National Instruments 24-Bit Half/Full-Bridge Analog Input Module NI... User manual

OPERATING INSTRUCTIONS AND SPECIFICATIONS
NI 9237
4-Channel, 24-Bit Half/Full-Bridge Analog Input Module
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NI 9237 Operating Instructions and Specifications 2 ni.com
This document describes how to use the National Instruments 9237
and includes specifications and pin assignments for the NI 9237.
Visit ni.com/info and enter rdsoftwareversion to determine
which software youneed for the modules youare using. For
information about installing, configuring, and programming the
system, refer to the system documentation. Visit ni.com/info
and enter cseriesdoc for information about C Series
documentation.
Note The safety guidelines and specifications in this
document are specific to the NI 9237. The other
components in the system might not meet the same safety
ratings and specifications. Refer to the documentation for
each component in the system to determine the safety
ratings and specifications for the entire system. Visit
ni.com/info and enter cseriesdoc for information
about C Series documentation.

© National Instruments Corp. 3 NI 9237 Operating Instructions and Specifications
Safety Guidelines
Operate the NI 9237 only as described in these operating
instructions.
Hot Surface This icon denotes that the component may be
hot. Touching this component may result in bodily injury.
Safety Guidelines for Hazardous Locations
The NI 9237 is suitable for use in Class I, Division 2, Groups A, B,
C, D, T4 hazardous locations; Class I, Zone 2, AEx nC IIC T4, and
Ex nC IIC T4 hazardous locations; and nonhazardous locations
only. Follow these guidelines if youare installing the NI 9237 in a
potentially explosive environment. Not following these guidelines
may result in serious injury or death.
Caution Do not disconnect I/O-side wires or connectors
unless power has been switched off or the area is known
to be nonhazardous.
Caution Do not remove modules unless power has been
switched off or the area is known to be nonhazardous.

NI 9237 Operating Instructions and Specifications 4 ni.com
Caution Substitution of components may impair
suitability for Class I, Division 2.
Caution For Zone 2 applications, install the system
in an enclosure rated to at least IP 54 as defined by
IEC 60529 and EN 60529.
Caution For Zone 2 applications, connected signals must
be within the following limits:
Capacitance .......................... 0.2 μF max
Inductance ............................ 80 mH max
Special Conditions for Hazardous Locations Use in Europe
This equipment has been evaluated as EEx nC IIC T4 equipment
under DEMKO Certificate No. 03 ATEX 0324020X. Each module
is marked II 3G and is suitable for use in Zone 2 hazardous
locations. If youare using the NI 9237 in Gas Group IIC hazardous
locations or in ambient temperatures of –40 °C ≤Ta ≤70 °C, you
must use the device in an NI chassis that has been evaluated as
EEx nC IIC T4, Ex nA IIC T4, or Ex nL IIC T4 equipment.

© National Instruments Corp. 5 NI 9237 Operating Instructions and Specifications
Special Conditions for Marine Applications
Some modules are Lloyd’s Register (LR) Type Approved for
marine applications. To verify Lloyd’s Register certification, visit
ni.com/certification and search for the LR certificate, or
look for the Lloyd’s Register mark on the module.
Caution To meet radio frequency emission requirements
for marine applications, use shielded cables and install
the system in a metal enclosure. Suppression ferrites
must be installed on power supply inputs near power
entries to modules and controllers. Power supply and
module cables must be separated on opposite sides of the
enclosure and must enter and exit through opposing
enclosure walls.

NI 9237 Operating Instructions and Specifications 6 ni.com
Connecting the NI 9237
The NI 9237 has four RJ-50 receptacles that provide connections
for four half or full bridges.
Figure 1. NI 9237 Pin Assignments
SC
AI+
AI–
RS+
RS–
EX+
EX–
T+
T–
SC
1
2
3
4
5
6
7
8
9
10
Ch0 – Ch3
EX+ EX+
EX– EX–
Ch 0
Ch 1
Ch 2
Ch 3

© National Instruments Corp. 7 NI 9237 Operating Instructions and Specifications
When youconnect a half bridge to the NI 9237, youmust connect
the AI+, EX–, and RS– signals, but youdo not connect the AI–
signal. When youconnect a full bridge, youmust connect the AI+,
AI–, EX–, and RS– signals. Refer to Figure 2 for an illustration of
how to connect half and full bridges to the NI 9237.
Note Youcan use a quarter bridge with the NI 9237 if
youadd a resistor externally to create a half bridge. You
also can use a quarter bridge with the NI 9237 if youuse
the NI 9944 or NI 9945 Quarter Bridge Completion
Accessory. Visit ni.com and search for the NI 9944 and
NI 9945 Quarter Bridge Completion Accessories for
more information about these accessories and how to
purchase them.
Note National Instruments does not recommend using
an RJ-45 cable with the NI 9237 because it can physically
damage pins 1 and 10 on the device, and thus
permanently disable shunt calibration no matter what
connector youuse.

NI 9237 Operating Instructions and Specifications 8 ni.com
The NI 9237 has a four-terminal external excitation voltage source
connector. Youcan use the EX+ and EX– terminals on the
connector to connect one external excitation voltage source to the
module. Youcan use the additional EX+ and EX– terminals on the
connector to wire multiple NI 9237 modules together in a daisy
chain.
Note When youinsert or remove a new sensor from the
NI 9237, slight changes in the excitation voltages can
cause a mismatch between the internal half-bridge
completion resistors and the half-bridge sensors, which
results in a change in the measurement offsets.
National Instruments recommends performing bridge
calibrations of quarter- or half-bridge sensors after
connecting all sensors to the NI 9237 and after removing
or attaching any additional sensor. Visit ni.com/info
and enter the info code rdw9237 for more information
about changes in voltage offsets in the NI 9237.

© National Instruments Corp. 9 NI 9237 Operating Instructions and Specifications
Figure 2. Connecting a Half or Full Bridge to the NI 9237
EX+
EX–
SC
SC
RS+
RS–
T+
T–
AI–
AI+
TEDS
NI 9237

NI 9237 Operating Instructions and Specifications 10 ni.com
Each channel on the NI 9237 has an independent 24-bit ADC
and an input amplifier that enable youto sample signals from all
four channels simultaneously.
The NI 9237 is isolated from earth ground. However, the individual
channels are not isolated from each other. The EX+, EX–, and
T– signals are common among all channels. Youcan connect the
NI 9237 to a device that is biased at any voltage within the NI 9237
rejection range of earth ground. Refer to the Specifications section
for more information about the common-mode rejection
ratio (CMRR).
Yo ualso can connect floating signals to the NI 9237. If you
connect floating signals to the NI 9237, National Instruments
recommends connecting the EX– signal to the earth ground or
shield for better noise rejection.
The NI 9237 also includes filters to prevent aliasing. The filters
on the NI 9237 filter according to the data rate. Refer to the
Understanding NI 9237 Filtering section for more information
about filtering.

© National Instruments Corp. 11 NI 9237 Operating Instructions and Specifications
Wiring TEDS Channels
Ensure that neither the TEDS data (T+) nor the TEDS return (T–)
signal is tied in common to any AI signals on the NI 9237. The
NI 9237 connects all the T– signals together internally. Visit
ni.com/info and enter the info code rdtedsfor more
information about TEDS sensors.
NI 9237 Connection Options
Wiring resistance can create errors in bridge circuits. The NI 9237
provides two mechanisms to correct for these errors: remote
sensing and shunt calibration.
Remote Sensing
Remote sensing continuously and automatically corrects for errors
in excitation leads, and generally is most appropriate for half- and
full-bridge sensors.
Long wire and small gauge wire have greater resistance, which can
result in gain error. The resistance in the wires that connect the
excitation voltage to the bridge causes a voltage drop, which is a
source of gain error. The NI 9237 includes remote sensing to
compensate for this gain error. Connect remote sense wires to the
points where the excitation voltage wires connect to the bridge

NI 9237 Operating Instructions and Specifications 12 ni.com
circuit. Refer to Figure 3 for an illustration of how to connect
remote sense wires to the NI 9237.
Figure 3. Connecting Remote Sense Wires to the NI 9237
EX+
EX–
RS+
RS–
Rbridge
Rlead
Rlead
Rbridge Rbridge
Rbridge
NI 9237
AI–
AI+

© National Instruments Corp. 13 NI 9237 Operating Instructions and Specifications
The actual bridge excitation voltage is smaller than the voltage at
the EX+ and EX– leads. If youdo not use remote sensing of the
actual bridge voltage, the resulting gain error is:
for half-bridge sensors and
for full-bridge sensors.
If youconnect the remote sense signals directly to the bridge
resistors, the NI 9237 senses the actual bridge voltage and
eliminates the gain errors caused by the resistance of the EX+ and
EX– leads.
Shunt Calibration
Shunt calibration can correct for errors from the resistance of both
the excitation wiring and wiring in the individual resistors of the
bridge. Remote sensing corrects for resistances from the EX pins
on the NI 9237 to the sensor, and shunt calibration corrects for
these errors and for errors caused by wire resistance within an arm
of the bridge. Shunt calibration is most useful with quarter-bridge
Rlead
Rbridge
-----------------
2Rlead ⋅
Rbridge
--------------------

NI 9237 Operating Instructions and Specifications 14 ni.com
sensors because there may be significant resistance in the wiring to
the active resistor in the bridge.
The NI 9237 shunt calibration circuitry consists of a precision
resistor and a software-controlled switch. Refer to the software
help for information about enabling the shunt calibration switch for
the NI 9237.
Shunt calibration involves simulating the input of strain by
changing the resistance of an arm in the bridge by some known
amount. This is accomplished by shunting, or connecting, a large
resistor of known value across one arm of the bridge, creating a
known strain-induced change in resistance. Youcan then measure
the output of the bridge and compare it to the expected voltage
value. Youcan use the results to correct gain errors in the entire
measurement path, or to simply verify general operation to gain
confidence in the setup.
Use a stable signal, which is typically the unloaded state of the
sensor, first with the shunt calibration switch off and then again
with the switch on. The difference in these two measurements
provides an indication of the gain errors from wiring resistances.
Yo ucan design the software application to correct subsequent
readings for this gain error.

© National Instruments Corp. 15 NI 9237 Operating Instructions and Specifications
Excitation Voltages
Although the sensor industry does not recognize a single standard
excitation voltage level, excitation voltage levels of between
2.5 V and 10 V are common. Youcan program the NI 9237 to
supply 2.5 V, 3.3 V, 5 V, 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 youset the excitation voltage to a value that
keeps the total power below 150 mW. The NI 9237 automatically
reduces internal excitation voltages as needed to stay below
150 mW total power.
The power consumed by a single bridge is where Ris the total
resistance of the bridge.
For a quarter or half bridge, Ris equal to two times the resistance
of each element. For a full bridge, Ris equal to the resistance of
each element.
Vex
2
R
---------

NI 9237 Operating Instructions and Specifications 16 ni.com
The 150 mW limit allows youto power half and full bridges as
follows:
•Four 350 Ωhalf bridges at 5.0 V
•Four 350 Ωfull bridges at 3.3 V
•Four 120 Ωhalf bridges at 2.5 V
If youneed an excitation voltage that causes more than 150 mW to
dissipate across all the bridges, use the EX+ and EX– terminals on
the external excitation voltage connector to connect an external
excitation source to the NI 9237.
Understanding NI 9237 Filtering
The NI 9237 uses a combination of analog and digital filtering to
provide an accurate representation of in-band signals while
rejecting out-of-band signals. The filters discriminate between
signals based on the frequency range, or bandwidth, of the signal.
The three important bandwidths to consider are the passband, the
stopband, and the alias-free bandwidth.
The NI 9237 represents signals within the passband, as quantified
primarily by passband flatness and phase nonlinearity. All signals
that appear in the alias-free bandwidth are either unaliased signals

© National Instruments Corp. 17 NI 9237 Operating Instructions and Specifications
or signals that have been filtered by at least the amount of the
stopband rejection.
Passband
The signals within the passband have frequency-dependent gain or
attenuation. The small amount of variation in gain with respect to
frequency is called the passband flatness. The digital filters of the
NI 9237 adjust the frequency range of the passband to match the
data rate. Therefore, the amount of gain or attenuation at a given
frequency depends on the data rate. Figure 4 shows typical
passband flatness for the NI 9237.

NI 9237 Operating Instructions and Specifications 18 ni.com
Figure 4. Typical Passband Flatness for the NI 9237
Stopband
The filter significantly attenuates all signals above the stopband
frequency. The primary goal of the filter is to prevent aliasing.
Therefore, the stopband frequency scales precisely with the data
Frequency/Data Rate
0.50.4
0.025
0.000
–0.025
–0.050
0.30.20.10
Gain (dB)

© National Instruments Corp. 19 NI 9237 Operating Instructions and Specifications
rate. The stopband rejection is the minimum amount of attenuation
applied by the filter to all signals with frequencies within the
stopband.
Alias-Free Bandwidth
Any signal that appears in the alias-free bandwidth of the NI 9237
is not an aliased artifact of signals at a higher frequency. The
alias-free bandwidth is defined by the ability of the filter to reject
frequencies above the stopband frequency and equals the data rate
minus the stopband frequency.
Understanding NI 9237 Data Rates
The frequency of a master timebase (fM) controls the data rate (fs)
of the NI 9237. The NI 9237 includes an internal master timebase
with a frequency of 12.8 MHz, but the module also can accept an
external master timebase or export its own master timebase. To
synchronize the data rate of an NI 9237 with other modules that use
master timebases to control sampling, all of the modules must
share a single master timebase source. Refer to the software help
for information about configuring the master timebase source for
the NI 9237. Visit ni.com/info and enter cseriesdoc for
information about C Series documentation.

NI 9237 Operating Instructions and Specifications 20 ni.com
The following equation provides the available data rates of the
NI 9237:
fs=
where nis any integer from 1 to 31.
However, the data rate must remain within the appropriate data rate
range. Refer to the Specifications section for more information
about the data rate range. When using the internal master timebase
of 12.8 MHz, the result is data rates of 50 kS/s, 25 kS/s, 16.67 kS/s,
and so on down to 1.613 kS/s, depending on the value of n. When
using an external timebase with a frequency other than 12.8 MHz,
the NI 9237 has a different set of data rates.
Note The cRIO-9151 R Series Expansion chassis does
not support sharing timebases between modules.
Sleep Mode
This module supports a low-power sleep mode. Support for sleep
mode at the system level depends on the chassis that the module is
plugged into. Refer to the chassis manual for information about
fM256÷
n
---------------------