National Instruments 9235 Manual

NI 9235/9236 Calibration Procedure
This document contains information about calibrating National
Instruments 9235/9236 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
Connecting the Calibrator to the Module ........................................ 6
Verification Procedures ................................................................... 8
Measurement Accuracy Verification........................................ 8
Shunt Calibration Accuracy Verification ................................. 12
Excitation Voltage Verification................................................ 16
Specifications .......................................................................................... 17
Test Limits .............................................................................................. 18
Calibrator Output ............................................................................. 18
Nominal Value ................................................................................. 18
1-Year Limits ................................................................................... 18
Where to Go for Support......................................................................... 20
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NI 9235/9236 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 8.8 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 9235/9236. NI-DAQmx, available at ni.com/downloads, configures
and controls the NI 9235/9236.
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
Program Files\National Instruments\NI-DAQ\Examples
directory.

©National Instruments Corporation 3 NI 9235/9236 Calibration Procedure
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 guides 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 9235/9236 Operating Instructions and Specifications—This
document describes how to use the NI 9235/9236 and includes
specifications and terminal assignments for the NI 9235/9236. The
limits you use to verify the accuracy of the devices are based on the
specifications found in this document. You can download the latest
version of this document from the NI Web site at ni.com/manuals.
Calibration Interval
The NI 9235/9236 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.

NI 9235/9236 Calibration Procedure 4 ni.com
Test Equipment
National Instruments recommends that you use the following equipment
for calibrating the NI 9235/9236.
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 ±5 °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 5500A If this instrument is unavailable, use a calibrator with
generation of resistances in the ranges of 108 Ω– 132 Ω
and 315 Ω–385Ωwith 0.01 Ωresolution, an accuracy
of at least 90 ppm, and automatic lead wire
compensation.
DMM NI 4070 If this instrument is unavailable, use a multi-ranging
6 1/2 digit DMM with an accuracy of at least 40 ppm.
Chassis NI cDAQ chassis —
Connecting
wires
—Connections between the module and the calibrator are
insensitive to connection resistance except between the
calibrator LO terminal and the module RC terminal. The
resistance of this connection must be less than 200 mΩto
limit bridge linearity errors. The verification test limits in
Tables 5 and 6 assume this requirement is met.

©National Instruments Corporation 5 NI 9235/9236 Calibration Procedure
Calibration Procedure
This section provides instructions for verifying the performance of the
NI 9235/9236.
Calibration Process Overview
The calibration process consists of the following steps:
1. Initial Setup—Configure the device in NI-DAQmx.
2. Verification Procedures—Verify the existing operation of the device.
This step confirms whether the device is operating within its specified
range and whether it needs adjustment.
3. Adjustment—If the device does not fall within the desired
specifications, submit the device to NI for a factory calibration to
adjust the calibration constants.
4. Verification Procedures—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 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 9235/9236 Calibration Procedure 6 ni.com
Connecting the Calibrator to the Module
Complete the following steps to connect the calibrator to the module for
both the measurement accuracy verification and the shunt calibration
verification:
1. Set the calibrator to standby mode (STBY).
2. Connect the AUX LO terminal of the calibrator to the LO terminal of
the calibrator.
3. Using discrete connections, connect the LO terminal of the calibrator
to the AI and RC terminals of the module channel you want to verify.
Leave all other channels unconnected.
Note The three connections to the calibrator LO terminal must be connected at the
calibrator output jack.
Note Ensure that the connection resistance between the calibrator LO terminal and the
module RC terminal meets the requirement listed in Table 1.
4. Connect the HI terminal of the calibrator to the EXC0 terminal of the
module.
5. Connect the AUX HI terminal of the calibrator to the EXC1 terminal
of the module.
Note You do not have to change the EXC terminal connections for each channel you
verify, because the EXC terminals all connect to a common excitation supply inside the
NI 9235/9236.

©National Instruments Corporation 7 NI 9235/9236 Calibration Procedure
Refer to Figure 1 for an illustration of the calibrator to module connections.
Figure 1. Calibrator to Module Connections
Refer to Figure 2 for the terminal assignments of the NI 9235/9236.
Figure 2. NI 9235/9236 Terminal Assignments
Fluke
5500A
AUX HI EXC1
EXC0
AI0
RC0AUX LO
HI
LO
NI 9235/9236
EXC0
AI0
RC0
EXC2
AI2
RC2
EXC4
AI4
RC4
EXC6
AI6
RC6
EXC1
AI1
RC1
EXC3
AI3
RC3
EXC5
AI5
RC5
EXC7
AI7
RC7
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24

NI 9235/9236 Calibration Procedure 8 ni.com
6. Set the calibrator resistance output to 120 Ωfor the NI 9235 or 350 Ω
for the NI 9236.
7. Set the calibrator to operate mode (OPR).
8. Wait one minute to allow the bridge elements formed by the calibrator
and the module completion resistor to thermally stabilize.
Verification Procedures
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. The NI 9235/9236 has eight independent analog input
channels. You can conduct verification on any or all of them, depending
upon your desired test coverage.
You can verify the following for NI 9235/9236 modules:
• measurement accuracy
• shunt calibration accuracy
• excitation voltage
Measurement Accuracy Verification
Complete the following steps to test the measurement accuracy of the
device:
1. Zero the calibrator with the ohms-only zero (OHMS ZERO) operation.
Refer to the calibrator documentation for more information about
zeroing the calibrator.
2. Set the calibrator output for two-wire compensation (COMP 2-wire).
This automatically compensates for the lead wire resistance between
the HI terminal of the calibrator and the EXC terminal of the module.
3. Connect the calibrator to the channel you want to verify, as explained
in the Connecting the Calibrator to the Module section.
4. Set the calibrator resistance output to a Calibrator Output value
indicated in Table 5 for the NI 9235 and Table 6 for the NI 9236,
beginning with the 0 V/V nominal value.

©National Instruments Corporation 9 NI 9235/9236 Calibration Procedure
5. 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 6 in LabVIEW.
6. Create and configure an AI Custom Voltage with Excitation channel using
the DAQmx Create Virtual Channel VI, as shown in the following table.
Note Throughout the procedure, refer to the NI-DAQmx function parameters for the
LabVIEW input values. Refer to the block diagram images for the correct instance to use
for polymorphic VIs.
LabVIEW Block Diagram NI-DAQmx Function Call
LabVIEW does not require this step.
Call DAQmxCreateTaskwith
the following parameters:
taskName:
AIVerificationTask
taskHandle: &taskHandle
LabVIEW Block Diagram NI-DAQmx Function Call
Call
DAQmxCreateAIVoltageChan
WithExcit with the following
parameters:
taskHandle: taskHandle
physicalChannel: dev1/ai x
*
nameToAssignToChannel:
myVoltageChannel
terminalConfig:
DAQmx_Val_Cfg_Default
minVal: –0.027
maxVal: 0.027
units: DAQmx_Val_Volts
bridgeConfig:
DAQmx_Val_QuarterBridge
voltageExcitSource:
DAQmx_Val_Internal
voltageExcitVal:
2.0 for the NI 9235
3.3 for the NI 9236
useExcitforScaling: TRUE
customScaleName: NULL
* xrefers to the channel number.

NI 9235/9236 Calibration Procedure 10 ni.com
7. Configure the timing properties for the acquisition using the DAQmx
Timing VI, as shown in the following table.
8. Turn off the shunt calibration for the channel using the DAQmx
Channel Property Node, as shown in the following table.
9. 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: 5000
sampleMode:
DAQmx_Val_FiniteSamps
sampsPerChan: 5000
LabVIEW Block Diagram NI-DAQmx Function Call
Call
DAQmxSetAIBridgeShuntCal
Enable with the following
parameters:
taskHandle: taskHandle
channel: NULL
data: FALSE
LabVIEW Block Diagram NI-DAQmx Function Call
Call DAQmxStartTaskwith the
following parameter:
taskHandle: taskHandle