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SCXI 8-Channel Lowpass
Filter Modules
SCXI-1141, SCXI-1142, SCXI-1143
SCXI-1143
Butterworth filter response
Recommended for maximum flatness
in the passband
8 channels
333 kS/s (any gain)
Programmable amplifier per channel
8th-order programmable filter setting
per module
Cutoff frequencies from 10 to 25 kHz
Overvoltage protection to ±30 V
powered on
Signal input levels from ±50 mV to ±5 V
Driver Software
NI-DAQ
Windows 2000/NT/Me/9x
Mac OS (1141 only)
SCXI-1141
Elliptical filter response
Recommended for sharp filter rolloff
Application Software
LabVIEW
Measurement Studio
Lookout
SCXI-1142
Bessel filter response
Recommended for maintaining
phase integrity
Cascade with the SCXI-1140 for
applications requiring lowpass
filtering and simultaneous sampling
Calibration Certificate Included
inputs are protected to ±30 V powered on
and ±15 V powered off.
The signal is then passed through an
8th-order lowpass filter. See Figures 2, 3,
and 4 for stopband and rolloff
characteristics for the different filter types.
These filters are a hybrid of SCFs and
continuous time filters (CTFs). With SCF
technology, the cutoff frequency of the
filters is easily configured through software.
However, SCFs are sampling systems, and
are subject to the same aliasing effects as
digitizers. The hybrid design uses a CTF
prefilter to prevent aliasing in the SCF and
a CTF postfilter to reconstruct the sampled waveform.
Input Signal Characteristics
EXPRESS
CODES
Module
±50 mV to ±5 V
8th Order Filter Type
SCXI-1141
Elliptical
SCXI-1142
Bessel
SCXI-1143
Butterworth
For information or to
buy products online,
visit ni.com/catalog
and enter:
Table 1. SCXI Module Compatibility
Overview
National Instruments offers three different 8-channel, programmable
lowpass filter SCXI modules that are ideal for antialiasing applications.
You can choose from elliptical, Bessel, or Butterworth filter types. These
modules have a common architecture, with a programmable
instrumentation amplifier and programmable 8th-order lowpass filter
on each channel. While all channels must be set to the same cutoff
frequency, you can programatically bypass the lowpass filter on a per-
channel basis. By using a combination of analog filters and switched
capacitor filters (SCFs), these modules offer the flexibility of 10,000
discrete software programmable cutoff frequencies while maintaining
the low-noise performance of traditional, continuous time-active filters.
An external clock input and output are also provided so that you can
set the filter cutoff frequency with an external clock in tracking filter
applications. Each module can condition signals ranging from ±50 mV
to ±5 V, and offers multiplexed and parallel modes of operation.
scxi1141
scxi1142
scxi1143
Modes of Operation
Multiplexer
In multiplexed mode the SCXI-1141, SCXI-1142, or SCXI-1143
multiplexes all eight conditioned signals onto channel MCH0 of the
rear signal connector, to which you can connect 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. Several SCXI
modules can operate in this manner with a single DAQ device.
Each module also has internal counters for the high-speed
automatic scanning mode with any E Series product. A scan clock
signal from the DAQ device synchronizes the scanning.
Description
Analog Input
These modules share a common architecture. Each channel has its own
programmable instrumentation amplifier with differential inputs that you
can individually program for signals ranging from ±50 mV to ±5 V. These
518
National Instruments
Tel: (512) 794-0100 • Fax: (512) 683-9300 • info@ni.com • ni.com
SCXI 8-Channel Lowpass
Filter Modules
Calibration
Each of these modules contain calibration hardware to
null out error sources. With automatic input zeroing,
software-programmable analog switches ground the
inputs of each of the instrumentation amplifiers for
offset error calibration. By providing an external
reference voltage, you can calibrate the system to your
exact operating conditions. In addition, DC
compensation circuits remove the offset errors of the
filters, guaranteeing excellent DC accuracy regardless
of whether lowpass filtering is enabled. An onboard
EEPROM stores the calibration constants for each
channel and each input range in a user-defined area.
The EEPROM also stores a set of factory calibration
constants in permanent memory, which cannot be
modified. NI-DAQ driver software transparently uses
the calibration constants to correct for gain errors.
Figure 1. Block Diagram of the SCXI-1141, SCXI-1142, and SCXI-1143
Using this hardware scanning mode, you can scan channels at rates
up to 333 kS/s (3 µs per channel).
0.5
0
Parallel
In parallel mode, you can pass each conditioned channel directly to
the analog inputs of the DAQ device. This configuration requires a
dedicated DAQ device and cable for each module operating in
parallel mode.
—0.5
Elliptic (1141)
—1
Bessel (1142)
Butterworth (1143)
—1.5
—2
—2.5
—3
—3.5
Lowpass Filters
Each module offers different filter response characteristics. The SCXI-1141
has 8th-order elliptical filters, which provide a very sharp rolloff.
Elliptical filters are ideal for blocking signals just above the Nyquist
frequency. The SCXI-1142 has 8th-order Bessel filters, which maintain
signal phase integrity. Bessel filters are ideal for applications requiring
simultaneous sampling. Finally, the SCXI-1143 has 8th-order
Butterworth filters, which have a flat passband. Butterworth filters are
ideal for applications where signal amplitude must be uniform
throughout the passband.
You can program the cutoff frequency of these filters for a range of
frequencies, from 10 Hz to 25 kHz. Specifically, the available
frequencies are f
c
= 100 kHz/n, where n = 4, 5, …,10,000. Optionally,
you can control the cutoff frequency with an external clock. In either
case, all eight channels operate with the same cutoff frequency.
However, you can individually bypass each of the eight filters in a
module under software control to leave the channel unfiltered.
0
0.2
0.4
0.6
0.8
1
Frequency (normalized)
Figure 2. Frequency Response
1.4
1.2
1
Elliptic (1141)
0.8
Bessel (1142)
Butterworth (1143)
0.6
0.4
0.2
0
0
2
4
6
8
10
Time (seconds)
Figure 3. Step Response
300
200
Elliptic (1141)
Bessel (1142)
Butterworth (1143)
100
0
—100
0
0.5
1
1.5
2
Frequency (normalized)
Figure 4. Phase Error
National Instruments
Tel: (512) 794-0100 • Fax: (512) 683-9300 • info@ni.com • ni.com
519
SCXI 8-Channel Lowpass
Filter Modules
Terminal Block
Type
Special Functions
Page
SCXI-1304
Screw terminals
AC coupling
538
(777687-04)
front-mounting
Signal ground referencing
SCXI-1305
BNC connectors
BNC connectors
538
(777687-05)
front-mounting
AC coupling
Signal ground referencing
SCXI-1310
Solder pins
Low-cost connector-and-
538
(777687-10)
shell assembly
shell assembly
Table 2. Terminal Block Options for SCXI-1141, SCXI-1142, and SCXI-1143
Signal Connection
Signals from transducers and signal sources connect to screw
terminals located in the SCXI-1304 terminal block or to BNC
connectors on the SCXI-1305, both of which plug into the module
front connector. These terminal blocks provide circuitry for AC or DC
coupling and signal referencing (through bias resistors to ground) for
floating signal connections. You can set coupling and ground
referencing on each of the eight channels using switches within the
SCXI-1304 and SCXI-1305 terminal blocks. When enabled, the AC
coupling circuitry can reject up to 50 VDC. A general-purpose 96-pin
connector and shell, the SCXI-1310, is available as a low-cost
alternative for custom wiring of the input signals. See page 538 for
more information.
Alternatively, you can use the SCXI-1352 cable to connect
the conditioned parallel outputs of one SCXI-1125 or up
to two SCXI-1121 modules configured for parallel mode to an auxillary
16-pin input connector located within the SCXI-1141, SCXI-1142, and
SCXI-1143 module. In this configuration, an SCXI-1304 or SCXI-1305
terminal block is required only for access to the external clock I/O signal.
Ordering Information
SCXI-1141
..........................................................776572-41
SCXI-1142
..........................................................776572-42
SCXI-1143
..........................................................776572-43
Extended Warranty
and Value Added Services
........................................page 880
See page 470 to configure your complete SCXI system.
520
National Instruments
Tel: (512) 794-0100 • Fax: (512) 683-9300 • info@ni.com • ni.com
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