The NI SCXI-1520 is an 8-channel universal strain gage input module that
offers all of the features you need for simple or advanced strain- and bridge-
based sensor measurements. With this single module, you can read signals
from strain, load, force, torque, and pressure sensors. Each SCXI-1520
is shipped with a NIST-traceable calibration certificate, and includes an
onboard reference for automatic calibration in changing environments.
For accurate 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 to 10 V programmable excitation
source with remote sense per channel. 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. The SCXI-1520 offers an automatic null
compensation circuit, remote sensing, and two shunt calibration circuits
per channel. In addition, the SCXI-1520 includes the simultaneous sample-
and-hold feature using track-and-hold (T/H) circuitry for simultaneous
sampling applications.
Each SCXI-1520 can multiplex its signals into a single channel of the
controlling data acquisition device, and you can add modules to increase
channel count. In NI-DAQ 7.0, parallel mode operation is available for
high-speed acquisitions. In this mode, each channel is routed to a unique
analog input channel of the data acquisition device to which it is cabled.
Parallel mode is not available in NI-DAQ Traditional (Legacy).
Analog Input
Each of the eight SCXI-1520 analog inputs consists of a programmable
instrumentation amplifier, 4-pole Butterworth filter, and simultaneous
sample-and-hold circuit.You can program the gain of each channel
individually to one of 49 input ranges from ±10 mV to ±10 V. You can also
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 provides random scanning capability, so you acquire data
from the channels you select in any order, thereby reducing your overall
scan times. For applications requiring fewer than eight strain gages,
you can use the extra analog input channels for general-purpose
analog signals.
Simultaneous Sampling
Each SCXI-1520 channel includes T/H 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 data acquisition 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 data acquisition device then
digitizes each frozen signal sequentially, giving you simultaneous
sampling between channels.
Module
SCXI-1520
Quarter-Bridge
(120
Ω,
350
Ω)
Half-Bridge
(120
Ω,
350
Ω)
Full-Bridge
Force, Load,
(120
Ω,
350
Ω)
Torque, Pressure
Table 1. Signal Compatibility
SCXI Universal Strain Gage Input
SCXI-1314
Terminal Block
SCXI-1520 Module
SO– CH–
Inst. amp.
x1 or x20
Lowpass
Filter
Lowpass
Filter
Analog
Multiplexer
SO+ CH+
Buffer
Variable
Gain
x1 to x50
Track
and
hold, x1
CH 0+
CH 0–
CH 7+
CH 7–
Scan Clock
Screw Terminals to Field Wiring
RSO+
PO+
Quarter-
Bridge
Completion
Half-Bridge
Completion
QTR
+
PO–
RSO–
Shunt Cal A
SCA0
SCA0
SCB0
SCB0
Shunt Cal B
1 kW
–
1 kW
–
+
Digital
Interface and Control
Calibration EEPROM
Output
Amps
–X1
Shunt Calibration Switches
Channel 0 of Channels 0 through 7
Figure 1. SCXI-1520 Block Diagram
Excitation
All SCXI-1520 channels have 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 automatically compensate for voltage drops due to
lead resistance. This circuitry corrects the excitation level on the fly so
the programmed excitation level is accurately applied at the sensor.
You can also monitor these excitation sources to detect open
or fault situations.
Strain Gage
120
Ω
350
Ω
Table 2. Excitation Values
Quarter-Bridge
6.25 V
10.00 V
Half-Bridge
6.9 V
10.0 V
Full-Bridge
3.125 V
10.000 V
Bridge Completion
The SCXI-1520 accepts quarter-, half-, and full-bridge sensors. Half-
bridge completion is provided in the SCXI-1520, 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.
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
and located in the SCXI-1314 front-mounting terminal block. You enable
or disable the shunt resistors through software commands.
Automatic Null Compensation
Each input channel of the SCXI-1520 includes a circuit to remove bridge
offset voltage. Driver software nulls the offset voltage 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.
BUY ONLINE
at
ni.com
or
CALL 800 813 3693 (U.S.)
SCXI Bus Connector
Null
Compensation
Setting
Excitation
Voltage
Setting
Multiplexer
Control
To
Analog
Analog
Bus
Bus
Switch
Rear Signal Connector
Calibration
Source
2
SCXI Universal Strain Gage Input
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 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
Traditional (Legacy) and NI-DAQ 7.0 transparently use the calibration
constants to correct for gain and offset errors for each channel.
Terminal
Block
SCXI-1314
(777687-14)
Type
Screw
terminals
CJ
Sensor
–
Compatible
Modules
SCXI-1520
Front-mounting
Cabling
–
Special
Functions
Quarter-bridge
completion
Table 3. Terminal Block Options for the SCXI-1520
Ordering Information
NI SCXI-1520 ........................................................................777966-20
BUY NOW!
For complete product specifications, pricing, and accessory
information, call 800 813 3693 (U.S.) or go to
ni.com/scxi.
BUY ONLINE
at
ni.com
or
CALL 800 813 3693 (U.S.)
3
SCXI Universal Strain Gage Input
Specifications
Typical for 25 °C unless otherwise noted.
Complete Accuracy Table, Voltage
Module
SCXI-1520
Nominal Range
1
±10.0 V
±5.0 V
±1.8 V
±1.0 V
±500.0 mV
±180.0 mV
±100.0 mV
±50.0 mV
±18.0 mV
±10.0 mV
Overall Gain
1
1.0
2.0
4.2
10.0
20.0
42.0
100.0
200.0
420.0
1000.0
Percent of Reading
1
±0.1
±0.1
±0.1
±0.1
±0.1
±0.1
±0.1
±0.1
±0.1
±0.1
Offset
1
±3.0 mV
±1.5 mV
±0.5 mV
±0.3 mV
±150.0 µV
±75.0 µV
±50.0 µV
±50.0 µV
±50.0 µV
z±50.0 µV
System Noise (peak, 3 sigma)
1
Single Point
100-Point Average
10.0 mV
1.0 mV
5.0 mV
0.5 µV
2.0 mV
0.2 mV
1.0 mV
0.1 mV
0.5 mV
50.0 µV
0.2 mV
20.0 µV
100.0 µV
10.0 µV
50.0 µV
5.0 µV
20.0 µV
2.0 µV
20.0 µV
2.0 µV
Temperature Drift
Gain Drift (%/°C)
Offset (µV/°C)
±0.03
±25
±0.03
±25
±0.03
±25
±0.03
±25
±0.03
±5
±0.03
±5
±0.03
±5
±0.03
±5
±0.03
±5
±0.03
±5
1
Absolute Accuracy (15 to 35 °C). Absolute accuracy is (voltage reading) x (% of reading) + (offset error) + (system noise). To include the effects of temperature drift outside the range 15 to 25 ˚C, add the term
∆T
x (gain drift) x (range) +
∆T
x (offset drift), where
∆T
is the 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. To calculate the absolute accuracy for the SCXI-1520, visit
Hardware Nulling Range Single Point 100-Point Average
±80,000 µε
±40 µε
±4 µε
±80,000 µε
±7 µε
±2 µε
±80,000 µε
±4 µε
±1 µε
±40,000 µε
±2 µε
±0.5 µε
±20,000 µε
±1 µε
±0.2 µε
Temperature Drift
Gain Drift (%/°C) Offset (µε/°C)
±0.03
±80
±0.03
±16
±0.03
±8
±0.03
±4
±0.03
±2
Half-bridge
Full-bridge
1
Absolute Accuracy (15 to 35 °C). Absolute accuracy is (voltage reading) x (% of reading) + (offset error) + (system noise). To include the effects of temperature drift outside the range 15 to 25 ˚C, add the
term
∆T
x (gain drift) x (range) +
∆T
x (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. To calculate the absolute accuracy for the SCXI-1520, visit
ni.com/accuracy.
Analog Input Characteristics
Number of channels............................ 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; 240;
270; 310; 360; 420; 560; 650;
750; 870; 1,000
Input signal ranges ............................. See Complete Accuracy table
Input coupling ..................................... DC
Maximum working voltage ................. Either input should remain
within ±10 V of ground.
Both inputs should be within
±10 V of one another.
Overvoltage protection ....................... ±35 V powered on,
