National Instruments Module SCXI-1125 Manual

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™,NI-DAQ

™, and SCXI™are trademarks of National Instruments Corporation. Product and company

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CALIBRATION PROCEDURE

SCXI -1125

Introduction

This document contains information and step-by-step instructions for

verifying and calibrating the National Instruments SCXI-1125 signal

conditioning module.

What Is Calibration?

Calibration is a procedure of reading offset and gain errors from a device

and updating special analog calibration circuitry that corrects these errors.

National Instruments calibrates every SCXI-1125 module at the factory.

During the factory calibration process, the calibration constants are stored

in the non-volatile memory of the device. These values are loaded from

memory and used as needed by the device.

Why Should You Calibrate?

Offset and gain errors drift with time and temperature changes, which could

invalidate the factory-set calibration of a device and increase measurement

uncertainty. Calibration restores the device to its specified accuracy.

How Often Should You Calibrate?

The measurement accuracy requirements of your application

determine how often you should calibrate your SCXI-1125 module.

National Instruments recommends you perform a complete calibration

at least once every year. You can shorten this interval to six months or

90 days, based on the demands of your application.

™

SCXI-1125 Calibration Procedure 2 ni.com

Equipment and Other Test Requirements

This section describes the equipment, software, documentation, and test

conditions required for calibrating the SCXI-1125.

Test Equipment

Calibration requires a high-precision voltage source with at least 50 ppm

accuracy, a multiranging 5 1/2 digit digital multimeter (DMM) with

15 ppm accuracy, and a National Instruments E Series DAQ device.

National Instruments recommends the following instruments for

calibrating the SCXI-1125:

• Calibrator—Fluke 5700A

• DMM—NI 4060 or HP 34401A

• National Instruments E Series DAQ device

If these instruments are not available, use the accuracy requirements listed

above to select a substitute calibration standard.

Note If you do not have custom connection hardware, you will need a connector block

such as the National Instruments SCXI-1320, a shielded 68-pin connector cable, a 50-pin

ribbon cable, a 50-pin breakout box, and an SCXI adapter. These components give easy

access to the individual pins on the SCXI-1125 front and rear connectors.

Software and Documentation

You will need the following software and documentation to calibrate

the SCXI-1125 signal conditioning modules. You can find these on the

National Instruments Web site at ni.com/calibration:

• The latest version of the National Instruments NI-DAQ driver

•NI-DAQ Function Reference Online Help file

• NI-DAQ User Manual for PC Compatibles

Software

The SCXI-1125 calibration procedure requires the latest version of the

National Instruments NI-DAQ driver on the calibration system. The driver

supports a number of programming languages, including LabVIEW,

LabWindows/CVI, Microsoft Visual C++, Microsoft Visual Basic, and

Borland C++. When you install the driver, you only need to install support

for the programming language you will use.

©National Instruments Corporation 3 SCXI-1125 Calibration Procedure

Documentation

The NI-DAQ Function Reference Online Help file and NI-DAQ User

Manual for PC Compatibles containdetailedinformationonusingthe

NI-DAQ driver. The function reference online help file includes detailed

information on the driver functions. You can access the online help by

clicking Start»Programs»National Instruments DAQ»NI-DAQ Help.

The user manual provides instructions on installing and configuring

National Instruments DAQ devices. This manual also includes detailed

information on creating applications that use the NI-DAQ driver. These are

your primary references for writing your calibration utility. For further

information on the products you are calibrating, refer to your device user

manuals.

Test Conditions

Follow these guidelines to optimize the connections and the environment

during calibration:

•Keep connections to the SCXI module short. Long cables and wires act

as antennae, picking up extra noise and thermal offsets 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 the temperature between 18 and 28 °C.

•Keep relative humidity below 80%.

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

30 minutes for the E Series device to ensure the measurement circuitry

is at a stable operating temperature.

Calibration

The calibration process consists of the following steps:

1. Setting up the device for testing.

2. Verifying the existing operation of the device to determine whether it

is operating within its specifications.

3. Adjusting the device with respect to a known voltage source.

4. Verifying that the device is operating within its specifications after

adjustments.

SCXI-1125 Calibration Procedure 4 ni.com

Setting Up Your Module

Complete the following steps to set up your module for testing:

1. Install the SCXI-1125 in slot 1 of the SCXI chassis.

2. Connect a 68-to-68-pin cable between the SCXI module and the

E Series DAQ device installed in the host computer.

3. Configure the hardware with Measurement & Automation Explorer

(MAX). For more information on configuring the system, refer to the

E Series device and SCXI module documentation.

Verifying Gain and Offset Errors

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

meeting its specifications. You can use this information to select the

appropriate calibration interval for your application.

Complete the following steps to verify gain and offset errors in the

SCXI-1125:

1. Read the Test Conditions section earlier in this document.

2. Refer to the specifications in Table 1 at the end of this document.

3. Connect the E Series device to the SCXI-1125.

4. Call Calibrate_E_Series to minimize the uncertainty associated

with the E Series device measurements. Set the following parameters:

•deviceNumber—The value assigned by MAX

•calOp—ND_SELF_CALIBRATE

•setOfCalConstants—ND_USER_EEPROM_AREA

•calRefVolts—0.0

5. Call AI_Configure to put the E Series device in bipolar mode. Set the

following parameters:

•deviceNumber—The value assigned by MAX

•chan—0

•inputMode—0

•inputRange—0

•polarity—0

•driveAIS—0

6. Call MIO_Config to enable dithering on the E Series device. Set the

following parameters:

•deviceNumber—The value assigned by MAX

•dither—1

•useAMUX—0

©National Instruments Corporation 5 SCXI-1125 Calibration Procedure

7. Connect the calibrator to the appropriate analog input, starting

with channel 0. Refer to Figure 1 to determine which pins on the

96-pin front connector correspond to the positive and negative inputs

of the specified channel. For example, the positive input for channel 0

(CH0+) is pin A32. The negative input for channel 0 (CH0–)ispin

C32. If you are using an SCXI-1320 terminal block connected to the

SCXI-1125, connect the calibrator to the CH0+ and CH0–inputs.

8. Call SCXI_Single_Chan_Setup to configure the module for

single-channel measurements. Set the following parameters:

•SCXIchassisID—The value assigned by MAX

•moduleSlot—1

•moduleChan—The same channel number from step 7

•DAQdeviceNumber—The device number assigned by MAX for

the E Series DAQ device

9. Call SCXI_Set_Gain to set the gain you want to test on the module.

Set the following parameters:

•SCXIchassisID—The ID obtained when the chassis is configured

with MAX

•moduleSlot—1

•channel—The channel number from step 7

•gain—The first gain value you want to test—you can find this

gain value in Table 1.

10. Call SCXI_Configure_Filter to configure the filter setting on the

module. Set the following parameters:

•SCXIchassisID—The ID obtained when the chassis is configured

with MAX

•moduleSlot—1

•channel—The channel number from step 7

•filterMode—1

•cutoffFrequency—10,000

11. Set the calibrator to output the test point voltage from Table 1.

12. Call DAQ _Op. Set the following parameters:

•deviceNumber—ThedevicenumberoftheDAQdevice

•channel—0

•gain—1 for 16-bit E Series devices or –1 for 12-bit E Series

devices

•count—5,000

•sampleRate—10,000

SCXI-1125 Calibration Procedure 6 ni.com

13. Call SCXI_Scale. Set the following parameters:

•SCXIChassisID—The value assigned by MAX

•moduleSlot—1

•channel—The same channel number from step 7

•SCXIgain—The same gain value from step 9

•TBgain—1

•DAQboard—The device number for the DAQ device

•DAQChannel—0

•DAQgain—1 for 16-bit devices, –1 for 12-bit devices

•numPoints—5.000

•binArray—The array returned from the DAQ_Op function

The result is a set of scaled voltages that you read from the SCXI

module.

14. Average the results returned from the SCXI_Scale function. Compare

the averaged result to the upper and lower limits in Table 1. If the result

falls within these limits, your module has passed this portion of the

test. If it does not, your module needs to be adjusted using the steps in

the next section, Adjusting for Gain and Offset Errors.

15. Repeat steps 8 through 14 for the remaining gains.

16. Repeat steps 7 through 15 for the remaining channels.

You have now finished verifying your SCXI-1125.

Note If all channels fall within the upper and lower limits, your device does not need to

be adjusted.

Adjusting for Gain and Offset Errors

The following two sections explain how to determine the gain and offset

errors of your SCXI-1125 and how to adjust the calibration constants to

correct for these errors.

Determining Gain and Offset Errors

Complete the following steps to determine the gain and offset errors of the

SCXI-1125:

1. Read the Test Conditions section.

2. Connect the E Series device to the SCXI-1125.

3. Call Calibrate_E_Series to minimize the uncertainty associated

with the E series device measurements. Set the following parameters:

•deviceNumber—The value assigned by MAX

©National Instruments Corporation 7 SCXI-1125 Calibration Procedure

•calOp—ND_SELF_CALIBRATE

•setOfCalConstants—ND_USER_EEPROM_AREA

•calRefVolts—0.0

4. Call AI_Configure to put the E Series device in bipolar mode. Set the

following parameters:

•deviceNumber—The value assigned by MAX

•chan—0

•inputMode—0

•inputRange—0

•polarity—0

•driveAIS—0

5. Call MIO_Config to enable dithering on the E Series device. Set the

following parameters:

•deviceNumber—The value assigned by MAX

•dither—1

•useAMUX—0

6. Call SCXI_Single_Chan_Setup to configure the module for

single-channel measurements. Set the following parameters:

•SCXIchassisID—The value assigned by MAX

•moduleSlot—1

•moduleChan—The channel you want to adjust

•DAQdeviceNumber—The device number assigned by MAX for

the E Series DAQ device

7. Call SCXI_Set_Gain.Set the following parameters:

•SCXIchassisID—The ID of the SCXI chassis obtained when you

configure the chassis with MAX

•moduleSlot—1, because the SCXI-1125 should be installed in

slot 1

•channel—The same channel number from step 6

•gain—The gain value you want to adjust. Refer to Table 1 for

acceptable gain values

8. Call SCXI_Configure_Filter to configure the filter setting on the

module. Set the following parameters:

•SCXIchassisID—The ID of the SCXI chassis obtained when you

configure the chassis with MAX

•moduleSlot—1

•channel—The same channel number from step 7

SCXI-1125 Calibration Procedure 8 ni.com

•filterMode—1

•cutoffFrequency—10,000

9. Connect the calibrator to the analog input you want to adjust. Refer

to Figure 1 to determine the pins on the 96-pin front connector that

correspond to the positive and negative inputs of the specified channel.

For example, the positive input for channel 0 is pin A32, which is

labeled CH0+. The negative input for channel 0 is pin C32, which

is labeled CH0–. If you are using an SCXI-1320 terminal block

connected to the SCXI-1125, connect the calibrator to the CH0+

and CH0–inputs.

10. Connect the DMM to the positive output on pin 3 (MCH0+) and

negative output on pin 4 (MCH0–) on the rear panel connector.

Figure 2 shows the 50-pin rear panel connector.

Tip For easy access to individual pins, use a screw terminal adapter connected to a

50-pin terminal block.

11. Set the calibrator voltage to the positive test point value in Table 1.

12. Read the voltage from the DMM. Record the actual DMM reading,

which is output1, and the calibrator output voltage, which is volt1,for

later use.

13. Set the calibrator to the negative test point value for the same gain.

Skip any input limits that are specified as 0.0 V. You need only upper

and lower limits for adjustment.

14. Read the voltage from the DMM. Record the actual DMM reading,

output2, and the calibrator output voltage, voltage2.

You now have two pairs of data points, (volt1,output1) and (volt2,

output2), where volt is the calibrator reading and output is the DMM

reading.

15. Convert the DMM voltage readings (output1 and output2)tobinary

readings (binary1 and binary2) by using one of the following

equations:

For example, using a 12-bit product such as a PCI-MIO-16E-1 and

obtaining a DMM reading of –9.9 V, you would have the following

results:

binary reading =(–9.9/20) × 212 =–2027.52

binary reading for 12-bit E Series products (binary) output

-----------------





212

×=

binary reading for 16-bit E Series products (binary) output

-----------------





216

×=

©National Instruments Corporation 9 SCXI-1125 Calibration Procedure

16. Record binary1 with volt1 and binary2 with volt2 for later use.

17. Repeat steps 7 through 16 for the remaining gain values you want to

adjust on this channel.

18. Repeat steps 6 through 17 for the remaining channels you want to

adjust.

You have determined the new calibration constants. Use these values in the

the following section, Adjusting Calibration Constants.

Adjusting Calibration Constants

Complete the following steps to adjust the calibration constants on the

SCXI-1125:

1. Call SCXI_Cal_Constants to create and store the new calibration

constants in the SCXI-1125 memory. Set the following parameters:

•SCXIchassisID—TheIDassignedbyMAX

•ModuleSlot—1, because the SCXI-1125 should be installed in

slot 1

•channel—The channel on the SCXI-1125 you want to adjust

•OpCode—2

•calibrationArea—0

•rangeCode—0 (not used for the SCXI-1125)

•SCXIgain—The gain you want to adjust

•DAQboard—The device number assigned by MAX

•DAQChan —0

•DAQGain —1 for 16-bit devices, –1 for 12-bit devices

•Tbgain—1.0

•volt1 and volt2—The first and second voltage readings for the

gain values that you are about to save on the SCXI-1125

•binary1 and binary2—The first and second binary readings for

the gain values that you are about to save on the SCXI-1125

•calConst1 and calConst2—These are return values

2. Repeat step 1, changing calibrationArea to 1.

3. Repeat steps 1 and 2 for the next channel.

You have now finished adjusting the calibration constants of your

SCXI-1125.

SCXI-1125 Calibration Procedure 10 ni.com

Verifying Adjusted Values

After completing the adjustments to your SCXI-1125 module, it is

important that you verify the analog input operation by repeating the steps

listed in the Verifying Gain and Offset Errors section of this document.

Reverifying after adjustments ensures that your SCXI-1125 module is now

operating within its specifications.

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