Strain- and Bridge-Based Measurements

2

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Specifications

>>

For complete specifications, see the

NI USB-9237 User Guide and

Input Characteristics

Number of channels............................

4

Bridge completion

Full and half ....................................

Internal

Quarter ............................................

External

ADC resolution................................

24 bits

Type of ADC ....................................

Delta-sigma

(with analog prefiltering)

Sampling mode...............................

Simultaneous

Master timebase (internal)

Frequency........................................

12.8 MHz

Accuracy .........................................

±100 ppm max

Nominal full-scale range ................

±25 mV/V

Scaling coefficient ..............................

2.9802 nV/V per LSB

Overvoltage protection

between any two terminals ...........

±30 V

Accuracy

Gain drift .............................................

10 ppm/°C max

Offset drift

2.5 V excitation...............................

0.6

μV/V per °C

3.3 V excitation...............................

0.5

μV/V per °C

5 V excitation..................................

0.3

μV/V per °C

10 V excitation................................

0.2

μV/V per °C

Safety

Safety Voltages

Connect only voltages that are within these limits.

Between any two terminals ...............

±30 V max

Isolation

Channel-to-channel.............................

No isolation between channels

Channel-to-earth ground

Continuous ......................................

60 VDC, Measurement Category I

Withstand .......................................

dielectric withstand test

Excitation Voltages

Although the sensor industry does not recognize a single standard

excitation voltage level, excitation voltage levels of between 2.5 and

10 V are common. You can program the USB-9237 to supply 2.5, 3.3,

5, or 10 V of excitation voltage, and the module can provide up to

150 mW of excitation power. Unless you supply external excitation

voltage, National Instruments recommends that you set the excitation

voltage to a value that keeps the total power below 150 mW. The

USB-9237 automatically reduces internal excitation voltages as needed

to stay below 150 mW.

The power consumed by a single bridge is

where R is the total resistance of the bridge.

For a full bridge, R is equal to the resistance of each element. For a

half or quarter bridge, R is equal to two times the resistance of each

element.

With the 150 mW limit, you can power full and half bridges as

follows:

• Four 350

Ω half bridges at 5.0 V

• Four 350

Ω full bridges at 3.3 V

• Four 120

Ω half bridges at 2.5 V

If you need an excitation voltage greater than 150 mW, use the

four-position external excitation voltage connector to connect an

external excitation source to the USB-9237.

Channel Signal Names

RJ50 (10p10c) Modular Plug and

Receptacle Pin Numbers

RJ50

Pin

RJ45

Pin

Signal

Name

Signal Description

1

–

SC

Shunt calibration

2

1

AI+

Positive input signal

3

2

AI-

Negative input signal

4

3

RS+

Positive remote sense signal

5

4

RS-

Negative remote sense signal

6

5

EX+

7

6

EX-

8

7

T+

TEDS DATA

9

8

T-

10

–

SC

Shunt calibration

1

These signals are shared by all four RJ50 connectors on the USB-9237.

Percent of Reading

Calibrated max (-40 to 70 °C)

0.20%

0.25%

Calibrated typ (25 °C, ±5 °C)

0.05%

0.05%

Uncalibrated max (-40 to 70 °C)

0.60%

0.35%

Uncalibrated typ (25 °C, ±5 °C)

0.20%

0.1%

1

V

ex

2

R

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