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SCXI Universal Strain Gauge
Input Module
SCXI-1520
Driver Software
NI-DAQ
Windows 2000/NT/Me/9x
8 channels
Programmable gain
(1 to 1000)
Programmable excitation
(0 to 10 V)
Programmable 4-pole Butterworth
filters (10, 100, 1000,
10000 Hz, bypass)
All settings programmable
per channel
Automatic null compensation
in hardware
Simultaneous sampling
Remote sensing
Quarter, half, and
full-bridge completion
Two shunt calibration circuits
per channel
Random Scanning
Onboard calibration reference
Application Software
LabVIEW
Measurement Studio
Lookout
Calibration Certificate Included
Module
Signal Compatibility
Quarter-Bridge
Half-Bridge
Full-Bridge
Force, Load,
EXPRESS
CODES
(120
, 350
)
(120
, 350
)
(120
, 350
)
Torque
SCXI-1520
For information or to
buy products online,
visit ni.com/catalog
and enter:
Table 1. Module Compatibility
Overview
The NI SCXI-1520 is an 8-channel universal strain gauge input module
that offers all of the features you need for simple or advanced strain
measurements. With this single module, you can read signals from strain
gauges, force sensors, torque sensors, and load cells. Each SCXI-1520 is
shipped with a NIST-traceable calibration certificate, and includes an on-
board reference for automatic calibration.
For simple strain measurements, the SCXI-1520 offers a
programmable amplifier and programmable 4-pole Butterworth filter
on each channel. Each channel also has an independent 0-10 V
programmable excitation source. In addition, the SCXI-1520 system
offers a half-bridge completion resistor network in the module, and a
socketed 350 Ω quarter-bridge completion resistor in the SCXI-1314
terminal block. A 120 Ω quarter-bridge completion resistor is also
included with the terminal block.
For more advanced applications, the SCXI-1520 also offers an
automatic null compensation circuit, remote sensing, and two shunt
calibration circuits per channel. In addition, the SCXI-1520 includes
track-and-hold circuitry for simultaneous sampling applications.
Each SCXI-1520 module multiplexes its signals into a single channel
of the controlling DAQ device, and you can add modules to increase
channel count. The SCXI-1520 also offers parallel-mode operation for
higher-speed scanning applications.
Analog Input
The eight analog inputs of the SCXI-1520
universal strain input module consist of
programmable instrumentation amplifiers,
4-pole Butterworth filters, and track-and-
hold circuitry. You can program the gain of each channel individually to
one of 48 input ranges from ±10 mV to ±10 V. You can program each
lowpass filter individually for 10 Hz, 100 Hz, 1 kHz, 10 kHz, or bypass
mode. The 4-pole Butterworth filters provide a sharp cutoff to block
noise while maintaining maximum flatness in the passband. Finally, the
SCXI-1520 implements random scanning, so you acquire data from only
the channels you select, thereby reducing your overall scan times. For
applications requiring fewer than eight strain gauges, you can use the
extra analog input channels for general-purpose analog signals.
scxi1520
Simultaneous Sampling
Each channel of the SCXI-1520 includes track-and-hold circuitry so you
can digitize simultaneous events with negligible skew time between
channels. The outputs of the T/H amplifiers follow their inputs until they
receive a hold signal from the DAQ device (typically at the start of a scan).
At the hold signal, the T/H amplifiers simultaneously freeze, holding the
input signal levels constant. The DAQ device then digitizes each frozen
signal sequentially, giving you simultaneous sampling between channels.
490
National Instruments
Tel: (512) 794-0100 • Fax: (512) 683-9300 • info@ni.com • ni.com
 
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SCXI Universal Strain Gauge
Input Module
SCXI-1314
Terminal
Block
SCXI-1520 Module
Calibration
Source
MCH0+
Buffer
SO+ CH+
MCH0–
Inst. amp.
x1 or x20
Lowpass
Filter
Variable Gain
x1 to x50
Lowpass
Filter
Track and
hold, x1
Analog
Multiplexer
SO– CH–
MCH7+
MCH7–
Scan Clock
To
Analog
Bus
Analog
Bus
Switch
Null
Compensation
Setting
Excitation
Voltage
Setting
Multiplexer
Control
RSO+
1 K
—
PO+
Output
Amp
Digital
Interface and Control
+
Screw
Terminals
to
Field
Wiring
Calibration EEPROM
Quarter-Bridge
Completion
Half-
Bridge
Completion
QTR
+
–X1
PO–
Output
Amp
—
1 K
RSO–
Shunt Calibration Switches
Shunt Cal A
SCA0
SCA0
Shunt Cal B
SCB0
SCB0
Channel 0 of Channels 0 Through 7
Figure 1. SCXI-1520 Block Diagram
Excitation
Each channel of the SCXI-1520 universal strain gauge input module
has an independent voltage excitation source. You can program each
excitation channel to one of 17 voltage excitation levels from 0 to 10 V.
These sources can drive a 350 Ω full bridge to the maximum 10 V
level. Each excitation channel incorporates remote sensing circuitry to
compensate for voltage drops due to lead resistance. This circuitry
boosts the excitation level so the programmed excitation level is
applied at the sensor. You can also monitor these excitation sources to
detect open or fault situations.
to zero in seconds. You do not need to manually adjust a
potentiometer. By removing this offset through the measurement
hardware, you can increase your system gain to achieve better
measurement sensitivity and resolution.
Bridge Completion
The SCXI-1520 universal strain gauge input module accepts quarter,
half, and full-bridge sensors. Half-bridge completion is provided in the
SCXI-1520 module, and you can enable it through software. The
RN-55 style quarter-bridge completion resistors are provided in the
SCXI-1314 front-mounting terminal block. They are socketed, so you
can replace them with your own resistors.
Strain Gauge
Maximum Excitation Voltage/Channel
Quarter-bridge
Half-bridge
Full-bridge
120
6.25 V
6.9 V
3.125 V
Shunt Calibration
Each input channel of the SCXI-1520 includes two independent shunt
calibration circuits, with which you can simulate two separate loading
effects on your strain-based device and compensate for any possible
gain errors. The RN-55 style shunt calibration resistors are in sockets
350
10 V
10 V
10 V
Table 2. Maximum Excitation Voltage Level per Channel
Automatic Null Compensation
Each input channel of the SCXI-1520 includes a circuit to
remove bridge offset voltage. Driver software nulls the offset voltage
National Instruments
Tel: (512) 794-0100 • Fax: (512) 683-9300 • info@ni.com • ni.com
491
 
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SCXI Universal Strain Gauge
Input Module
and located in the SCXI-1314 front-mounting terminal block. You
enable or disable the shunt resistors through software commands.
using simple software
commands, you perform
calibrations to compensate for
environmental changes
without connecting external
hardware. Each module has
an onboard calibration
EEPROM that stores
calibration constants for each
channel; factory calibration
constants are stored in a
protected area of the
EEPROM. Additional user-
modifiable locations mean
calibration can occur under
your exact operating
conditions. NI-DAQ driver
software transparently uses the calibration constants to correct for gain
and offset errors for each channel.
Multiplexer Operation
In multiplexed mode, the SCXI-1520 multiplexes all eight conditioned
input signals onto channel MCH0 of the rear signal connector, which
you can connect to a single input channel of the DAQ device.
Furthermore, you can connect the multiplexed output of each
module using programmable switches to analog bus 0 of the SCXIbus
and to other modules. Up to 96 SCXI modules can operate in this
manner with a single DAQ device.
Each module also has an internal counter and FIFO for the high-
speed automatic scanning mode with any E Series digitizer. You may
scan channels in any order, as a scan clock signal from the DAQ device
synchronizes the scanning. To determine the allowable scanning rate,
total the following
• SCXI-1520 acquisition time (7 µs for 12-bit accuracy)
• SCXI system scan interval (3 µs with a PCI-MIO-16E-1)
multiplied by the number of channels minus one.
For example, you can scan 16 channels of two SCXI-1520 modules
with the PCI-MIO-16E-1 in 7µs + 3µs(16-1) = 52 µs or 19.23 kHz.
Reduce Thermal Drift
Although you can connect
to a quarter bridge using
two wires, using three wires
eliminates temperature-
induced offset drift in the
wiring. The SCXI-1520
hardware manual discusses
methods of minimizing
temperature-related errors.
You may download the
SCXI-1520 user manual at
ni.com/info and enter exhesg
Signal Connection
You connect your sensors to the SCXI-1520 module using the
SCXI-1314 front-mounting terminal block. Each channel includes 11
screw terminals - two for input signal, two for excitation, two for
remote sensing, one for quarter-bridge completion, and four for two
independent shunt calibration circuits.
Calibration
The SCXI-1520 provides simple yet powerful calibration capabilities.
Each module includes a precision onboard calibration source, which
you can programmatically route to any analog input channel. By
Terminal Block
Type
CJ Sensor
Compatible Modules
Cabling
Special Functions
SCXI-1314
Screw terminals
N
SCXI-1520
-
Quarter-bridge completion
(777687-14)
Front-mounting
Table 3. Terminal Block Options for the SCXI-1520
Ordering Information
SCXI-1520 ..........................................................777966-20
See page 470 to configure your complete SCXI system.
Extended Warranty
and Value Added Services ...................................page 880
492
National Instruments
Tel: (512) 794-0100 • Fax: (512) 683-9300 • info@ni.com • ni.com
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SCXI Universal Strain Gauge
Input Module
SCXI-1314
SCXI-1520
SCXI-1314
SCXI-1520
P+
P+
S-
S+
S-
S+
Qtr
Quarter-Bridge
Configuration
P-
Half-Bridge
Configuration
SCXI-1314
SCXI-1314
SCXI-1314
Shunt Calibration
P+
RS+
P+
P+
S+
S-
S+
S-
S+
S-
P-
SC-
P-
P-
Full-Bridge
Configuration
Remote Sensing
RS-
SC+
Figure 2. Bridge Completion, Remote Sensing and Shunt Calibration Diagrams
Specifications
Complete Accuracy Table, Voltage
Absolute Accuracy (15 to 35˚ C)
Temperature Drift
Range
Gain
% of Reading Max
System Noise (Peak, 3 sigma)
Max Offset
Single Pt.
100 Pt. Average
Gain Drift
Offset Drift
±10 V
1
±0.1%
±3 mV
10 mV
1 mV
±0.03%/˚C
±25 µV/˚C
±5.0 V
2
±0.1%
±1.5 mV
5 mV
0.5 µV
±0.03%/˚C
±25 µV/˚C
±1.8 V
4.2
±0.1%
±0.5 mV
2 mV
0.2 mV
±0.03%/˚C
±25 µV/˚C
±1.0 V
10
±0.1%
±0.3 mV
1 mV
0.1 mV
±0.03%/˚C
±25 µV/˚C
±500 mV
20
±0.1%
±150 µV
0.5 mV
50 µV
±0.03%/˚C
±5 µV/˚C
±180 mV
42
±0.1%
±75 µV
0.2 mV
20 µV
±0.03%/˚C
±5 µV/˚C
±100 mV
100
±0.1%
±50 µV
100 µV
10 µV
±0.03%/˚C
±5 µV/˚C
±50 mV
200
±0.1%
±50 µV
50 µV
5 µV
±0.03%/˚C
±5 µV/˚C
±18 mV
420
±0.1%
±50 µV
20 µV
2 µV
±0.03%/˚C
±5 µV/˚C
±10 mV
1000
±0.1%
±50 µV
20 µV
2 µV
±0.03%/˚C
±5 µV/˚C
Absolute accuracy is (voltage reading) * (% of Reading) + (offset error) + (system noise). To include the effects of temperature drift outside the range 15 to 25 ˚C, add the term
T * (Gain drift) * (Range) * ∆ T * (Offset Drift), where ∆ T is temperature difference between the module temperature and 15 or 35 ˚C, whichever is smaller. Bandwidth
setting is 10 Hz and Scan rate for 100-point averages is 200 scans/s. Excitation is set to zero Volts.
Complete Accuracy Table, Strain, GF=2.0, Excitation = 5V
Absolute Accuracy (15 to 35˚ C)
Temperature Drift
Bridge
Range
Gain
% of Reading Max
Hardware
System Noise (Peak, 3 sigma)
Nulling Range
Single Pt.
100 Pt. Average
Gain Drift
Offset Drift
±40000 µ ε
±80,000 µ ε
±40 µ ε
±4 µ ε
±80 µ ε
Quarter Bridge
100
±0.1%
±0.03%/˚C
/˚C
±7000 µ ε
±80,000 µ ε
±7 µ ε
±2 µ ε
±16 µ ε
560
±0.1%
±0.03%/˚C
/˚C
±4000 µ ε
±80,000 µ ε
±4 µ ε
±1 µ ε
±8 µ ε
1000
±0.1%
±0.03%/˚C
/˚C
±2500 µ ε
±40,000 µ ε
±2 µ ε
±0.5 µ ε
±4 µ ε
Half Bridge
1000
±0.1%
±0.03%/˚C
/˚C
±1250 µ ε
±20,000 µ ε
±1 µ ε
±0.2 µ ε
±2 µ ε
Full Bridge
1000
±0.1%
±0.03%/˚C
/˚C
Absolute accuracy is (voltage reading) * (% of Reading) + (offset error) + (system noise). To include the effects of temperature drift outside the range 15 to 25 ˚C, add the term
T * (Gain drift) * (Range) + ∆ T * (Offset Drift), where ∆ T is temperature difference between the module temperature and 15 or 35 ˚C, whichever is smaller. Bandwidth
setting is 10 Hz and Scan rate for 100-point averages is 200 scans/s.
National Instruments
Tel: (512) 794-0100 • Fax: (512) 683-9300 • info@ni.com • ni.com
493
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SCXI Universal Strain Gauge
Input Module
Specifications (continued)
Typical for 25 ˚C unless otherwise noted
Null Compensation Characteristics
Range…………………………………….......
Analog Input Characteristics
Number of channels...................................
±4% of excitation voltage, 20000 counts
of resolution ( ±80,000 µe, 4 µe
resolution for quarter bridge, GF = 2.0)
8
Voltage gain settings ..................................
X1 to X1000 with the following gain
settings: 1; 1.15; 1.3; 1.5; 1.8; 2; 2.2;
2.4; 2.7; 3.1; 3.6; 4.2; 5.6; 6.5; 7.5; 8.7;
10; 11.5; 13; 15; 18; 20; 22; 24; 27; 31;
36; 42; 56; 65; 75; 87;100; 115; 130;
150; 180; 200; 220; 270; 310; 360; 420;
560; 750; 870; 1000
Excitation Characteristics
Type ..........................................................
constant voltage
Settings ......................................................
0.0 to 10.0 V in 0.625 V increments
Error ..........................................................
±20 mV +.3% absolute
±0.1% of EEPROM setting
Short circuit current limit .............................
50 mA minimum
Input coupling…………………………….....
DC
Regulation..................................................
no load to 120 Ω load: ±0.003%
with remote sense ±0.08% without
remote sense
Maximum working voltage ……………. .....
Either input should remain within ±10 V
of ground. Both inputs should be within
±10 V of one another.
Drift…………………………………………...
±0.005%/C ±30 µV/C max.
Overvoltage protection………………….. ....
±35 V powered on, ±25 V powered off
Noise………………………………………. ...
DC to 10 kHz: 200 µV
Inputs protected ………………………….....
<0...7>
Remote Sense……………………………......
Error less than ±0.02% per Ω
of lead resistance
Transfer Characteristics
Nonlinearity................................................
Protection………………………………….....
Surge arrestors in parallel with excitation
terminals, shunt to ground.
Better than 0.02%
Bridge Completion 1
Half bridge .................................................
Gain error...................................................
±.35% of setting, +0.1% of EEPROM value
Offset Error
Gain>20……………………………..........
2 k Ω precision resistor network internal
to module
150 µV max
Gain<20……………………………..........
3 mV max
Quarter bridge ...........................................
resistor in accessory terminal block
SCXI-1314
Amplifier Characteristics
Input Impedance (DC)…………………….. ..
Shunt Calibration 2
Type: ..........................................................
>1 G Ω
Input bias current…………………………....
±20 nA max
two independent points
Input offset current………….…………….. ..
±20 nA max
Resistor.......................................................
in terminal block
Switch Resistance........................................
32 Ω
CMRR
Switch off Leakage......................................
<1 nA
Switch break-down voltage
Power Requirements..................................
Gain
CMRR DC to 60 Hz
±60 VDC
<20
60 dB
V+
..........................................................
18.5 to 25 VDC, +170 mA min
20
85 dB
V–
..........................................................
–18.5 to –25 VDC, –170 mA min
Dynamic Characteristics
Scan Interval (per channel, any gain)
±0.0125 accuracy………………. ............
+5V ..........................................................
+4.75 to 5.25 VDC, 50 mA min
Physical
Dimensions ................................................
3 µs
±0.006 accuracy………………. ............
10 µs
3.0 by 17.2 by 20.3 cm (1.2 by 6.9 by
8.0 in.)
±0.0015 accuracy………………. ............
20 µs
Environment
Operating temperature...............................
Noise RTI, gain=200, 0.1 to 10 Hz ..............
2.0 µV p p
0 to 50 °C
Spot noise RTI, gain=200, 1000 Hz .............
16 nV/ √
Hz
Storage temperature...................................
–20 to 70 °C
Relative humidity ........................................
10 to 90% noncondensing
Filter Characteristics
Low pass filter type .....................................
Certifications and Compliances
European Compliance
EMC ..........................................................
4-pole Butterworth
(24 dB octave rolloff)
Low pass filter settings ................................
10 Hz, 100 Hz, 1 kHz, 10 kHz, or bypass
EN 61326 Group I Class A, 10m, Table 1
Immunity
Bandwidth, filter bypassed..........................
-3 dB at 20 kHz
Safety ........................................................
EN 61010-1
Track and Hold Characteristics
Hold mode settle time ................................
North American Compliance
EMC ..........................................................
1 µs typ
FCC Part 15 Class A using CISPR
Interchannel skew ......................................
±50 nS max
Australia & New Zealand Compliance
EMC ..........................................................
Intermodule skew .......................................
±100 nS max
AS/NZS 2064.1/2 (CISPR-11)
Droop rate .................................................
30 mV/s typ, 100mV/s max
Analog Input Stability
Recommended warm-up time ....................
15 minutes
Gain drift:...................................................
±40 ppm/C max
Offset drift
Gain>20.................................................
2 µV/˚C typ, ±5 µV/ ˚C max
Gain<20.................................................
10 µV/˚C typ, ±25 µV/ ˚C max
Notes
1
Half-bridge completion is inside the module and configured under software control. Quarter-bridge completion resistor is in SCXI-1314 terminal block and socketed. Resistors
shipped with SCXI-1314 are 120 Ω
and 350 Ω
RN-55 style (0.25 W) Tolerance is ±0.1%. Temperature coefficient is ±10 ppm/˚C max.
2
Shunt calibration resistors are in SCXI-1314 terminal block and socketed. Resistors shipped with SCXI-1314 are 100 k Ω
RN-55 style (0.25 W) Tolerance is ±0.1%.
Temperature coefficient is ±10 ppm/˚C max
494
National Instruments
Tel: (512) 794-0100 • Fax: (512) 683-9300 • info@ni.com • ni.com
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