SCXI 8-Channel Isolated Analog Input Modules
SCXI 8-Channel Isolated Analog Input
NI SCXI-1125, NI SCXI-1120, NI SCXI-1120D
• 8 channels
• 333 kS/s maximum sampling rate
• Gain and lowpass filter settings
per channel
• Up to 300 V
rms
working isolation
per channel
• Signal inputs from ±2.5 mV
to ±1000 VDC with TBX-1316
• NI-DAQ driver software simplifies
configuration, measurement and scaling
SCXI-1120, SCXI 1120D
• Jumper selectable filter per channel
•
4 Hz and 10 kHz filter (SCXI-1120)
•
4.5 kHz and 22.5 kHz (SCXI-1120D)
• 250 V
rms
working isolation per channel
Operating Systems
• Windows 2000/NT/XP
Recommended Software
• LabVIEW
• LabWindows/CVI
• Measurement Studio
• VI Logger
SCXI-1125
• Programmable gain and filter settings
• 300 V
rms
working isolation per channel,
Driver Software
• NI-DAQ 7
Calibration Certificate Included
See page 21.
Overview
The National Instruments SCXI-1125, SCXI-1120, and SCXI-1120D
are 8-channel isolated analog input modules. These modules share a
common architecture, providing 250 to 300 V
rms
of working isolation
and lowpass filtering for each analog input channel. This architecture
is ideal for amplification and isolation of millivolt, volt, 0 to 20 mA,
4 to 20 mA, and thermocouple signals. Each module can multiplex
these eight channels into a single channel of the DAQ device, and you
can add modules to increase channel count. These modules also offer
parallel mode operation for increased scanning rates.
SCXI-1120, SCXI-1120D
The analog inputs of the NI-1120/D consist of eight isolation
amplifiers. You can configure each amplifier using jumpers for input
ranges from ±2.5 mV to ±5 V (SCXI-1120) or ±5 mV to ±10 V
(SCXI-1120D). With the SCXI-1327 high-voltage attenuator terminal
block, the input range is extended to ±250 V. With the TBX-1316, the
input range is extended to ±1000 VDC (680 V
rms
). Each channel also
includes a lowpass filter that is jumper configurable for 4 Hz or 10 kHz
(SCXI-1120), or for 4.5 or 22.5 kHz (SCXI-1120D). Each channel is
individually isolated with a working common-mode voltage of 250 V
rms
between channels or channel to earth. In addition, the SCXI-1120
and SCXI-1120D are CE certified as double insulated, Category II,
for 250 V
rms
of operational isolation.
Data Acquisition and
Signal Conditioning
Analog Input
SCXI-1125
The analog inputs of the NI SCXI-1125 consist of eight
programmable isolation amplifiers. You can program each channel
independently for input ranges from ±2.5 mV to ±5 V. With the
SCXI-1313 high-voltage attenuator terminal block, the input range is
extended to ±300 V. With the TBX-1316, the input range is extended
to ±1000 VDC (680 V
rms
). Each channel also includes a programmatic
lowpass filter that you can configure for 4 Hz or 10 kHz. With the
SCXI-1125 you can perform random scanning meaning you can
select only the channels from which you want to acquire data as well
as scan channels in any order. Each channel is individually isolated
with a working common-mode voltage of 300 V
rms
between channels
or channel to earth. In addition, the SCXI-1125 is CE certified as
double insulated, Category II, for 300 V
rms
of operational isolation.
Cold-Junction Compensation
Each of these modules can read the cold-junction sensor from the
SCXI-1320, SCXI-1321, SCXI-1327, SCXI-1328, and TBX-1328
terminal blocks. The SCXI-1125 can scan the sensor along with other
channels, but the SCXI-1120/D must read the cold-junction sensor
as a separate analog input operation. This is commonly done once
before the start of a continuous acquisition.
Module
SCXI-1125
SCXI-1120
SCXI-1120D
±2.5 mV
✓
✓
–
±5 mV to ±5 V
✓
✓
✓
±10 V
–
–
✓
±1000 V
✓
*
✓
✓
0 to 20 mA
✓
✓
✓
Thermocouple
✓
✓
✓
*Using attenuating terminal block.
Table 1. Module Compatibility
296
National Instruments • Tel: (800) 433-3488 • Fax: (512) 683-9300 • info@ni.com • ni.com
SCXI 8-Channel Isolated Analog Input Modules
SCXI 8-Channel Isolated Analog Input
Gain Select
Input Channel 0
Lowpass
Filter
Isolation
Barrier
Buffer
MCHO
Front Signal Connector
Analog
Multiplexer
MCH7
Jumper
Multiplexer
Control
Gain Select
Input Channel 7
Lowpass
Filter
Lowpass
Filter
Analog
Bus
Switch
CH O+
CH O–
Rear Signal Connector
Lowpass
Filter
1120 Series
Only
CH 7+
CH 7–
Scan Clock
Digital Interface
and Control
MTEMP
Figure 1. SCXI-1125, SCXI-1120, and SCXI-1120D Block Diagram
Terminal Block
SCXI-1313
SCXI-1320
SCXI-1327
SCXI-1328
SCXI-1338
SCXI-1305
1
TBX-1316
TBX-1328
Part Number
777687-13
777687-20
777687-27
777687-28
777687-38
777687-05
777207-16
777207-28
Type
Screw terminals
Front-mounting
Screw terminals
Front-mounting
Screw terminals
Front-mounting
Screw terminals
Front-mounting
Screw terminals
Front-mounting
BNC connectors
Front-mounting
Screw terminals
DIN-rail mount
Screw terminals
DIN-rail mount
Screw terminals
DIN-rail mount
Solder pins
Front-mounting
CJ Sensor
✓
✓
✓
✓
✓
–
Compatible Modules
SCXI-1125
SCXI-1125
SCXI-1120
SCXI-1120D
Cabling
–
–
–
–
–
–
SH32-32-A
(183230-01)
SH32-32-A
(183230-01)
SH32-32-A
(183230-01)
–
Special Functions
Programmable
100:1 attenuator
IC Sensor for CJC
100:1 attenuator
Isothermal construction
Prewired ground referencing
For current inputs
AC coupling
200:1 attenuator
DIN-rail mount
Isothermal construction
Prewired ground referencing
DIN-rail mount
AC coupling
Low-cost connector and
shell assembly
SCXIbus Connector
To
Analog
Bus
Page
328
329
329
Data Acquisition and
Signal Conditioning
329
330
328
331
331
✓
TBX-1329
SCXI-1330
1
The
777207-29
777687-30
–
–
331
329
SCXI-1305 is not intended for high-voltage (>42 V) usage.
Table 2. Terminal block options for SCXI-1125, SCXI-1120, and SCXI-1120D.
Calibration
The SCXI-1125 contains calibration hardware to null out error sources.
With programmable offset calibration, software-programmable
analog switches ground the inputs of each of the instrumentation
amplifiers for offset error calibration. An onboard EEPROM stores
the calibration constants for each channel for each input range in a
user-defined area. The EEPROM also stores a set of factory calibration
constants in permanent memory, and cannot be modified. NI-DAQ
driver software transparently uses the calibration constants to correct
for gain and offset errors.
Ordering Information
NI SCXI-1125 ................................................................776572-25
NI SCXI-1120 ................................................................776572-20
NI SCXI-1120D..........................................................776572-20D
Accessories
SCXI current resistors (4-pack) ..................................776582-01
For information on extended warranty and value-added
services, see page 20.
BUY ONLINE!
Visit
ni.com/info
and enter scxi1120, scxi1120d and/or scxi1125.
See page 276 to configure your complete system.
National Instruments • Tel: (800) 433-3488 • Fax: (512) 683-9300 • info@ni.com • ni.com
297
SCXI 8-Channel Isolated Analog Input Modules
SCXI 8-Channel Isolated Analog Input
Specifications
Absolute Accuracy Table
Percent of Reading
*
Typical
Max
0.3996
1.2489
0.2548
0.6498
0.2548
0.6498
0.2548
0.6498
0.2548
0.6498
0.2548
0.6498
0.2478
0.6478
0.2478
0.6478
0.2478
0.6478
0.2478
0.6478
0.2478
0.6478
0.2478
0.6478
0.2478
0.6478
0.2478
0.6478
0.2478
0.6478
0.2478
0.6478
0.2478
0.6478
0.2478
0.6478
0.2478
0.6478
0.3996
1.2489
0.2548
0.6498
0.2548
0.6498
0.2548
0.6498
0.2548
0.6498
0.2548
0.6498
0.2478
0.6498
0.2478
0.6498
0.2478
0.6498
0.2478
0.6498
0.2478
0.6478
0.2478
0.6478
0.2478
0.6478
0.2478
0.6478
0.2478
0.6478
0.2478
0.6478
0.2478
0.6478
0.2478
0.6478
0.2478
0.6478
System Noise (peak, 3 sigma)
*
Single Point
Average
4 Hz
10 kHz or FBW
4 Hz
10 kHz or FBW
115 mV
1.62 V
24.5 mV
401 mV
57.7 mV
946 mV
12.7 mV
203 mV
29.9 mV
478 mV
6.26 mV
100 mV
12.0 mV
183 mV
2.51 mV
40.1 mV
5.67 mV
111 mV
1.27 mV
20.3 mV
2.82 mV
47.9 mV
641 uV
10.1 mV
1.05 mV
19.1 mV
238 µV
4.06 mV
528 µV
8.59 mV
122 µV
2.03 mV
254 µV
4.25 mV
59.7 µV
1.01 mV
109 µV
1.68 mV
23.7 µV
403 µV
68.2 µV
882 µV
12.2 µV
202 µV
32.0 µV
474 µV
6.26 µV
101 µV
10.9 µV
180 µV
2.37 µV
40.4 µV
6.20 µV
88.2 µV
1.24 µV
20.3 µV
2.58 µV
47.9 µV
0.593 µV
10.4 µV
2.25 µV
37.1 µV
0.499 µV
8.57 µV
1.27 µV
21.8 µV
0.268 µV
4.69 µV
0.713 µV
14.9 µV
0.170 µV
3.13 µV
0.420 µV
11.2 µV
0.099 µV
2.49 µV
162 mV
1.94 V
38.6 mV
488 mV
86.5 mV
972 mV
18.8 mV
244 mV
37.3 mV
503 mV
9.11 mV
122 mV
15.3 mV
199 mV
3.68 mV
48.4 mV
7.73 mV
98.9 mV
1.79 mV
24.4 mV
4.28 mV
54.6 mV
895 µV
12.3 mV
1.57 mV
26.2 mV
375 µV
4.92 mV
840 µV
10.8 mV
188 µV
2.41 mV
385 µV
5.00 mV
88.7 µV
1.20 mV
157 µV
2.22 mV
36.4 µV
482 µV
80.2 µV
993 µV
18.5 µV
241 µV
45.0 µV
518 µV
9.18 µV
123 µV
15.5 µV
221 µV
3.61 µV
49.3 µV
7.74 µV
108 µV
1.82 µV
24.9 µV
4.21 µV
54.9 µV
0.940 µV
13.3 µV
3.38 µV
50.6 µV
0.788 µV
11.6 µV
1.97 µV
29.3 µV
0.454 µV
7.03 µV
0.962 µV
25.5 µV
0.260 µV
5.58 µV
0.908 µV
22.4 µV
0.314 µV
5.07 µV
Temperature Drift
Percent of
Offset
Reading/°C
(µV/°C)
0.0034
132 mV
0.0029
44 mV
0.0029
44 mV
0.0029
22 mV
0.0029
11 mV
0.0029
4.4 mV
0.0029
2.2 mV
0.0027
1.12 mV
0.0027
460 µV
0.0027
240 µV
0.0027
130 µV
0.0027
64 µV
0.0027
42 µV
0.0027
31 µV
0.0027
24.4 µV
0.0027
22.2 µV
0.0027
21.1 µV
0.0027
20.9 µV
0.0027
20.3 µV
0.0034
132 mV
0.0029
44 mV
0.0029
44 mV
0.0029
22 mV
0.0029
11 mV
0.0029
4.4 mV
0.0029
2.2 mV
0.0027
1.12 mV
0.0027
460 µV
0.0027
240 µV
0.0027
130 µV
0.0027
64 µV
0.0027
42 µV
0.0027
31 µV
0.0027
24.4 µV
0.0027
22.2 µV
0.0027
21.1 µV
0.0027
20.9 µV
0.0027
20.3 µV
Module
SCXI-1125
Nominal Range
*
±1000 V
rms4
±300 V
3
±250 V
3
±100 V
3
±50 V
3
±25 V
3
±10 V
3
±5 V
±2.5 V
±1 V
±500 mV
±250 mV
±100 mV
±50 mV
±25 mV
±20 mV
±10 mV
±5 mV
±2.5 mV
±1000 V
rms4
±500 V
rms4
±250 V
2
±100 V
2
±50 V
2
±25 V
2
±10 V
2
±5 V
±2.5 V
±1 V
±500 mV
±250 mV
±100 mV
±50 mV
±25 mV
±20 mV
±10 mV
±5 mV
±2.5 mV
Overall Gain
*
0.005
0.01
0.02
0.05
0.1
0.2
0.5
1
2
5
10
20
50
100
200
250
500
1000
2000
0.005
0.01
0.02
0.05
0.1
0.2
0.5
1
2
5
10
20
50
100
200
250
500
1000
2000
Offset
Offset
*
854 mV
500 mV
250 mV
100 mV
50 mV
25 mV
10 mV
5.0 mV
2.5 mV
1.0 mV
508 µV
258 µV
108 µV
58 µV
33 µV
28 µV
18 µV
13 µV
11 µV
854 mV
337 mV
250 mV
132 mV
65.3 mV
31.9 mV
11.9 mV
11.3 mV
5.13 mV
2.02 mV
1.00 mV
487 µV
193 µV
93.6 µV
45.3 µV
35.6 µV
18.0 µV
13.0 µV
11.1 µV
SCXI-1120
Data Acquisition and
Signal Conditioning
*Absolute Accuracy (15 to 35 °C). To calculate the absolute accuracy for the SCXI-1125, SCXI-1120, and SCXI-1120D refer to page 194 or visit
ni.com/accuracy
Module
SCXI-1120D
Range
*
±1000 V
rms4
±500 V
rms4
±200 V
2
±100 V
2
±50 V
2
±20 V
2
±10 V
2
±5 V
±2 V
±1 V
±500 mV
±200 mV
±100 mV
±50 mV
±20 mV
±10 mV
±5 mV
Gain
*
0.01
0.02
0.05
0.1
0.2
0.5
1
2
5
10
20
50
100
200
500
1000
2000
Percent of Reading
*
Typical
Max
0.3533
0.8832
0.3533
0.8832
0.3533
0.8832
0.3533
0.8832
0.3533
0.8832
0.3533
0.8832
0.3525
0.8812
0.3525
0.8812
0.3525
0.8812
0.3525
0.8812
0.3525
0.8812
0.3525
0.8812
0.3525
0.8812
0.4192
1.0480
0.7800
1.9500
1.3036
3.2590
2.4008
6.0020
Offset
Offset
*
1.04 V
0.52 V
0.52 V
260 mV
104 mV
52.2 mV
21.0 mV
10.6 mV
5.4 mV
2.28 mV
1.25 mV
726 µV
414 µV
310 µV
258 µV
227 µV
216 µV
System Noise (peak, 3 sigma)
*
Single Point
Average
4.5 kHz
22.5 kHz
4.5 kHz
842 mV
4.29 V
206 mV
475 mV
3.15 V
103 mV
179 mV
2.46 V
47.3 mV
104 mV
2.32 V
30.4 mV
71.6 mV
2.23 V
26.1 mV
46.9 mV
1.96 V
21.4 mV
9.65 mV
40.9 mV
2.11 mV
4.38 mV
30.4 mV
1.04 mV
2.13 mV
23.5 mV
483 µV
1.03 mV
22.2 mV
300 µV
677 µV
21.5 mV
256 µV
448 µV
18.9 mV
208 µV
297 µV
13.2 mV
140 µV
271 µV
13.9 mV
140 µV
263 µV
9.50 mV
139 µV
252 µV
4.81 mV
136 µV
243 µV
2.42 mV
131 µV
22.5 kHz
1.53 V
1.45 V
1.45 V
1.45 V
1.45 V
1.33 V
14.9 mV
14.3 mV
14.3 mV
14.3 mV
14.3 mV
12.8 mV
9.45 mV
9.45 mV
6.35 mV
3.21 mV
1.61 mV
Temperature Drift
Percent of
Offset
Reading/°C
(V/°C)
0.0059
44 mV
0.0059
44 mV
0.0059
22 mV
0.0059
11 mV
0.0059
4.4 mV
0.0059
2.2 mV
0.0059
900 µV
0.0057
460 µV
0.0057
240 µV
0.0057
108 µV
0.0057
64 µV
0.0057
42 µV
0.0057
28.8 µV
0.0057
24.4 µV
0.0057
22.2 µV
0.0057
20.9 µV
0.0057
20.4 µV
*Absolute Accuracy (15 to 35 °C). To calculate the absolute accuracy for the SCXI-1125, SCXI-1120, and SCXI-1120D refer to page 194 or visit
ni.com/accuracy
1
V
rms
refers to sinusoidal waveform; V refers to
2
With SCXI-1327 high-voltage terminal block.
3
With
4
With
DC or AC peak.
SCXI-1313 high-voltage terminal block.
TBX-1316 high-voltage terminal block.
298
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SCXI 8-Channel Isolated Analog Input Modules
SCXI 8-Channel Isolated Analog Input
Specifications
Input Characteristics
Number of channels......................................... 8 differential
Input signal ranges
Module
Signal Ranges
SCXI-1125
±2.5 mV to ±5 V
SCXI-1120
±2.5 mV to ±5 V
SCXI-1120D
±5 mV to ±10 V
Input coupling................................................... DC (or AC with SCXI-1305 or TBX-1329)
Maximum working voltage (without SCXI-1313, 1327, or TBX-1316)
Module
Signal and Common Mode
SCXI-1125
±300 V
rms
SCXI-1120, SCXI-1120D
±250 V
rms
Dynamic Characteristics
Input signal bandwidth
Module
SCXI-1125
SCXI-1120
SCXI-1125/1120
SCXI-1125/1120
SCXI-1120D
Filter
4 Hz
10 kHz
10 kHz
2, 3
10 kHz
4
4.5 kHz
Input Range
All ranges
All ranges
All ranges
All ranges
± 250 V to ± 50 mV
± 20 mV to ± 10 mV
± 5 mV
± 250 V to ± 1 V
± 50 mV to ± 20 mV
± 10 V to ± 50 mV
± 10 mV
± 5 mV
Bandwidth
4 Hz
10 kHz
2.6 kHz
500 Hz
4.5 kHz
4 kHz
3.5 kHz
22.5 kHz
22 kHz
20 kHz
17 kHz
14 kHz
22.5 kHz
Module
SCXI-1125
SCXI-1120, SCXI-1120D
Powered On
±300 V
rms
±250 V
rms
Powered Off
±300 V
rms
±250 V
rms
Multiplexer performance
Module
SCXI-1125
SCXI-1120
SCXI-1120D
Scan Interval (Per Channel, Any Gain and Filter Setting)
Settle to ±0.012 %
5
Settle to ±0.006 %
6
Settle to ±0.0015 %
6
3 µs
10 µs
20 µs
Overvoltage protection
Inputs protected ............................................... CH0..CH7
Transfer Characteristics
Nonlinearity
Module
SCXI-1125
SCXI-1120, SCXI-1120D
Percent of Full Scale Range
±0.02%
±0.04%
Offset error ....................................................... See accuracy table
Gain error ......................................................... See accuracy table
Amplifier Characteristics
Input impedance
Module
SCXI-1125
SCXI-1120
SCXI-1120D
Normal Powered On
>1G
>1G
>1M
Powered Off/Overload
4.5 M
50 k
500 k
System noise .................................................... See accuracy table
Filter type
SCXI-1125......................................................... Third-order Butterworth
SCXI-1120, SCXI-1120D ................................... Third order RC
Cutoff frequency (-3dB)
SCXI-1125......................................................... 4 Hz, 10 kHz (programmable)
SCXI-1120......................................................... 4 Hz, 10 kHz (jumper selectable)
SCXI-1120D ...................................................... 4.5 kHz, 22.5 kHz (jumper selectable)
Stability
Module
Gain Temperature Coefficient
Offset Coefficient
SCXI-1125
20 ppm/°C
± 0.2 ± 220/gain) µV/°C
SCXI-1120
20 ppm/°C
± .42 ± 250/gain) µV/°C
SCXI-1120D
50 ppm/°C
± 20 ± 220/gain) µV/°C
Data Acquisition and
Signal Conditioning
Input bias current
SCXI-1125......................................................... ±100 pA
SCXI-1120......................................................... ±80 pA
SCXI-1120D ...................................................... ±15 pA
NMR (Normal Mode Rejection Ratio)
SCXI-1125/1120/1120D ................................... 60 dB
CMRR (Common Mode Rejection Ratio) (DC to 60 Hz)
Module
SCXI-1125
SCXI-1120
SCXI-1120D
Filter
4 Hz
10 kHz
4 Hz
10 kHz
4.5 kHz
10 kHz
CMRR 50 or 60 Hz
160 dB
100 dB
160 dB
100 dB
110 dB
98 dB
Physical
Dimensions....................................................... 3.1 by 17.3 by 20.3 cm
(12.2 by 6.8 by 8.0 in)
I/O Connector
Rear ................................................................. 50-pin male ribbon cable rear connector
Front ................................................................. 32-pin male DIN C connector
Environment
Operating temperature..................................... 0 to 50 ˚C
Storage temperature ........................................ -20 to 70 ˚C
Relative humidity ............................................. 5 to 90% noncondensing
Certification and Compliance
SCXI-1120/D..................................................... 250 V, Cat II working voltage
SCXI-1125......................................................... 300 V, Cat II working voltage
Output range .................................................... ± 5 V
Output impedance
Module
SCXI-1125, SCXI-1120, SCXI-1120D
1
V
rms
European Compliance
Parallel Mode
330
EMC EN 61326 Group I Class A, 10m, Table 1 Immunity
Safety .............................................................. EN 61010-1
Multiplexed Mode
100
North American Compliance
EMC ................................................................. FCC Part 15 Class A using CISPR
Safety ............................................................... UL Listed to UL 3111-1
CAN/CSA C22.2 No. 1010.1
refers to sinusoidal waveform; V refers to DC or AC peak.
2
With SCXI-1327 high-voltage terminal block.
3
With SCXI-1313 high-voltage terminal block.
4
With TBX-1316 high-voltage terminal block.
5
Includes effects of AT-MIO-16E-2 with 1 or 2 m SCXI cable assembly.
6
Includes effects of AT-MIO-16X or AT-AI-16XE-10 with 1 or 2 m SCXI cable assembly.
Australia & New Zealand Compliance
EMC ................................................................. AS/NZS 2064.1/2 (CISPR-11)
For a definition of specific terms, please visit
ni.com/glossary
National Instruments • Tel: (800) 433-3488 • Fax: (512) 683-9300 • info@ni.com • ni.com
299
Multifunction DAQ and SCXI Signal Conditioning
Accuracy Specifications Overview
Multifunction DAQ Accuracy Specifications
Every Measurement Counts
There is no room for error in your measurements. From sensor to
software, your system must deliver accurate results. NI provides
detailed specifications for our products so you do not have to guess
how they will perform. Along with traditional data acquisition
specifications, our E Series multifunction data acquisition (DAQ)
devices and SCXI signal conditioning modules include accuracy tables
to assist you in selecting the appropriate hardware for your application.
To calculate the accuracy of NI measurement products,
visit
ni.com/accuracy
Below is the Absolute Accuracy at Full Scale calculation for the
NI PCI-6052E DAQ device after one year using the ±10 V input
range while averaging 100 samples of a 10 V input signal. In all the
Absolute Accuracy at Full Scale calculations, we assume that the
ambient temperature is between 15 and 35 °C. Using the Absolute
Accuracy table on the next page, we see that that the calculation for
the ±10 V input range for Absolute Accuracy at Full Scale yields
4.747 mV. This calculation is done using the parameters in the same
row for one year Absolute Accuracy Reading, Offset and Noise +
Quantization, as well as a value of 10 V for the input voltage value.
You can then see that the calculation is as follows:
Absolute Accuracy = ±[(10 X 0.00037) + 947.0 µV + 87 µV] = ±4.747 mV
Absolute Accuracy
Absolute accuracy is the specification you use to determine the
overall maximum tolerance of your measurement. Absolute accuracy
specifications apply only to successfully calibrated DAQ devices
and SCXI modules. There are four components of an absolute
accuracy specification:
•
Percent of Reading
– is a gain uncertainty factor that is multiplied
by the actual input voltage for the measurement.
•
Offset
– is a constant value applied to all measurements.
•
System Noise
– is based on random noise and depends on
the number of points averaged for each measurement
(includes quantization error for DAQ devices).
•
Temperature Drift
– is based on variations in your
ambient temperature.
•
Input Voltage
– the absolute magnitude of the voltage input
for this calculation. The fullscale voltage is most commonly used.
Based on these components, the formula for calculating absolute
accuracy is:
Absolute Accuracy = ±[(Input Voltage X % of Reading) +
(Offset + System Noise + Temperature Drift)]
Absolute Accuracy RTI
1
= (Absolute Accuracy Input Voltage)
1
RTI = relative to input
In many cases, it is helpful to calculate this value relative to the input
(RTI). Therefore, you do not have to account for different input
ranges at different stages of your system.
Absolute Acuracy RTI = (±0.004747/10) = ±0.0475%
Data Acquisition and
Signal Conditioning
The following example assumes the same conditions except that the
ambient temperature is 40 °C. You can begin with the calculation
above and add in the Drift calculation using the % Drift per °C from
Table 2 on page 196.
Absolute Accuracy = 4.747 mV + ((40 – 35 °C) x 0.000006 /°C X 10 V) = ±5.047 mV
Absolute Acuracy RTI = (±0.005047/10) = ±0.0505%
Absolute Accuracy for SCXI Modules
Below is an example for calculating the absolute accuracy for the
NI SCXI-1102 using the ±100 mV input range while averaging
100 samples of a 14 mV input signal. In this calculation, we assume
the ambient temperature is between 15 and 35 °C, so Temperature
Drift = 0. Using the accuracy table on page 313, you find the
following numbers for the calculation:
Input Voltage = 0.014
% of Reading Max = 0.02% = 0.0002
Offset = 0.000025 V
System Noise = 0.000005 V
Absolute Accuracy = ±[(0.014 x 0.0002) + 0.000025 + 0.000005] V = ±32.8 µV
Absolute Accuracy RTI = ±(0.0000328 / 0.014) = ±0.234 %
Temperature drift is already accounted for unless your ambient
temperature is outside 15 to 35 °C. For instance, if your ambient
temperature is at 45 °C, you must account for 10 °C of drift. This is
calculated by:
Temperature Drift = Temperature Difference x % Drift per °C x Input Voltage
Absolute Accuracy for DAQ Devices
Absolute Device Accuracy at Full Scale is a calculation of absolute
accuracy for DAQ devices for a specific voltage range using the
maximum voltage within that range taken one year after calibration,
the Accuracy Drift Reading, and the System Noise averaged value.
The following example assumes the same conditions, except the
ambient temperature is 40 °C. You can begin with the Absolute
Accuracy calculation above and add in the Temperature Drift.
Absolute Accuracy = 32.8 µV + (0.014 x 0.000005 + 0.000001) x 5 = ±38.15 µV
Absolute Accuracy RTI = ±(0.00003815 / 0.014) = ±0.273 %
194
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