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The Lawson Labs, Inc. Model 301 24-Bit Data Acquisition System is a high-resolution device for
translating voltages into digital form. The Model 301 communicates with a host computer using USB (Universal Serial Bus).
It has two multiplexed differential analog input channels and eight each digital input and digital output lines. The analog
input and digital I/O sections are optically isolated from the computer.
The Model 301 is intended for DC and low frequency applications. The data rate is programmable from 50
to 1000 samples/second. At a data rate of 50 samples/second, the RMS noise approaches one count, providing 23 bits effective
resolution. (The converter is guaranteed monotonic to 23 bits.) Effective resolution decreases with increasing data rates.
Even so, 20-bit effective resolution is maintained at 1000 samples/second.
The Model 301 requires a single DC supply in the range of 9 to 15 volts to power the isolated circuitry. With the optional
preregulator, the range is 15 to 30 VDC. Current draw is typically 25 milliamps in normal operation. The power and analog
inputs are protected against substantial overvoltages.
Both polled and scanning modes are available. In scanning mode, the Model 301 maintains its own time base and transmits
a pre-defined scan of one or two input channels at a preset interval. Self-calibration can remove offset and gain errors
under software control.
The exceptional resolution, stability, flexibility and price are achieved by combining an accurate, but complex,
delta-sigma type A/D converter with a microcontroller supervisor. The microcontroller simplifies the task of interfacing to
the converter itself.
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The Model 301 interconnections consist of a DB25 cable
connector, a USB connector, and a 2-terminal power connector
NOTE: Always handle circuit cards by the edges. Static
electricity can damage computer circuitry, so care
should be taken to control static discharge.
For operational checks, only the power supply and serial cable need be connected. The power supply
voltage can range from 9 to 15 VDC and does not need to be regulated. Power is connected to the terminals on the orange
terminal block. The power terminals are labeled "+" and "-". The wall-mounted transformer supplied has a white stripe on
the positive wire. A battery, or other DC supply (in the correct voltage range), can be substituted. The board is protected
against reverse voltage but will not operate without a properly connected supply. The power can be connected before or after
the serial interface connection is made. Note that the computer chassis ground is not connected to ground at the Model 301
because of the optical isolation. If your Model 301 has been mounted in the standard enclosure it will have a 47K ohm resistor
placed between the computer's ground and the Model 301's chassis ground. This resistor should be removed if the two grounds
are to be more than 100 volts apart. (Contact the factory for details.)
For maximum accuracy the board should be enclosed in the shielded box. Open cell is placed against both
sides of the board to minimize air currents. Although a copper/solder junction is not considered a good thermocouple, there
are many such junctions and collectively, they can have an effect on the least significant bits.
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FIGURE 1:
MODEL 301 BLOCK DIAGRAM
The software drivers provided include a system-level
USB driver, a DLL and sample application code, with source,
in VC or VB. Win98 and Win2000 are supported. Refer to the
read_1st.txt file on the disk before installing the
drivers. Do not forget to follow the steps described there for
adding your Model 301's ID number to the device list. The latest
version of the driver software and sample application code can
be downloaded at no charge from www.lawsonlabs.com. In some cases,
an updated microcontroller chip may be required to take advantage
of new features.
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Establishing Communications
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Once the drivers are installed, run the executable version of the VC or VB sample application. (The VB
code has a more-developed user interface.) Before any other communications can succeed, the Model 301 needs to be initialized.
Each Model 301 has a unique identification number permanently programmed into the controller and printed on its label. You
will need to enter that number. The driver will then locate the requested device and assign it to the application for as long
as the application is loaded.
If you ever cannot regain access to a Model 301, unplug the USB cable, wait for the hourglass to disappear
(if you see it), then re-plug in the USB cable. This process will re-enumerate the device.
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In polled mode, individual conversions are requested with individual commands. Other commands set the
digital outputs, change input channels, recalibrate the system, or change to scanning mode. To maintain compatibility with
earlier products, the offset correction channel is designated channel 7. The full scale calibration channel is designated
as channel 6. The two available analog input channels are called channels 0 and 1.
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To confirm proper operation, select channel 7 and you should see a voltage in the general vicinity of
zero. The offset portion of the system calibration removes errors caused by the input signal conditioning circuitry as well
as offset errors in the A/D converter itself. Confirm that the reading is close to zero volts. Channel 6 should read close
to +5 volts after a system calibration. The full-scale calibration removes gain and signal conditioning errors.
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A battery is a convenient voltage source for checking the
Model 301. Connect the positive and negative ends of the battery
to a pair of positive and negative analog input pins on the analog
input connector. (Pins 14 and 1, respectively for channel 0). You
will also need a wire from one end (normally the negative) of the
battery to ground at the Model 301 to insure that the input
voltage at both input terminals is within 6.5 volts of ground. The
ground at pin 3 is provided for the purpose. This extra ground is
for common-mode requirements only, and while necessary, it is non-
critical. Remember that the Model 301 is optically isolated and
floats in comparison to the computer chassis ground. A typical D
cell should read about 1.5 volts. Reverse the wires and note the
polarity change. Connecting the input wires directly together will
cause a potential of zero volts. An open circuit will read
unpredictably. A positive overvoltage will read 5 volts. A
negative overvoltage will read -5 volts. The analog input channels
are protected against continuous overvoltage up to 60 volts.
Because of the extreme resolution possible with the Model 301
it is necessary to carefully shield your input signals from
electrical noise. Electrical noise can be radiated through the
air and picked up by wiring and/or circuitry. It can also be
introduced via the power connections. Also, air currents can
create sufficient temperature changes to cause thermal noise.
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FIGURE 2:
25-PIN CONNECTOR PINOUT
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PIN 13
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Digital Ground
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PIN 25
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Digital Out 7
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PIN 12
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Digital In 0
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PIN 24
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Digital Out 6
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PIN 11
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Digital In 1
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PIN 23
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Digital Out 5
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PIN 10
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Digital In 2
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PIN 22
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Digital Out 4
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PIN 9
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Digital In 3
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PIN 21
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Digital Out 3
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PIN 8
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Digital In 4
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PIN 20
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Digital Out 2
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PIN 7
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Digital In 5
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PIN 19
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Digital Out 1
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PIN 6
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Digital In 6
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PIN 18
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Digital Out 0
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PIN 5
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Digital In 7
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PIN 17
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Digital Ground
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PIN 4
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Digital Ground
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PIN 16
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Analog Ground
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PIN 3
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Analog Ground
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PIN 15
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CHANNEL 1+
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PIN 2
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CHANNEL 1-
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PIN 14
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CHANNEL 0+
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PIN 1
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CHANNEL 0-
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Note: For maximum protection, any unused input terminals
should be connected to ground. This is done to protect
the circuitry from static discharges which can be of
extremely high voltage. Open inputs can also pick up
noise. Strain-relief is recommended for all permanent
wiring on the connector. Otherwise, physical stress may
cause the failure of an electrical connection. The
connector hood provided has a strain-relief clamp.
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The digital input word, from pins 5 through 12 on the
input/output connector, is updated with each conversion in polled
mode and with each packet of ten conversions in normal scanning mode.
For digital input scanning, the digital input byte is updated and
transmitted once per conversion. The digital inputs are active low.
They can be activated by a contact closure to digital ground or by
any 5 volt logic signal referenced to the same ground.The digital
input word is displayed as an integer between 0 and 255. You may
want to view it in binary format so thatindividual inputs show as
ones and zeros.
The digital output command puts a latched 8-bit digital word at pins
18 through 25 on the input/output connector. Valid values are in the
range of 0 to 255. The digital outputs are buffered to source or sink
20 MA. Note that when sinking current, the voltage may rise significantly
above 0 volts.
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The software drivers provided include a system-level
USB driver, a DLL and sample application code, with source,
in VC or VB. Win98 and Win2000 are supported. Refer to the
read_1st.txt file on the disk before installing the
drivers. Do not forget to follow the steps described there for
adding your Model 301's ID number to the device list. The latest
version of the driver software and sample application code can
be downloaded at no charge from www.lawsonlabs.com. In some cases,
an updated microcontroller chip may be required to take advantage
of new features.
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The Model 301 will send data back continuously at the preset
rate if a start scan command is sent. There are six different
types of scan, single-channel, single-channel with digital input,
multi-channel, multi-channel with digital input, multi-channel
with calibration data, and multi-channel with digital input and
calibration data. For single-channel scanning, the Model 301 will
take data on whatever channel was selected when the scan was
started. Channel change commands can be sent at any time during
single-channel scanning. For multi-channel scanning, the Model 301
controls the channel selection. Do not send channel change
commands during multi-channel scanning. You can send digital
output commands at any time during either type of scan without
altering the data acquisition timing.
Calibration scan adds a reading of channel 7, the offset
channel, at the end of each scan. The application then subtracts
the offset value from the other data to provide active zero-drift
supression.
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The Model 301 features fully differential inputs. A basic
understanding of differential measurements will help you to use
your card to best advantage. The plus and minus input pins should
be wired directly to the voltage being measured. In this way, it
is assured that the only current flowing in the wires will be the
input current of the A/D converter. Because the wires have finite
resistance, any current flowing will cause a voltage drop and a
corresponding error. The Model 301 requires a vanishingly small
input current so the error caused by even very long wires is
negligible. For proper operation it is necessary that the positive
and negative inputs both be within 6 volts of ground. For a
floating voltage source, this is generally accomplished by
connecting a third wire between the Model 301 ground and a ground
terminal at the source of the measured voltage. Ground currents
may flow in this wire, but the resulting voltage drop will not
cause a measurement error. Redundant grounding can cause ground
loops. Ground loops can cause erratic behavior because currents
will flow through different paths at different times causing
unpredictable voltage drops.
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FIGURE 3:
TYPICAL INPUT CONNECTIONS
Best results are obtained with filtered, buffered voltages.
Electrical noise travels through the air and can be picked up
by interconnecting wires. The first defense against noise is
shielding. Use shielded wire with the shield connected at one
or both ends to ground. (See above.) The lower the impedance
of the voltage source, the less susceptible the wiring will be
to electrical noise. If noise problems persist, try to locate
the source of the interference and shield it. Electric motors,
electric heaters and flickering fluorescent lights are potential
sources of interference.
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FIGURE 4:
EFFECT OF DATA RATE ON EFFECTIVE RESOLUTION
AND SETTLING TIME
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DATA RATE (Hz.)
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EFFECTIVE RESOLUTION (Bits)
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SETTLING TIME (ms.)
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50
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22.5
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160
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100
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22
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80
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250
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21.5
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32
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500
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20.5
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16
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1000
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20
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8
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Settling time is the time in milliseconds required to obtain
a fully valid reading after an instantaneous full-scale step
Effective resolution is defined as total resolution minus
RMS noise. Numbers above use an oversampling factor of four.
See the FuncDesc.doc file and the sample application source
code for programmer's information. We will continue to add
operating system and language support for the Model 301 as
appropriate.
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Can't Initialize the Model 301
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A.) Make sure that power is connected properly to the 301.
B.) If hubs are being used, make sure they are powered.
C.) Double-check your cabling.
D.) Try unplugging the USB cable from the Model 301. If you
see the hourglass, wait for it to disappear, then plug
the cable back in. Close and reopen the application.
You may also need to cycle power to the Model 301 itself.
E.) It may be possible under some circumstances for your
Model 301 to be marked as belonging to an application
that is not responding. It may be necessary to explicitly
free the device. Select "utilities" in our sample
application and free the device in question. Then unplug
and replug the USB cable to re-enumerate the device.
F.) Make sure the ID number of the device you want to
initialize is listed in the configuration file. See the
read_1st.txt file for details.
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Won't respond to a data request
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A.) Remember to wait for the result from the previous request
before issuing another.
B.) Occasional USB traffic does not go through. You can request
again if the initial request fails.
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Data is consistent, but wrong
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A.) Do a system calibration.
B.) Make sure another A/D input channel isn't badly over-
range.
C.) Resend the rate information. If power is momentarily lost
to the isolated circuitry, the A/D rate will be forgotton.
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Data fluctuates wildly during scanning
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A.) If either the DLL's data buffer overflows, your data may
lose registration, that is, high, middle and low bytes of
the answer may be confused. Stop and restart the scan, or
reset the data buffer pointers to correct the problem.
B.) If "A" persists, slow the scanning rate, or locate and
suspend the other application or system driver that is
causing the USB driver to not be serviced frequently
enough.
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A.) Lower the data rate or increase averaging.
B.) Check shielding and grounding. Check that the DC common-
mode range of +/-6 volts is being observed.
C.) Make sure another A/D input channel isn't badly over-
range.
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Hardware calibration is set at the factory and should never
need adjustment. The software should always be able to calibrate
to yield peak performance.
There are two potentiometers on the board. The potentiometer
closest to the power input connector is the common-mode rejection
adjustment. The other adjusts the reference voltage. Changing the
reference voltage has the effect of changing the gain.
If you wish to reset the common-mode adjustment, first
connect the + and - input pins of a channel to a ground on the
analog input connector. Zero the channel by using the offset
command. Now remove the connection to ground and connect both
input pins to the 5 volt reference on pin 8 of the analog input
connector. Adjust the common-mode potentiometer for a reading of
zero. Repeat for best results.
The A/D gain is set by connecting a known voltage to an analog
channel. Do a system calibration, then adjust the gain
potentiometer to obtain the desired reading. Repeat for best
results.
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A/D TYPE
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24-bit delta-sigma converter with
microcontroller supervisor and
optical isolation.
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MONOTONICITY
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23 bits
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LINEARITY
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+/-0.002% of full scale.
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DIFFERENTIAL INPUT RANGE
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+/-5 volts
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DC COMMON MODE RANGE
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+/-6.5 volts
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DC COMMON MODE REJECTION
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-100 dB typical
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ANALOG INPUTS
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2, multiplexed true differential
protected to +/-60 volts.
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INPUT IMPEDANCE
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10^13 ohms typical
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PROGRAMMABLE DATA RATE
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50 to 1000 Hz, lower rates are
generated through digital averaging.
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EFFECTIVE RESOLUTION
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Effective resolution is defined as total
resolution in bits minus RMS noise in
bits.
Figures below use an oversampling
ratio of four.
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Rate
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Effective Resolution in Bits
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1000
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20
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500
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21
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200
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22
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50
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22.5
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SCANNING MODE
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Three scan types are available, single channel, multi-channel, and a calibration
scan that adds a reading of the zero channel at the end of each scan for offset
drift compensation. The max number of data points per second in multi-channel
scanning mode is 200. Any of the three scan types can add a digital input into
the data stream after each conversion. That feature allows maximum timing
accuracy for digital events.
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DIGITAL INPUTS
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8 bits, contact closure or 5 volt logic compatible.
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DIGITAL OUTPUTS
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8 latched, ruggedized, double-buffered 5V outputs.
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POWER REQUIREMENT
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11 to 15 VDC, regulated or unregulated, for isolated circuitry
or 15 to 30 VDC with pre-regulator option
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TYPICAL POWER CONSUMPTION
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The microcontroller operates as a low-power
USB bus-powered device.
The analog input and digital I/O requires 25 MA
(add drive current for active digital outputs, up
to 20 MA each).
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SIZE
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Board only 5.5 x 3.7 x 0.75 inches
Enclosure 6.0 x 4.2 x 1.25 inches
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The Lawson Labs, Inc. Model 301 is guaranteed against
defects in materials and workmanship for a period of one year
from the date of delivery. Products must be returned to Lawson
Labs for warranty service. Contact Lawson Labs at "800 321-5355" for
return authorization before returning anything for service.
The above warranty is in lieu of all warranties express or
implied. Lawson Labs will not be liable for indirect or
consequential damages caused by any defect in this product. Some
states do not allow the limitation of consequential
damages, so the above exclusion may not apply to you.
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lawsnlab@lawsonlabs.com
last reviewed/modified: 1-7-04 (Tim Van Dusen)
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