-
-
The Lawson Labs, Inc. Model 302 24-Bit Data Acquisition
System is a high-resolution device for translating voltages
into digital form. The Model 302 communicates with a host
computer using USB (Universal Serial Bus). It has six
multiplexed differential analog input channels, three analog
outputs, four digital inputs and eight digital output lines.
All the input/output circuitry is optically isolated from the
computer.
The Model 302 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 nearly 23 bits
effective resolution. The converter is guaranteed monotonic
to 23 bits. Effective resolution decreases with increasing
data rates. Even so, over 20 bits of effective resolution are
maintained at 500 samples/second.
The Model 302 requires a single DC supply in the range of
15 to 30 volts to power the isolated circuitry. With the
preregulator removed (a factory option), the range is 9 to 15 VDC.
Current draw is typically 30 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 302 maintains its own time base and transmits
a pre-defined scan of one to eight input channels at a preset
interval. The last two channels are for gain and offset
correction. Self-calibration can remove drift 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.
|
-
-
The Model 302 interconnections consist of a 14-position analog
input screw terminal block, a 16-position screw terminal block for
other Input/Output, a 2-terminal power connector, and a USB
connector. An extension cable is available, as an option, to
connect between a 4-terminal polarized header on the Model 302
board and a bulkhead-mounting USB connector. The bulkhead fitting
is made to mount in a DB9 serial connector openning to aid in
retrofitting existing hardware.
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 15 to 30 VDC and does not need to be regulated. Power
is connected to the 2-terminal block. The power terminals are
labeled "+V" and "GD". 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 302 because of the optical isolation. We normally
recommend placing a 5K to 100K ohm resistor between the two
grounds near the Model 302 if there is no other common ground
point.
For maximum accuracy the board should be enclosed in a
shielded box. Open cell foam can be placed firmly 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.
-
FIGURE 1:
MODEL 302 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 302's ID number to the device list.
|
Establishing Communications
|
-
-
Once the driver is 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 302 needs to be initialized. Each Model 302 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 302, 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.
|
-
-
In polled mode, individual conversions are requested with
individual commands. Other commands set the analog outputs or
digital outputs, change input channels, recalibrate the system, or
change to scanning mode. The offset correction channel is
designated channel 7. The full scale calibration channel is
designated as channel 6. The six available analog input channels
are called channels 0 through 5. Note that labels on the board
itself are 1 through 6, etc. The assumption is that from a
developer or programmer's perspective, zero-based references
are more appropriate, but that the user would prefer counting to
start from 1.
|
-
-
To confirm proper operation, select channel 7 and look for 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.
|
-
-
A battery is a convenient voltage source for checking the
Model 302. Connect the positive and negative ends of the
battery to a pair of positive and negative analog input terminals
on the analog input connector. You will also need a wire from one
end (normally the negative) of the battery to ground at the Model
302 to insure that the input voltage at both input terminals is
within 6.5 volts of ground. Several ground terminals are provided
for the purpose. This third wire ground is for common-mode
requirements only, and, while necessary, it is non-critical.
Remember that the Model 302 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 302
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.
-
FIGURE 2:
Analog Input Connector (14 terminals)
-
|
+1
|
-1
|
+2
|
-2
|
+3
|
-3
|
+4
|
-4
|
+5
|
-5
|
+6
|
-6
|
GD
|
GD(ground)
|
|
|
-
Other Input/Output Connector (16 terminals)
-
|
Analog Out
|
|
Digital In
|
|
Digital Output
|
|
1
|
2
|
3
|
|
1
|
2
|
3
|
4
|
|
GD
|
A
|
B
|
C
|
D
|
E
|
F
|
G
|
H
|
|
|
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.
|
-
-
The digital input word is updated with each conversion
in polled mode, with each packet of ten conversions in normal
scanning mode, or with each conversion in digital input scanning
modes. 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 that individual inputs show as ones and zeros.
The digital output command puts a latched 8- bit digital word
at terminals A through H 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.
|
-
-
The A/D converts constantly at the selected rate. That rate
determines the frequency response of the converter. Rates from
50 to 1000 Hz are supported. There is a low-pass filter intrinsic
to the conversion process. The cut-off frequency of that filter
is the data rate times .262. For maximum effective resolution
use the lowest data rate that meets your needs. For rates lower
than 50 Hz, average in the application, or discard the extra data.
Note that 1 / rate must be an integer multiple of the A/D's
clock period. Therefore, the actual rate selected and displayed
may be slightly different than the rate you enter.
The application can keep a running average of N consecutive
conversions. The Model 302 is specified with the running
average set to four.
|
-
-
The Model 302 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 302 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
302 controls the channel selection. It sequentially selects
the input channels and converts the data. Do not send channel
change commands during multi-channel scanning. You can send analog
or digital output commands at any time during either type of scan
without altering the data acquisition timing.
|
-
-
The Model 302 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 302 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 302 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.
|
-
-
-
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.
|
-
-
-
FIGURE 4:
EFFECT OF DATA RATE ON EFFECTIVE RESOLUTION
AND SETTLING TIME
-
-
|
DATA RATE (Hz.)
|
EFFECTIVE RESOLUTION (Bits)
|
SETTLING TIME (ms.)
|
|
50
|
22.5
|
160
|
|
100
|
22
|
80
|
|
250
|
21.5
|
32
|
|
500
|
20.5
|
16
|
|
1000
|
20
|
8
|
|
|
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 302 as
appropriate.
|
Can't Initialize the Model 302
|
-
-
A.) Make sure that power is connected properly to the 302.
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 302. 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 302 itself.
E.) It may be possible under some circumstances for your
Model 302 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.
|
Won't respond to a data request
|
-
-
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.
|
Data is consistent, but wrong
|
-
-
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.
|
Data fluctuates wildly during scanning
|
-
-
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.
|
-
-
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.
|
-
-
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.
|
-
-
-
-
-
A/D TYPE
-
24-bit delta-sigma converter with
microcontroller supervisor and
optical isolation.
-
MONOTONICITY
-
23 bits
-
LINEARITY
-
+/-0.002% of full scale.
-
DIFFERENTIAL INPUT RANGE
-
+/-5 volts
-
DC COMMON MODE RANGE
-
+/-6.5 volts
-
DC COMMON MODE REJECTION
-
-100 dB typical
-
DIFFERENTIAL ANALOG INPUT CHANNELS
-
6 Multiplexed true differential
protected to +/-60 volts.
-
ANALOG OUTPUTS
-
3 at 14-bit resolution, settling
to 99% of full scale in 1 second.
-
INPUT IMPEDANCE
-
10^13 ohms typical
-
PROGRAMMABLE DATA RATE
-
50 to 1000 Hz, lower rates are
generated through digital averaging.
-
EFFECTIVE RESOLUTION
-
Effective resolution is defined as total
resolution in bits minus RMS noise in
bits. Figures below use an oversampling
ratio of four.
Rate Effective Resolution in Bits
-----------------------------------------------
1000 20
500 21
200 22
50 22.5
-
SCANNING MODE
-
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.
-
DIGITAL INPUTS
-
4, contact closure or 5 volt logic compatible.
-
DIGITAL OUTPUTS
-
8 latched, ruggedized, double-buffered 5V outputs.
-
POWER REQUIREMENT
-
11 to 15 VDC, regulated or unregulated, for isolated circuitry
or 15 to 30 VDC with pre-regulator option
-
TYPICAL POWER CONSUMPTION
-
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).
-
SIZE
-
6.5 x 4.8 x 0.8 inches
|
-
-
The Lawson Labs, Inc. Model 302 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.
|
lawsnlab@lawsonlabs.com
last reviewed/modified: 1-7-04 (Tim Van Dusen)
|
