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Analog Input (App Note)

Analog Input Overview

A common question is "can this sensor/signal be measured with a LabJack". Unless the signal has a voltage (referred to the device's ground) beyond the device's limits, it can be connected without damaging the LabJack. However more thought is required to determine what is necessary to make useful measurements with a LabJack or any measurement device.

A few important factors should be considered to be sure that you don't harm the device and get useful information.

  • Signal Voltage
  • Signal's Source Impedance
  • A Devices Resolution (and Accuracy)
  • Speed

Signal Voltage:

Whether you are trying to make single-ended analog readings or differential analog readings (if your device permits) you need to make sure that the voltage on each channel with respect to ground is still within the common mode limits. When measuring parameters other than voltage, or voltages too big or too small for a particular LabJack, some sort of sensor or transducer is required to produce the proper voltage signal. Examples are a temperature sensor, amplifier, resistive voltage divider, or perhaps a combination of such things.

Notable piece of information: Our U6 devices feature Fully-Differential Inputs that output calibrated voltage readings.

Impedance:

When connecting a LabJack, or any measuring device, to a signal source, it must be considered what impact the measuring device will have on the signal. The main consideration is whether the currents going into or out of the device's analog input will cause noticeable voltage errors due to the impedance of the source. To maintain consistent results reference the appropriate devices datasheet or Users Guide Appendix A for the recommended maximum source impedance. Our devices have relatively high input impedance, however in some very low current applications problems may arise.

Resolution (and Accuracy):

Each LabJack device has its own specifications regarding how precise and accurate they are. Please refer to the relevant LabJack datasheet or Appendix A for more information.

Speed:

Speed is an important attribute when trying to acquire analog signals. If you are trying to read steady state signals (constant voltages like those given by potentiometers) its fairly straight forward however if you are trying to read and interpret a waveform it is much more complicated. Some of the information needed is the frequency, amplitude, offset, and a knowledge of what you expect to see (its shape). Answers to these questions will help decide how fast you need to collect data points and determine how complicated your code will need to be. Our LabJack devices have two general data acquisition modes, normal mode (command and response) and streaming mode (configuration and listening, a periodic bulk transfer of data). It is also important to consider how many channels you are expecting to read and if you are trying to read them simultaneously. Most of our devices use dedicated ADCs with analog MUXs to switch between input channels (Very common in the world of electronics). This means that there is a delay in-between "multiple channel reads" called scan rate.

Important Links:

Analog Input pages:
Useful for finding device specific information and their capabilities
Appendix A in the Datasheets:
Useful for device specifications and reference numbers

2 comments

Robert W. Welch's picture

I am trying to upgade teh DAQ system from a circa 1960's piece of test equipment for a client of mine. I am using a U6 Pro to log J type TCs that will only be used in the 250 - 350 degrees F range. I am uncertain how to construct the scaling equations to provide the temperatures in degrees C and have read the section on Scaling Equation but can't quite figure it out. The NIST website lists 250F (121.11C) at 6.425 mV, and 350F (176.66C) at 9.535 mV, giving 55.978 x 10^-6 V / degree C for the range. Is the equation: y=TCVoltsToTemp[J:c:55.978*a-642.5] correct? And do I need to subtract the initial voltage offset from the c: channel? Appreciate any help you could provide. So far the U6 Pro is the right DAQ system for this equipment.  I will have additional questions on reading the strain responses taken from an oscillating sensor at 1.67 cps.

 

 

LabJack Support's picture

Start by going through the U6 Thermocouple Tutorial and let us know if that does not clear up your questions:

https://labjack.com/support/app-notes/thermocouples

https://labjack.com/support/app-notes/thermocouples/u6

You are calling the special thermocouple conversion function provided by the UD library.  This TCVoltsToTemp function is documented in Section 4.2.15 of the U6 Datasheet.  You pass in the thermocouple type, thermocouple voltage, and cold junction temperature in degrees K, and the function outputs the temperature of the remote end of the thermocouple in degrees K.