National Instruments SCXI-1520 Manual

CALIBRATION PROCEDURE

SCXI -1520

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

calibrating the National Instruments SCXI-1520 module.

Equipment and Other Test Requirements

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

conditions required for calibrating the SCXI-1520.

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 16-bit data acquisition (DAQ) device.

NI recommends the following instruments for calibrating the SCXI-1520.

• Calibrator—Fluke 5700A

• DMM—NI 4060 or Agilent 34401A

• 16-bit 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 need an SCXI-1314 terminal

block and a shielded 68-pin connector cable. These components give easy access to the

individual pins on the SCXI-1520 front connectors. You also need an SCXI-1349 cable

adapter to connect the SCXI-1520 to the DAQ device.

Software and Documentation

Use the following software and documentation to calibrate the SCXI-1520.

You can find them on the NI Web site at ni.com/support.

• The latest version of the NI-DAQ driver software

•NI-DAQ Function Reference Help

™

SCXI-1520 Calibration Procedure 2 ni.com

• NI-DAQ User Manual for PC Compatibles

• SCXI-1314 Installation Guide

• SCXI-1349 Installation Guide

Software

The SCXI-1520 calibration procedure requires the latest version of the

NI-DAQ driver on the calibration system. The driver supports a number

of programming languages, including LabVIEW, Measurement Studio,

Microsoft Visual C++, Microsoft Visual Basic, and Borland C++. When

you install the driver, you need to install support only for the programming

language you are using.

Documentation

The NI-DAQ Function Reference Help and the NI-DAQ User Manual for

PC Compatibles contain detailed information on using the NI-DAQ driver.

The online help includes detailed information on the driver functions.

You can access the help by selecting Start»Programs»National

Instruments DAQ»NI-DAQ Help. The user manual provides instructions

on installing and configuring DAQ devices. The user 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

more information on the SCXI-1520, refer to the SCXI-1520 User Manual.

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 reduce noise.

• 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 DAQ device to ensure that the measurement

circuitry is at a stable operating temperature.

©National Instruments Corporation 3 SCXI-1520 Calibration Procedure

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.

Setting Up the SCXI-1520 for Testing

Complete the following steps to set up the SCXI-1520 for testing.

1. Install the SCXI-1520 in slot 1 of the SCXI chassis.

2. Using an SCXI-1349 cable adapter, connect a 68-to-68-pin cable

between the SCXI-1520 module and the DAQ device installed in the

host computer, as shown in Figure 1.

Figure 1. Connecting an SCXI Chassis to a DAQ Device

1 DAQ Device

2 SH68-68 Cable

3 SCXI-1349 Cable Adapter

4 Thumbscrews

5 SCXI-1520 Module

6 SCXI-1314 Terminal Block

7 SCXI Chassis Power Switch

8 SCXI-1000 Chassis

ADDRESS

SCXI1100

SCXI-1520 Calibration Procedure 4 ni.com

3. Configure the hardware with Measurement & Automation Explorer

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

documentation for the DAQ device and the SCXI-1520.

Verifying Gain and Offset Errors

The verification procedure determines how well the SCXI-1520 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 of the

SCXI-1520.

1. Read the Test Conditions section.

2. Refer to the specifications in Table 2.

3. Call Calibrate_E_Series to minimize the uncertainty associated

with the DAQ device measurements. Set the following parameters:

•deviceNumber—The value assigned by MAX

•calOP—ND_SELF_CALIBRATE

•setOfCalConst—ND_USER_EEPROM_AREA

•calRefVolts—0.0

4. Call AI_Configure to put the DAQ 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 SCXI_Track_Hold_Setup to set up the track-and-hold circuitry

of the SCXI-1520. Set the following parameters:

•SCXIchassisID—The value assigned by MAX

•moduleSlot—1

•inputMode—2

•source—0

•send—0

•holdCount—1

•DAQdeviceNumber—The value assigned by MAX

©National Instruments Corporation 5 SCXI-1520 Calibration Procedure

6. Call SCXI_MuxCtr_Setup to configure the DAQ device counter for

use as a multiplexer counter during SCXI channel scanning. Set the

following parameters:

•deviceNumber—The value assigned by MAX

•enable—1

•scanDiv—1

•ctrValue—1

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

with channel 0. Refer to Table 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 (S0+)

is pin C32. The negative input for channel 0 (S0–) is pin B32. If you

are using an SCXI-1314 terminal block, connect the calibrator to the

S0+ and S0– inputs.

8. Call SCXI_SCAN_Setup to set up the SCXI chassis to perform a

multiplexed scanning data acquisition. Set the following parameters:

•SCXIchassisID—The value assigned by MAX

•numModules—1

•moduleList—{1}1

•numChans—{1}

•startChans—{The channel number from step 7}

•DAQdeviceNumber—The value assigned by MAX

•modeFlag—0

9. Call SCXI_Configure_Filter to configure the filter on the module.

Set the following parameters:

•chassisID—The value assigned by MAX

•moduleSlot—1

•channel—The channel number from step 7

•filterMode—0

•freq—0

•cutoffDivDown—0

•outClkDivDown—0

10. Call SCXI_Set_Potentiometer to set the fine potentiometer to the

center position. Set the following parameters:

•SCXIchassisID—The value assigned by MAX

•moduleSlot—1

1 Array notation: {X} denotes a one-element array initialized with the value X; [Y] denotes an uninitialized array of size Y.

SCXI-1520 Calibration Procedure 6 ni.com

•channel—The channel number from step 7

•value—2048

•pottype—1

11. Call SCXI_Set_Potentiometer to set the coarse potentiometer

to the center position. Set the following parameters:

•SCXIchassisID—The value assigned by MAX

•moduleSlot—1

•channel—The channel number from step 7

•value—62

•pottype—0

12. Call SCXI_Set_Excitation to set the excitation voltage to zero.

Set the following parameters:

•SCXIchassisID—The value assigned by MAX

•moduleSlot—1

•moduleChan—The channel number from step 7

•excitationType—2

•excitationValue—0

SCXI_Set_Excitation returns a 32-bit floating-point number,

actualExcitationValuePtr.

13. Call SCXI_Configure_Connection to configure the channel for

a full-bridge connection. Set the following parameters:

•SCXIchassisID—The value assigned by MAX

•moduleSlot—1

•moduleChan—The channel number from step 7

•connectionType—6

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

Set the following parameters:

•SCXIchassisID—The value assigned by MAX

•moduleSlot—1

•channel—The channel number from step 7

•gain—The gain value you want to test; you can find valid gain

values in Table 2.

15. Set the calibrator to output the test point voltage from Table 2.

16. Call SCAN_Op to acquire the data. Set the following parameters:

•deviceNumber—The value assigned by MAX

•numChans—{1}

©National Instruments Corporation 7 SCXI-1520 Calibration Procedure

•chans—{0}

•gains—1

•buffer—[5000]

•count—5000

•sampleRate—10000

•scanRate—5000

17. Call SCXI_Scale to scale the data returned by SCAN_Op. Set the

following parameters:

•SCXIChassisID—The value assigned by MAX

•moduleSlot—1

•channel—The channel number from step 7

•SCXIgain—The gain value from step 14

•TBgain—1

•DAQboard—The device number of the DAQ device

•DAQchannel—0

•DAQgain—1

•numPoints—5000

•binArray—The array returned by SCAN_Op in step 16

The result is a set of scaled voltages called scaledArray.

18. Average the results returned by SCXI_Scale. Compare the averaged

result to the upper and lower limits in Table 2. If the result falls within

these limits, your module has passed this portion of the test. If the

result is outside these limits, adjust the module using the steps in the

Adjusting for Gain, Offset, and Excitation Errors section.

19. Repeat steps 14 through 18 for the remaining gains.

20. Repeat steps 7 through 19 for the remaining channels.

Note If all channels fall within the upper and lower limits, the gain and offset do not need

adjustment.

Verifying Voltage Excitation

This section describes how to verify the voltage excitation of the

SCXI-1520 and how to calculate new voltage excitation constants.

The calibration constants for the voltage excitation on the SCXI-1520

are calculated with respect to the onboard 2.5 V reference voltage. These

calibration constants are stored on the module EEPROM and can be read

by application software and used for software correction. You cannot

physically adjust the excitation on the SCXI-1520. If the measured values

of the excitation fall outside the limits given in Table 3, the module is not

SCXI-1520 Calibration Procedure 8 ni.com

operating within specifications. Return it to NI for repair. For more

information about how to contact NI for repair, refer to the Signal

Conditioning Technical Support Information document.

1. Read the Test Conditions section.

2. Connect the DAQ device to the SCXI-1520 using an SCXI-1349 cable

adapter and a 68-to-68-pin cable.

3. Connect the DMM to the appropriate excitation output terminals,

starting with channel 0. Refer to Table 1 to determine which pins

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

excitation terminals of the specified channel. For example, the positive

excitation terminal for channel 0 (P0+) is pin C30. The negative

excitation terminal for channel 0 (P0–) is pin B30. If you are using

an SCXI-1314 terminal block, connect the DMM to the P0+ and P0–

output terminals of the SCXI-1314.

4. Call SCXI_Set_Excitation to set the excitation voltage to the

desired value. Set the following parameters:

•SCXIchassisID—The value assigned by MAX

•moduleSlot—1

•moduleChan—The channel number from step 3

•excitationType—2

•excitationValue—The excitation value that you want to measure.

Refer to Table 3 for a list of valid values.

SCXI_Set_Excitation returns a 32-bit floating-point number

called actualExcitationValuePtr. This number was stored in the

SCXI-1520 EEPROM when the module was last calibrated, and

represents the voltage excitation measured at that time. Record this

value as Vexe.

5. Read the voltage from the DMM and record it as Vexm. You will use

Vexm to verify the voltage excitation and, if necessary, to calculate new

calibration constants in the Adjusting Calibration Constants section.

6. Compare Vexm from step 5 to the upper and lower limits in Table 2.

If Vexm is within these limits, the module has passed this portion of the

test. If Vexm is outside these limits, the module is not operating within

specifications. Return it to NI for repair.

7. Divide Vexe from step 4 by Vexm from step 5. If the resulting value is

between 0.999 and 1.001, the calibration constants are within the

required specifications. Do not use 0.0 V excitation for this step.

8. Repeat steps 4 through 7 for the remaining excitation levels you want

to test on the module.

9. Repeat steps 3 through 8 for the remaining channels you want to test

on the module.

©National Instruments Corporation 9 SCXI-1520 Calibration Procedure

Adjusting for Gain, Offset, and Excitation Errors

The Determining Gain Errors and Adjusting Calibration Constants

sections explain how to determine the gain, offset, and excitation errors of

the SCXI-1520 and how to adjust the calibration constants to correct for

these errors.

Determining Gain Errors

Complete the following steps to determine the gain errors of the

SCXI-1520.

1. Read the Test Conditions section.

2. Refer to the specifications in Table 2.

3. Call Calibrate_E_Series to minimize the uncertainty associated

with the DAQ device measurements. Set the following parameters:

•deviceNumber—The value assigned by MAX

•calOP—ND_SELF_CALIBRATE

•setOfCalConst—ND_USER_EEPROM_AREA

•calRefVolts—0.0

4. Call AI_Configure to put the DAQ 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 SCXI_Track_Hold_Setup to configure the track-and-hold

circuitry of the SCXI-1520. Set the following parameters:

•SCXIchassisID—The value assigned by MAX

•moduleSlot—1

•inputMode—2

•source—0

•send—0

•holdCount—1

•DAQdeviceNumber—The value assigned by MAX

SCXI-1520 Calibration Procedure 10 ni.com

6. Call SCXI_MuxCtr_Setup to configure the DAQ device counter for

use as a multiplexer counter during SCXI channel scanning. Set the

following parameters:

•deviceNumber—The DAQ device number assigned by MAX

•enable—1

•scanDiv—1

•ctrValue—1

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

channel 0. Refer to Table 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 (S0+)

is pin C32. The negative input for channel 0 (S0–) is pin B32. If you

are using an SCXI-1314 terminal block, connect the calibrator to the

S0+ and S0– inputs on the SCXI-1314.

8. Call SCXI_SCAN_Setup to set up the SCXI chassis to perform

a multiplexed scanning data acquisition. Set the following parameters:

•SCXIchassisID—The value assigned by MAX

•numModules—1

•moduleList—{1}

•numChans—{1}

•startChans—{The channel from step 7}

•DAQdeviceNumber—The value assigned by MAX

•modeFlag—0

9. Call SCXI_Configure_Filter to configure the filter on the module.

Set the following parameters:

•chassisID—The value assigned by MAX

•moduleSlot—1

•channel—The channel from step 7

•filterMode—0

•freq—0

•cutoffDivDown—0

•outClkDivDown—0

SCXI_Configure_Filter returns the actual cutoff frequency that

the module uses as a 64-bit floating-point number, actualFreq.

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