National Instruments NI 9235 User manual

OPERATING INSTRUCTIONS AND SPECIFICATIONS
NI 9235/9236
8-Channel, 24-Bit Quarter-Bridge Analog Input
Module
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NI 9235/9236 2ni.com
This document describes how to use the National Instruments 9235
and National Instruments 9236 and includes specifications and
terminal assignments. In this document, the NI 9235 and NI 9236
are referred to inclusively as the NI 9235/9236. 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 9235/9236. 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. 3NI 9235/9236
Safety Guidelines
Operate the NI 9235/9236 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 9235/9236 is suitable for use in Class I, Division 2, Groups
A, B, C, D, T4 hazardous locations; Class I, Zone 2,
AEx nA IIC T4, and Ex nA IIC T4 hazardous locations; and
nonhazardous locations only. Follow these guidelines if youare
installing the NI 9235/9236 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 9235/9236 4ni.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 limit:
Capacitance .......................... 0.2 μF max
Special Conditions for Hazardous Locations Use in Europe
This equipment has been evaluated as Ex nA IIC T4 equipment
under DEMKO Certificate No. 07 ATEX 0626664X. Each module
is marked II 3G and is suitable for use in Zone 2 hazardous
locations. If youare using the NI 9235/9236 in Gas Group IIC
hazardous locations or in ambient temperatures of
–40 °C ≤Ta ≤70 °C, youmust 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. 5NI 9235/9236
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 9235/9236 6ni.com
Connecting the NI 9235/9236
The NI 9235/9236 has a 24-terminal detachable spring-terminal
connector that provides connections for 8 analog input channels.
Figure 1. NI 9235/9236 Terminal Assignments
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

© National Instruments Corp. 7NI 9235/9236
Yo ucan connect a quarter-bridge sensor to each channel. Each
channel has an EXC terminal that provides the excitation voltage
stimulus, an AI terminal that measures the bridge voltage, and an
RC terminal that provides the quarter-bridge completion. Refer to
Figure 2 for an illustration of how to connect quarter-bridge
sensors to the NI 9235/9236.
Figure 2. Quarter-Bridge Connections for the NI 9235/9236
*For best system accuracy, set up the connections to EXC and RC with
equal lengths of an identical wire type and gauge.
** For best system accuracy, set up the connection to AI directly at the
sensor instead of shorting AI to RC directly at the terminals.
EXC*
AI**
RC*NI 9235/9236

NI 9235/9236 8ni.com
Connecting Wires to the NI 9235/9236 Connector
Use a flathead screwdriver with a blade smaller than 2.3 × 1.0 mm
(0.09 × 0.04 in.) to connect wires to the detachable spring-terminal
connector. Insert the screwdriver into a spring clamp activation slot
and press a wire into the corresponding connector terminal, then
remove the screwdriver to clamp the wire into the terminal. Refer
to the Specifications section for more information about
spring-terminal wiring.
Figure 3. Connecting Wires to the NI 9235/9236 Connector

© National Instruments Corp. 9NI 9235/9236
Wiring for High-Vibration Applications
If an application is subject to high vibration, National Instruments
recommends that youuse the NI 9965 backshell kit to protect the
connections. Refer to Figure 4 for an illustration of the NI 9965
connector backshell.
Figure 4. NI 9965 Connector Backshell

NI 9235/9236 10 ni.com
NI 9235/9236 Circuitry
The NI 9235/9236 is isolated from earth ground. However, the
individual channels are not isolated from each other. The EXC
terminals all connect internally to a common excitation supply.
Yo umust connect each EXC terminal to only one gage to maintain
the channel-to-channel crosstalk performance of the module.
Each channel on the NI 9235/9236 has an independent 24-bit ADC
and input amplifier that enables youto sample signals from all
eight channels simultaneously. Refer to Figures 5 and 6 for
illustrations of the input circuitry for one channel of the
NI 9235/9236.

© National Instruments Corp. 11 NI 9235/9236
Figure 5. Input Circuitry for One Channel of the NI 9235
EXC
AI
RC
+
–
Bridge
Completion
Resistor
Filtered
Differential
Amplifier
2.0 V
Bridge
Excitation
120 Ω50 kΩ
Shunt
Resistor
+
–
ADC
NI 9235

NI 9235/9236 12 ni.com
Figure 6. Input Circuitry for One Channel of the NI 9236
The NI 9235/9236 also includes filters to prevent aliasing. The
filters on the NI 9235/9236 filter according to the sampling rate.
Refer to the Understanding NI 9235/9236 Filtering section for
more information about filtering.
EXC
AI
RC
+
–
Bridge
Completion
Resistor
Filtered
Differential
Amplifier
3.3 V
Bridge
Excitation
350 Ω100 kΩ
Shunt
Resistor
+
–
ADC
NI 9236

© National Instruments Corp. 13 NI 9235/9236
Quarter-bridge measurements are inherently sensitive to accuracy
degradation due to the lead resistance of wiring from the sensor to
the measurement device. For a given change in the gage resistance,
the total effective resistance changes slightly less. Accordingly, the
measured mV/V reading is less than its true value. However, you
can use shunt calibration to quantify the lead wire desensitization,
and can then design the software application to correct subsequent
readings for this gain error. The gain error caused by a lead wire
equals RL/RG, where RLis the lead wire resistance, and RGis the
quarter-bridge completion resistance.
Shunt Calibration
The NI 9235/9236 shunt calibration circuitry consists of a
precision resistor and a software-controlled switch, connected
across the internal quarter-bridge completion resistor. Refer to the
software help for information about enabling the shunt calibration
switch for the NI 9235/9236. Each input channel has a unique
shunt calibration resistor that can operate independently, as shown
in Figures 5 and 6.

NI 9235/9236 14 ni.com
Shunt calibration simulates strain input by changing the resistance
of an arm in the bridge by a known amount. By shunting, or
connecting, a large resistor across one arm of the bridge, a specific
change occurs in the bridge voltage ratio. With the connected
sensor in a stable, typically unloaded, state, youcan measure the
output of the bridge before and after the shunt calibration. Youcan
compare the measured reading change to the shunt calibration
output value to verify system setup or compensate for
quarter-bridge lead wire desensitization error. Refer to the
Specifications section for the shunt calibration output value. Visit
ni.com/info and enter lwcomp for information about lead wire
compensation.
Excitation Voltage
The NI 9235/9236 provides a constant excitation supply voltage to
each channel. The excitation supply provides sufficient output
current to power all eight channels at minimum resistance.
The excitation supply retains regulation even if one channel
experiences a gage short. If more than one channel has a gage
short, the excitation supply enters a current limit state and the
excitation voltage falls accordingly.

© National Instruments Corp. 15 NI 9235/9236
Understanding NI 9235/9236 Filtering
The NI 9235/9236 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 9235/9236 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 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 9235/9236 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 7 shows typical
passband flatness for the 10 kS/s data rate.

NI 9235/9236 16 ni.com
Figure 7. Typical Passband Response for the NI 9235/9236
Frequency (kHz)
54
0.025
0.000
–0.025
–0.050
3210
Gain (dB)

© National Instruments Corp. 17 NI 9235/9236
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
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 9235/9236 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 9235/9236 Data Rates
The frequency of a master timebase (fM) controls the data rate (fs)
of the NI 9235/9236. The NI 9235/9236 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 9235/9236 with
other modules that use master timebases to control sampling, all of

NI 9235/9236 18 ni.com
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 9235/9236. Visit ni.com/info and
enter cseriesdoc for information about C Series documentation.
The following equation provides the available data rates of the
NI 9235/9236:
fs=
where nis any integer in the set {2; 4, 5, ..., 63}.
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 10 kS/s, 8.333 kS/s,
7.143 kS/s, and so on down to 794 S/s, depending on the value of
n. When using an external timebase with a frequency other than
12.8 MHz, the NI 9235/9236 has a different set of data rates.
Note The cRIO-9151 R Series Expansion chassis does
not support sharing timebases between modules.
fM256÷
n
---------------------

© National Instruments Corp. 19 NI 9235/9236
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
support for sleep mode. If the chassis supports sleep mode, refer to
the software help for information about enabling sleep mode. Visit
ni.com/info and enter cseriesdoc for information about
CSeriesdocumentation.
Typically, when a system is in sleep mode, youcannot
communicate with the modules. In sleep mode, the system
consumes minimal power and may dissipate less heat than it does
in normal mode. Refer to the Specifications section for more
information about power consumption and thermal dissipation.

NI 9235/9236 20 ni.com
Specifications
The following specifications are typical for the range –40 to 70 °C
unless otherwise noted. The specifications are the same for the
NI 9235 and the NI 9236 unless otherwise noted.
Input Characteristics
Number of channels .......................... 8 analog input channels
Quarter-bridge completion
NI 9235....................................... 120 Ω, 10 ppm/°C max
NI 9236....................................... 350 Ω, 10 ppm/°C max
ADC resolution ................................. 24 bits
Type of ADC..................................... Delta-Sigma (with analog
prefiltering)
Sampling mode ................................. Simultaneous
Internal master timebase (fM)
Frequency ................................... 12.8 MHz
Accuracy..................................... ±100 ppm max