LabJack U12 Frequently Asked Questions

• What is a LabJack?
• What kind of things can be done with a LabJack?
• How easy is it to install and operate a LabJack?
• What is USB?
• What is an analog input?
• What is an analog output?
• What is a counter?
• What are digital I/O?
• What is an I/O module?
• What do single-ended and differential mean?
• What does 12-bit resolution mean?
• What is an amplifier, PGA or PGIA?

What Is A LabJack?

LabJacks are USB 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 Kinds 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.

How Easy Is It To Install And Operate A LabJack

As easy as it gets. Connect the LabJack to a USB port using the included cable. The computer will automatically recognize the device and install the drivers which are already on your computer (Windows 98SE, ME, and 2000 are supported). Install and run the application LJlogger which comes with the LabJack, and you will immediately be able to monitor inputs and control outputs. Sensors etc. are easily connected to the LabJack with the built-in screw terminals.

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, and PCI cards are available for under $25 to add USB to older computers. USB is designed to be robust, easy to use, and high speed.

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 allows various types of events to be counted. 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?

A digital input provides a voltage thresholding operation. If the voltage is higher than some value, the computer will detect the digital input as "ON". If the voltage is lower than some value, the computer will detect the digital input as "OFF".

A digital output allows you to control a voltage with a computer. If the computer instructs the output to be "ON", the output will produce a voltage (generally about 5 volts). If the computer instructs the output to be "OFF", it is connected to ground and produces no voltage.

What Is An I/O Module?

I/O modules such as the Grayhill 70 series, sold as a LabJack accessory, provide an isolated interface between digital I/O and high voltage/current connections. For instance the AC Output Module (70-OAC5A-L) allows you to turn on and off circuits with AC voltages of 24-280 volts and currents up to 3.5 amps. The optical isolation protects the LabJack from high voltage levels.

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.

On the LabJack U12, analog inputs can set as single-ended or differential via software. Both analog outputs are single-ended only.

What Does 12-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:

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.

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.

In the case of the LabJack U12, there is a PGA available for all differential channels which provides a variable gain between 1 and 20. When set to a gain of 1, the differential input voltage range is +20 volts to -20 volts and the voltage resolution is 0.00977 volts per bit (9.77 mV/bit). When set to again of 20, the differential input voltage range is +1 volt to -1 volt and the voltage resolution is 0.000488 volts per bit (0.488 mV/bit).