The Model 302 is a complete, Delta-Sigma high-resolution data acquisition system for the Universal Serial
Bus (USB). It serves as a general purpose A/D converter or as a built-in interface for a wide variety of
instrumentation. Up to 32 Model 302s can be connected to a single USB port using inexpensive hubs. Six
analog input channels are provided, along with three analog outputs, four digital inputs and eight digital
outputs. An expandable version was introduced in 2016. Standard and custom expansion boards are offered.
Lawson Labs has been designing and building high-resolution Delta-Sigma Data Acquisition Systems since
1991. The Model 302 represents our sixth generation of Delta-Sigma design. It incorporates the optical
isolation and input protection that have long allowed our high resolution systems to function reliably
in the industrial environment.
The standard analog input range is +/- 5 volts, true differential. The maximum single-channel data rate is 1000 Hz
at 20 bits effective resolution. Two channels can be scanned at 100 Hz with effective resolution
of 20 bits. As the unit is slowed down, it quickly gains effective bits to 22 bits at 200 Hz and 22.5 at
50 Hz for single-channel operation. The input impedance is 1013 ohms and the DC common mode rejection is
typically -100 dB. The isolated portion of the Model 302 operates from a single unregulated, low-power supply.
Drivers are provided for Win98 through Win10, with sample application code for
VisualBasic, VisualC++, .NET (VisualBasic, VisualC++, VisualC#), LabView®, Linux, and Python.
The VB sample code includes a real-time graphic presentation of data, and is powerful enough for many general-purpose
data logging applications. Multiple Model 302s can be split
between 32 applications, or all used by one, or anything in between. Units can be connected or disconnected
on the fly, without interrupting other ongoing acquisition. In order to obtain fully independent operation,
even under fault conditions, we found it beneficial to write our own low level USB device drivers.
These signed drivers provide high-reliability operation for critical acquisition tasks.
Each Lawson Labs USB data system is programmed with a unique ID number to eliminate ambiguity when multiple data
systems are connected. The unique ID also provides the additional benefit that ID numbers can be used for software
protection. We can reserve a range of ID numbers for a particular customer. Application software can then verify the
ID number to guarantee that the software installation is authorized.
SPECIFICATIONS
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 standard
+/-2, +/-1, +1/-0.4v available
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
First output 0-5 volts.
Other outputs selectable, 0-5 or +/-5volts
INPUT IMPEDANCE
1013 ohms typical
PROGRAMMABLE DATA RATE
50 to 1000 Hz, lower rates are
generated through digital averaging
EFFECTIVE RESOLUTION
The effective resolution is defined as total
resolution in bits minus RMS noise in
bits. The 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).
