USER GUIDE
SCC-LP Series Lowpass Filter
Modules
The SCC-LP Series lowpass filter modules contain fourth-order
Butterworth filter circuitry. They accept two differential input signals
within a ±10 V range. A differential amplifier attenuates each signal by
a factor of two. The output of the amplifier passes through a fourth-order
Butterworth filter circuit.
The SCC-LP Series consists of the following modules:
• SCC-LP01—25 Hz cutoff frequency
• SCC-LP02—50 Hz cutoff frequency
• SCC-LP03—150 Hz cutoff frequency
• SCC-LP04—1 kHz cutoff frequency
Conventions
The following conventions are used in this guide:
<> Angle brackets that contain numbers separated by an ellipsis represent
a range of values associated with a bit or signal name—for example,
P0.<3..0>.
» The » symbol leads you through nested menu items and dialog box options
to a final action. The sequence File»Page Setup»Options directs you to
pull down the File menu, select the Page Setup item, and select Options
from the last dialog box.
This icon denotes a note, which alerts you to important information.
This icon denotes a caution, which advises you of precautions to take to
avoid injury, data loss, or a system crash. When this symbol is marked on
the product, refer to the Read Me First: Safety and Radio-Frequency
Interference document, shipped with the product, for precautions to take.
When symbol is marked on a product, it denotes a warning advising you to
take precautions to avoid electrical shock.
When symbol is marked on a product, it denotes a component that may be
hot. Touching this component may result in bodily injury.
bold Bold text denotes items that you must select in software, such as menu
items and dialog box options. Bold text also denotes parameter names.
italic Italic text denotes variables, emphasis, a cross reference, or an introduction
to a key concept. This font also denotes text that is a placeholder for a word
or value that you must supply.
monospace Text in this font denotes text or characters that you should enter from the
keyboard, sections of code, programming examples, and syntax examples.
This font is also used for the proper names of disk drives, paths, directories,
programs, subprograms, subroutines, device names, functions, operations,
variables, filenames, and extensions.
SC-2345 SC-2345 refers to both the SC-2345 connector block and the SC-2345 with
configurable connectors.
SCC SCC refers to any SCC Series signal conditioning module.
SCC-LPXX SCC-LPXX refers to any SCC-LP Series lowpass filter signal conditioning
module.
SCC-LP Series Lowpass Filter Modules User Guide 2 ni.com
What You Need to Get Started
To set up and use the SCC-LPXX, you need the following items:
❑ SC-2345/2350 with one of the following:
– SCC-PWR01
– SCC-PWR02 and the PS01 power supply
– SCC-PWR03—requires a 7 to 42 VDC power supply (not
included)
❑ One or more SCC-LPXX
❑ SCC-LP Series Lowpass Filter Modules User Guide
❑ SC-2345/2350 User Manual, available at ni.com
❑ SCC Quick Start Guide, available at ni.com
❑ Read Me First: Safety and Radio-Frequency Interference
❑ SC-2345 Quick Reference Label
❑ 68-pin E Series DAQ device, documentation, and 68-pin cable
❑ 1/8 in. flathead screwdriver
❑ Numbers 1 and 2 Phillips screwdrivers
❑ Wire insulation strippers
❑ NI-DAQ (current version) for Windows 2000/NT/XP/Me
Note Software scaling of measurements is not supported on the Macintosh operating
system. Refer to the Using the SCC-LPXX when Scaling Voltage Measurements section for
more information.
© National Instruments Corporation 3 SCC-LP Series Lowpass Filter Modules User Guide
Device Specific Information
Note For general SCC module installation and signal connection information, and
information about the SC-2350 carrier, refer to the SCC Quick Start Guide, available
for download at
Installing the Module
Caution Refer to the Read Me First: Safety and Radio-Frequency Interference document
before removing equipment covers or connecting/disconnecting any signal wires.
ni.com/manuals.
You can plug the SCC-LPXX into any analog input socket on the SC-2345.
It can function as a single-stage module or as the first or the second stage
of a dual-stage signal conditioning configuration. The socket you choose
determines which E Series DAQ device channels receive the SCC-LPXX
output signals, as explained in the Connecting the Input Signals section.
For single-stage conditioning, plug the SCC-LPXX into any socket J(X+1),
where X is 0 to 7, and connect the input signals to the module as described
in the Connecting the Input Signals section.
The SCC-LPXX can function as either the first or the second stage of a
dual-stage configuration. Plug the first-stage SCC into any socket J(X+9)
and plug the second-stage SCC into socket J(X+1), where X is 0 to 7.
Connect the input signals to the first-stage SCC. The SC-2345 connects the
output signals of the first-stage SCC to the inputs of the second-stage SCC.
An example of dual-stage conditioning is an SCC-A10 voltage attenuator
module followed by an SCC-LPXX.
Sockets J9 to J16 also are available for digital input/output (DIO)
conditioning or control. Refer to the SC-2345 User Manual and SCC Quick
Start Guide for more information on configuring, connecting, and
installing SCC modules.
Connecting the Input Signals
Note The signal names have changed. Refer to ni.com/info and enter rdtntg to
confirm the signal names.
Each screw terminal is labeled by pin number <1..4>. Pins 1 and 2 form
a differential channel routed to E Series DAQ device channel X+8, and
pins 3 and 4 form a second differential channel routed to E Series DAQ
device channel X, where X is 0 to 7 depending on the socket where you plug
in the SCC-LPXX.
SCC-LP Series Lowpass Filter Modules User Guide 4 ni.com
The signal source can be floating or ground-referenced. The SCC-LPXX
input circuitry includes high-impedance bias resistors typically required for
floating sources. Therefore, floating signal sources do not require external
bias resistors connected to ground.
Note For floating signal sources in high-noise environments, connect the negative
terminal of the signal source to the AI GND terminal on the SC-2345 screw-terminal block
to reduce common-mode noise.
Signal
Sources
+
–
+
–
SCC-LP
4
1 kΩ
3
2
1
1 kΩ
10 MΩ
1 kΩ
1 kΩ
10 MΩ
Signal source may be floating or ground-referenced.
+
–
+
–
Fourth-Order
Butterworth
Lowpass
Filter
Fourth-Order
Butterworth
Lowpass
Filter
E Series DAQ Device
AI (X )
AI SENSE
AI GND
AI (X+8)
Figure 1. SCC-LPXX Signal Connections
For information about configuring the SCC-LPXX module using
NI-DAQmx, refer to the SCC Quick Start Guide.
© National Instruments Corporation 5 SCC-LP Series Lowpass Filter Modules User Guide
Using the SCC-LPXX when Scaling Voltage Measurements
If you configured the SCC-LPXX using Measurement & Automation
Explorer (MAX) and you are using NI-DAQ, the voltage reading you get
from the E Series DAQ device accounts for the voltage scaling effect of the
SCC-LPXX. Otherwise, you must scale the readings as follows:
1. Read the SCC-LPXX channel on the E Series DAQ device
V
ESERIES
(CHX).
2. Calculate the SCC-LPXX voltage using this formula:
V
= 2V
LP
ESERIES
where
V
is the SCC-LPXX input voltage.
LP
V
is the E Series DAQ device input voltage.
ESERIES
Specifications
These ratings are typical at 25 °C unless otherwise stated.
Amplifier Characteristics
Number of input channels.......................2 DIFF
Input signal range ...................................±10 V
Output signal range.................................±5 V
Gain ........................................................0.5
Input impedance .....................................10 GΩ in parallel with 10 pF
Gain error................................................Adjustable to 0% of reading
Offset-voltage error ................................350 µV typ
1
This specification is calculated relative to the input range of the module.
2
Applicable at 25 °C.
(powered on)
10 kΩ
(powered off or overloaded)
(referred to input, RTI)
1.5 mV max
2
1
SCC-LP Series Lowpass Filter Modules User Guide 6 ni.com
Input bias current ................................... ±2 nA typ
Nonlinearity ........................................... 0.004% of full scale
Filter Characteristics
Filter type ............................................... Fourth-order Butterworth
Rolloff rate ............................................. 80 dB/decade
–3 dB cutoff frequency (fc)
Passband ripple
±5 nA max
1
lowpass
SCC-LP01.......................................25 Hz
SCC-LP02.......................................50 Hz
SCC-LP03....................................... 150 Hz
SCC-LP04.......................................1 kHz
Input Signal Typica l Maximum
DC to 1/3 fc ±0.04 dB 0 ± 0.1 dB
DC to 1/2 fc ±0.06 dB 0 ± 0.2 dB
DC to 2/3 fc –0.2 ± 0.25 dB –0.2 ± 0.4 dB
DC to fc –3 ± 0.3 dB –3 ± 0.5 dB
System Noise
Total harmonic distortion (THD) at fc ... < –90 dB
Wide-band noise
(DC to 1 MHz, RTI)............................... 100 µV
Narrow-band noise
(DC to 33 kHz, RTI) .............................. 6 µV
1
Applicable at 25 °C.
© National Instruments Corporation 7 SCC-LP Series Lowpass Filter Modules User Guide
rms
rms
Stability
Power Requirement
Physical
Gain temperature coefficient ..................10 ppm/°C typ
20 ppm/°C max
Offset-voltage
temperature coefficient ...........................3.4 µV/°C typ (RTI)
27 µV/°C max
Analog power
SCC-LP01, SCC-LP02....................135 mW max
+15 V........................................4.5 mA max
–15 V ........................................4.5 mA max
SCC-LP03, SCC-LP04....................475 mW max
+15 V........................................15.8 mA max
–15 V........................................15.8 mA max
Digital power ..........................................0.0 mW max
Dimensions .............................................8.89 cm × 2.92 cm × 1.85 cm
(3.50 in. × 1.15 in. × 0.73 in.)
Mass........................................................37 g (1.3 oz)
I/O connectors.........................................One 20-pin right-angle
male connector,
one 4-pin screw terminal
Field-wiring diameter .............................28 to 16 AWG
Maximum Working Voltage
Maximum working voltage refers to the signal voltage plus the
common-mode voltage.
Channel-to-earth (inputs)........................±15 V, Installation Category I
Channel-to-channel (inputs) ...................±15 V, Installation Category I
Environmental
Operating temperature ............................0 to 50 °C
Storage temperature ................................–20 to 70 °C
SCC-LP Series Lowpass Filter Modules User Guide 8 ni.com
Humidity ................................................ 10 to 90% RH, noncondensing
Maximum altitude .................................. 2,000 m
Pollution Degree (indoor use only) ........ 2
Safety
The SCC-LPXX meets the requirements of the following standards
for safety and electrical equipment for measurement, control, and
laboratory use:
• IEC 61010-1, EN 61010-1
• UL 3111-1, UL 61010B-1
• CAN/CSA C22.2 No. 1010.1
Note For UL and other safety certifications, refer to the product label, or visit
ni.com/hardref.nsf, search by model number or product line, and click the
appropriate link in the Certification column.
Electromagnetic Compatibility
Emissions ............................................... EN 55011 Class A at 10 m
Immunity................................................ EN 61326:1997 + A2:2001,
FCC Part 15A above 1 GHz
Table 1
CE, C-Tick, and FCC Part 15 (Class A) Compliant
Note For full EMC compliance, operate this device with shielded cabling. In addition,
all covers and filler panels must be installed.
CE Compliance
This product meets the essential requirements of applicable European
Directives, as amended for CE marking, as follows:
Low-Voltage Directive (safety): ............ 73/23/EEC
Electromagnetic Compatibility
Directive (EMC) .................................... 89/336/EEC
Note Refer to the Declaration of Conformity (DoC) for this product for any additional
regulatory compliance information. To obtain the DoC for this product, visit
ni.com/hardref.nsf, search by model number or product line, and click the
appropriate link in the Certification column.
© National Instruments Corporation 9 SCC-LP Series Lowpass Filter Modules User Guide
Theory of Operation
SCC-LPXX Performance
The SCC-LPXX uses a Butterworth filter, which is characterized by
maximal flatness in the passband with very sharp monotonic rolloff. It has
a nonlinear phase response, the delay is not constant, and the step response
exhibits a moderate amount of overshoot (ringing). These characteristics
present no problems in applications where only the amplitude of signal
frequency components is of interest.
The Butterworth filter is a good general-purpose filter. Figures 2 through 5
show the typical gain response curve for each SCC-LPXX.
– 5.0
0.0
– 5.0
– 10.0
– 15.0
Amplitude (dB)
– 20.0
– 25.0
1234510202550
Frequency (Hz)
Figure 2. Typical SCC-LP01 Response Curve
SCC-LP Series Lowpass Filter Modules User Guide 10 ni.com
0.0
– 5.0
– 10.0
– 15.0
Amplitude (dB)
– 20.0
– 25.0
1 2 3 4 5 10 20 25 50 100
Frequency (Hz)
Figure 3. Typical SCC-LP02 Response Curve
0.0
– 5.0
– 10.0
– 15.0
Amplitude (dB)
– 20.0
– 25.0
1 2 3 4 5 10 20 25 50 300
Frequency (Hz)
150
Figure 4. Typical SCC-LP03 Response Curve
© National Instruments Corporation 11 SCC-LP Series Lowpass Filter Modules User Guide
2.5
0.0
–5.0
–10.0
–15.0
Amplitude (dB)
–20.0
–25.0
1 2 3 4 5 10 20 50 1000 2000
Frequency (Hz)
Figure 5. Typical SCC-LP04 Response Curve
SCC-LP Series Lowpass Filter Modules User Guide 12 ni.com
Figure 6 shows the theoretical transfer characteristics of the SCC-LPXX.
a. Frequency Response
0
–20
–40
–60
Amplitude (dB)
–80
–100
0.10.15 0.5
Normalized Frequency (f/fc)
b. Group Delay
2
1
Delay (s)
1 1.5 5 10
0
0.1 0.15 0.5
1.0 1.5
Normalized Frequency (f/fc)
c. Step Response
2
)
IN
/V
OUT
1
Step Response (V
0
0
4123 5
Normalized Time (1/fc s)
Figure 6. Theoretical Transfer Characteristics
© National Instruments Corporation 13 SCC-LP Series Lowpass Filter Modules User Guide
The horizontal axes of the first two plots are normalized to the SCC-LPXX
cutoff frequency. When the input frequency (f) equals the cutoff frequency
(fc), the normalized frequency has a value of 1.
The vertical axis of the third plot is normalized to the magnitude of the step
input voltage. When the step-response output voltage equals the magnitude
of the step-input voltage, the normalized step response is 1 V
OUT/VIN
.
Figure 6a shows that the SCC-LPXX provides 80 dB attenuation above
ten times the cutoff frequency. Figure 6b shows variation in the group
delay of the SCC-LPXX. Figure 6c shows the SCC-LPXX response to a
step input. As shown, the peak voltage of the output is greater than the
magnitude of the step input. If you expect step inputs, choose a gain setting
and input range on the E Series DAQ device that allow for the effects of
ringing. Otherwise the DAQ device input may be saturated, resulting in
invalid data.
SCC-LP Series Lowpass Filter Modules User Guide 14 ni.com
Using the SCC-LPXX as an Antialiasing Filter
Aliasing, a phenomenon of voltage-sampling systems, causes a
high-frequency signal to take on the identity of a low-frequency signal.
1
–1
02 46 810
Input Signal
Sampled Points
Reconstructed Signal
Figure 7. Aliasing of an Input Signal Frequency of 0.8 Times the Sampling Rate
The solid line depicts a high-frequency signal being sampled at the
indicated points. However, when these points are connected to reconstruct
the waveform, as shown by the dotted line, the signal appears to have a
lower frequency. Any signal with a frequency greater than one half of the
sampling rate will be aliased and incorrectly analyzed as having a
frequency below one half of the sampling rate. This limiting frequency,
one half of the sampling rate, is known as the Nyquist frequency.
To prevent aliasing, you must remove all of the signal components with
frequencies greater than the Nyquist frequency from an input signal before
you sample it. When you sample the data and aliasing occurs, it is
impossible to accurately reconstruct the original signal.
The SCC-LPXX removes these high-frequency signals before they reach
the E Series DAQ device and cause aliasing. Because the SCC-LPXX
stopband begins at ten times the cutoff frequency (for an attenuation of
80 dB), the Nyquist frequency should be at least ten times the cutoff
frequency. Thus, the rate at which the E Series DAQ device samples a
channel should be at least 20 times the filter cutoff frequency.
For example, if you use the SCC-LP01, which has a cutoff frequency of
25 Hz, you can calculate the minimum scan rate used by the E Series DAQ
device to prevent aliasing—25 Hz × 20 = 500 samples per second per
channel.
© National Instruments Corporation 15 SCC-LP Series Lowpass Filter Modules User Guide
Calibrating Gain Errors
The SCC-LPXX is calibrated at the factory before shipment. If you want to
calibrate the SCC-LPXX in your system, you need a voltage source capable
of providing a DC voltage up to ±10 V that is several times more accurate
than the SCC itself.
To calibrate the SCC-LPXX, complete the following steps for each channel
of the module:
1. Select the desired SCC-LPXX channel (X or X+8) on the E Series DAQ
device.
2. Set the gain on the E Series DAQ device so that the E Series input
range is ±5V.
3. Connect the voltage source to the screw terminals of the desired
channel on the SCC-LPXX.
4. Input 9 VDC to the SCC-LPXX.
5. Using your software, have the E Series DAQ device read the desired
channel on the SCC-LPXX. Record the value.
6. Input 0 VDC to the SCC-LPXX.
7. Have the E Series DAQ device read the channel again and record the
new value.
8. Calculate the difference between the two values you recorded
(first reading – second reading).
9. Adjust the appropriate trimpot (X or X+8) on the top of the SCC-LPXX.
Repeat steps 4 through 8 until the difference you get in step 8
equals 9 V.
For example, you connect 9 VDC to the input of CH (X) and the E Series
DAQ device reads 9.05 V at the SCC output; then you connect 0 VDC to
the input of CH (X) and the E Series DAQ device reads –0.01 V at the SCC
output. Subtract the second value from the first (9.05 – (–0.01)) to get a
difference of 9.06 V. Because the difference is not equal to 9 V, you adjust
the trimpot until the difference in outputs equals 9 V.
Note In this example there may be an offset voltage such that the final voltages are 9.01 V
and 0.01 V for a difference of 9 V. The SCC-LPXX trimpot adjusted in step 9 adjusts only
for gain errors and does not compensate for this offset voltage.
SCC-LP Series Lowpass Filter Modules User Guide 16 ni.com
SCC-LPXX Module Pin Assignments
Figure 8 shows the I/O connector pins on the bottom of the module.
4
5
1Pin 1 2Pin 2 3PWB Key 4 Pin 19 5Pin 20
Figure 8. SCC Module Bottom View
1
2
3
© National Instruments Corporation 17 SCC-LP Series Lowpass Filter Modules User Guide
Table 1 lists the signal connection corresponding to each pin. AI (X) and
AI (X+8) are the analog input signal channels of the E Series DAQ device.
AI GND is the analog input ground signal and is the reference for AI (X)
and AI (X+8). A GND is the reference for the ±15 V supplies. AI GND and
A GND connect to the SC-2345 at the SCC-PWR connector. You can use
pins 17 to 20 for cascading channels. Refer to the Device Specific
Information section for more information on cascading configurations.
Table 1. SCC-LPXX Pin Signal Connections
Pin Number Signal
1 E Series AI (X)
2 E Series AI GND
3 —
4 E Series AI (X+8)
5 —
6 E Series AI GND
7 —
8 E Series AI GND
9 —
10 —
11 A GND
12 —
13 +15 V
14 –15 V
15 —
16 —
17 AI (X)– (from first stage)
18 AI (X+8)+ (from first stage)
19 AI (X)+ (from first stage)
20 AI (X+8)– (from first stage)
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*371071B-01*
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