM. The following
calibration specifications are for 23 ±10 °C.
Test Limits
Table 4 lists the specifications that the NI 9237 should meet if it has been
one year between calibrations.
Using the Table
The following definitions describe how to use the information from
Table 4.
Excitation Setting Value
The Excitation Setting Value is the voltage value to enter in the DAQmx
Create Virtual Channel VI in step 4.
Calibrator Output
The Calibrator Output is the voltage value that you source into the NI 9237
for verification purposes. This value is broken down into two
columns—Location and Value. Location refers to where the value fits
within the test range. Value refers to the voltage value to be verified. Max
refers to maximum value, Min refers to minimum value, and Mid refers to
mid-scale.
Range
The Range is the minimum and maximum voltage range of the input signal.
Table 3. NI 9237 Accuracy
Excitation Setting Value (V) Gain Error Offset Error (μV/V)
2.5 0.14% 32.5
3.3 0.12% 10
5.0 0.14% 27.5
10.0 0.16% 45

©National Instruments Corporation 13 NI 9237 Calibration Procedure
Test Point Value
The Test Point Value is an approximate voltage value that the NI 9237
should read given the corresponding calibrator output and excitation
setting.
1-Year Absolute Error
The 1-Year Absolute Error is the voltage value that you use to calculate
upper and lower limits.
Table 4. NI 9237 Verification Test Limits
Excitation
Setting
Value (V)
Calibrator Output Range (V/V)
Test Point
Value
(V/V)
1-Year
Absolute
Error
(V/V)
Location Value (V) Minimum Maximum
2.5 Max 0.055 –0.025 0.025 0.022000 0.000063
2.5 Mid 0.000 –0.025 0.025 0.000000 0.000033
2.5 Min –0.055 –0.025 0.025 –0.022000 0.000063
3.3 Max 0.073 –0.025 0.025 0.022000 0.000036
3.3 Mid 0.000 –0.025 0.025 0.000000 0.000010
3.3 Min –0.073 –0.025 0.025 –0.022000 0.000036
5.0 Max 0.110 –0.025 0.025 0.022000 0.000058
5.0 Mid 0.000 –0.025 0.025 0.000000 0.000028
5.0 Min –0.110 –0.025 0.025 –0.022000 0.000058
10.0 Max 0.220 –0.025 0.025 0.022000 0.000080
10.0 Mid 0.000 –0.025 0.025 0.000000 0.000045
10.0 Min –0.220 –0.025 0.025 –0.022000 0.000080

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