Basic
Information FAQ
What
is a LabJack?
LabJacks are USB/Ethernet based measurement and automation devices which
provide analog inputs/outputs, digital
inputs/outputs, and more. LabJacks provide an easy to use interface
between computers and the physical world.
What
kind of things can be done with a LabJack?
Read the output of sensors which measure voltage, current, power,
temperature, humidity, wind speed, force, pressure, strain, acceleration,
RPM, light intensity, sound intensity, gas concentration, position, and
many more. A LabJack brings this data into a PC where it can be stored and
processed as desired.
Control
things like motors, lights, solenoids, relays, valves, and more.
What
is USB?
USB (Universal Serial Bus)
allows the connection of up to 127 devices to a computer. All computers
sold today come with at least one USB port. USB is designed to be easy to
use.
What
is an analog input?
An analog input converts a voltage level into a digital value that can be
stored and processed in a computer. Why would you want to measure
voltages? There are a multitude of sensors available which convert things
like temperature, pressure, etc. into voltages.
What
is an analog output?
The LabJack's analog outputs convert digital values from a computer into a
voltage level. This provides an adjustable output.
What
is a counter?
A counter generally increments an internal register each time a voltage
pulse is detected at the counter input. This internal register is then
read periodically to determine how many events have occured. Typically
used to measure frequency or RPM, accumulate pulses from a tipping bucket
precipitation (rain) gauge, and similar counting applications.
What
are digital I/O?
Each digital I/O on a LabJack can be individually configured to one of 3
states: input, output-high, or output-low.
A
digital input provides a voltage thresholding operation. If the voltage is
higher than some value, the computer will detect the digital input as
high/set/1. If the voltage is lower than some value, the computer will
detect the digital input as low/clear/0.
A
digital output allows you to control a voltage with a computer. If the
computer instructs the output to be high, the output will produce a
voltage (generally about 5 or 3.3 volts). If the computer instructs the
output to be low, it is connected to ground and produces no voltage.
What
do single-ended and differential mean?
Single-ended and differential refer to the reference for a voltage. For a
single-ended measurement, the LabJack converts the difference between the
voltage at an input and ground. For a differential measurement, the
LabJack converts the difference between the voltage at one input and the
voltage at another input that is not necessarily ground.
On
the LabJack U12, analog inputs can set
as single-ended or differential via software. Analog inputs on the LabJack
UE9 are single-ended only, so an instrumentation amp is needed to convert
a differential signal to single-ended, or a pseudo-differential
measurement can be made by taking the difference of two single-ended
measurements.
What
does 12- or 16-bit resolution mean?
Resolution in this context refers to the conversion of an analog
voltage at the LabJack to a digital value in a computer (and vice
versa). A computer is a digital machine and thus stores a number as a
series of ones and zeroes. If you are storing a digital 2-bit number you
can store 4 different values: 00, 01, 10, or 11. Now, say you have a
device which converts an analog voltage between 0 and 10 volts into a
2-bit digital value for storage in a computer. This device will give
digital values as follows:
| Voltage |
2-Bit
Digital Representation |
|
0
to 2.5
2.5 to 5
5 to 7.5
7.5 to 10
|
00
01
10
11
|
So
in this example, the 2-bit digital value can represent 4 different
numbers, and the voltage input range of 0 to 10 volts is divided into 4
pieces giving a voltage resolution of 2.5 volts per bit. A 3-bit digital
value can represent 8 (2^3) different numbers. A 12-bit digital value can
represent 4096 (2^12) different numbers. A 16-bit digital value can
represent 65536 (2^16) different numbers.
It
might occur to you at this point that a digital
input could be thought of as a 1-bit analog to digital converter. Low
voltages give a 0 and high voltages give a 1.
In
the case of the LabJack U12, a single-ended
analog input has a voltage range of -10 volts to +10 volts (20 volt total
span) and 12-bit resolution. This gives a voltage resolution of 20/4096 or
0.00488 volts per bit (4.88 mV/bit). In the case of a differential
input voltage (with gain=1), either input can swing from -10 volts to +10
volts, so the difference between the two inputs can swing from +20 volts
to -20 volts (40 volt total span). This gives a voltage resolution of
40/4096 or 0.00977 volts per bit (9.77 mV/bit).
What
is an amplifier, PGA or PGIA?
A PGA (Programmable Gain Amplifier) provides variable gain which is
controlled in software. It is used to amplify low voltages on analog
inputs.
Most
LabJacks have an internal PGA that can be used to provide small gains in
some situations, but for larger gains an external amplifier must be used.
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