SCXI 8-Channel Lowpass
Filter Modules
SCXI-1141, SCXI-1142, SCXI-1143
8 channels SCXI-1143
333 kS/s (any gain) Butterworth filter response
Programmable amplifier per channel Recommended for maximum flatness
8th-order programmable filter setting in the passband
per module
Cutoff frequencies from 10 to 25 kHz
Driver Software
Overvoltage protection to Ä…30 V
NI-DAQ
powered on
Windows 2000/NT/Me/9x
Signal input levels from Ä…50 mV to Ä…5 V
Mac OS (1141 only)
SCXI-1141
Application Software
Elliptical filter response
LabVIEW
Recommended for sharp filter rolloff
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 Ä…30 V powered on
Input Signal Characteristics
and Ä…15 V powered off.
Module Ä…50 mV to Ä…5 V 8th Order Filter Type
EXPRESS
The signal is then passed through an
SCXI-1141 ' Elliptical
CODES
SCXI-1142 ' Bessel 8th-order lowpass filter. See Figures 2, 3,
SCXI-1143 ' Butterworth
and 4 for stopband and rolloff For information or to
buy products online,
Table 1. SCXI Module Compatibility characteristics for the different filter types.
visit ni.com/catalog
These filters are a hybrid of SCFs and and enter:
Overview continuous time filters (CTFs). With SCF scxi1141
scxi1142
National Instruments offers three different 8-channel, programmable technology, the cutoff frequency of the
scxi1143
lowpass filter SCXI modules that are ideal for antialiasing applications. filters is easily configured through software.
You can choose from elliptical, Bessel, or Butterworth filter types. These However, SCFs are sampling systems, and
modules have a common architecture, with a programmable are subject to the same aliasing effects as
instrumentation amplifier and programmable 8th-order lowpass filter digitizers. The hybrid design uses a CTF
on each channel. While all channels must be set to the same cutoff prefilter to prevent aliasing in the SCF and
frequency, you can programatically bypass the lowpass filter on a per- a CTF postfilter to reconstruct the sampled waveform.
channel basis. By using a combination of analog filters and switched
capacitor filters (SCFs), these modules offer the flexibility of 10,000 Modes of Operation
discrete software programmable cutoff frequencies while maintaining Multiplexer
the low-noise performance of traditional, continuous time-active filters. In multiplexed mode the SCXI-1141, SCXI-1142, or SCXI-1143
An external clock input and output are also provided so that you can multiplexes all eight conditioned signals onto channel MCH0 of the
set the filter cutoff frequency with an external clock in tracking filter rear signal connector, to which you can connect a single input
applications. Each module can condition signals ranging from Ä…50 mV channel of the DAQ device. Furthermore, you can connect the
to Ä…5 V, and offers multiplexed and parallel modes of operation. multiplexed output of each module using programmable switches to
analog bus 0 of the SCXIbus and to other modules. Several SCXI
Description modules can operate in this manner with a single DAQ device.
Analog Input Each module also has internal counters for the high-speed
These modules share a common architecture. Each channel has its own automatic scanning mode with any E Series product. A scan clock
programmable instrumentation amplifier with differential inputs that you signal from the DAQ device synchronizes the scanning.
can individually program for signals ranging from Ä…50 mV to Ä…5 V. These
518 National Instruments
Tel: (512) 794-0100 " Fax: (512) 683-9300 " info@ni.com " ni.com
SCXI-1141, SCXI-1142, SCXI-1143
Signal Conditioning
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
0.5
Parallel
Elliptic (1141)
1
In parallel mode, you can pass each conditioned channel directly to
Bessel (1142)
Butterworth (1143) 1.5
the analog inputs of the DAQ device. This configuration requires a
2
dedicated DAQ device and cable for each module operating in
2.5
parallel mode.
3
3.5
0 0.2 0.4 0.6 0.8 1
Frequency (normalized)
Lowpass Filters
Figure 2. Frequency Response
Each module offers different filter response characteristics. The SCXI-1141
has 8th-order elliptical filters, which provide a very sharp rolloff.
1.4
Elliptical filters are ideal for blocking signals just above the Nyquist
1.2
frequency. The SCXI-1142 has 8th-order Bessel filters, which maintain
1
signal phase integrity. Bessel filters are ideal for applications requiring
Elliptic (1141)
0.8
Bessel (1142)
simultaneous sampling. Finally, the SCXI-1143 has 8th-order
Butterworth (1143)
0.6
Butterworth filters, which have a flat passband. Butterworth filters are
0.4
ideal for applications where signal amplitude must be uniform
0.2
throughout the passband.
0
You can program the cutoff frequency of these filters for a range of 0 2 4 6 8 10
Time (seconds)
frequencies, from 10 Hz to 25 kHz. Specifically, the available
Figure 3. Step Response
frequencies are fc = 100 kHz/n, where n = 4, 5, & ,10,000. Optionally,
300
you can control the cutoff frequency with an external clock. In either
case, all eight channels operate with the same cutoff frequency. 200
Elliptic (1141)
However, you can individually bypass each of the eight filters in a
Bessel (1142)
100
Butterworth (1143)
module under software control to leave the channel unfiltered.
0
100
0 0.5 1 1.5 2
Frequency (normalized)
Figure 4. Phase Error
National Instruments 519
Tel: (512) 794-0100 " Fax: (512) 683-9300 " info@ni.com " ni.com
SCXI-1141, SCXI-1142, SCXI-1143
Signal Conditioning
Gain (dB)
Output Voltage (V)
Phase Error (degrees)
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 Ordering Information
connectors on the SCXI-1305, both of which plug into the module SCXI-1141 ..........................................................776572-41
front connector. These terminal blocks provide circuitry for AC or DC SCXI-1142 ..........................................................776572-42
coupling and signal referencing (through bias resistors to ground) for SCXI-1143 ..........................................................776572-43
floating signal connections. You can set coupling and ground
referencing on each of the eight channels using switches within the Extended Warranty
SCXI-1304 and SCXI-1305 terminal blocks. When enabled, the AC and Value Added Services........................................page 880
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.
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
SCXI-1141, SCXI-1142, SCXI-1143
Signal Conditioning
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