EDNE October 2012 - (Page 15)

The floating thermocouple circuit in Figure 2a does not provide a current path to ground or a bias-voltage reference for the INA’s input pins. Thus, the input-current leakages are not dissipated, and the two inputs can float to any undefined voltage. That situa- tion, in turn, causes the INA’s output to change to an invalid output voltage, usually in the middle of the INA’s output range. The invalid voltage can appear to be legitimate, making it difficult to detect the correctness of the INA input implementation. Figure 2b illustrates a correct thermocouple connection to an INA; both inputs have a path to ground—in this case, through a 10-kΩ resistor, biased to a voltage within the INA’s input range, or ground. C i r c u i t designers often misapply a thermocouple or even a two-wire microphone to Figure 2 An incorrect thermocouple connection to the INA the INA’s input produces erroneous outputs (a). A correct connection to the circuit; the INA’s input involves biasing the input stages to ground through problem arises a 10-kΩ resistor (b). when the INA inputs are connected without proper consideration to current paths or biasing. Heed those considerations for the INA’s input stage, and you can be confident that the INA’s output voltage is representative of the thermocouple’s voltage. But wait—there may be a problem with the output values of A1 and A2 (Figure 1). How would you solve this problem? Comment online at www. edn.com/4392026. I’d really like to hear from you! EDN RefeRences Kugelstadt, Thomas, “Getting the most out of your instrumentation amplifier design,” Analog Applications Journal, Texas Instruments, 4Q 2005, http://bit.ly/Peilok. 2 Trump, Bruce, “Instrumentation amplifiers—avoiding a common pitfall,” TI E2E Community, March 27, 2012, http://bit.ly/Nw4er2. 3 Trump, Bruce, “Instrumentation amplifiers—they’re not op amps but what are they?” TI E2E Community, April 2, 2012, http://bit.ly/NCsxHI. 1 Bonnie Baker is a senior applications engineer at Texas Instruments. Touch me 'cause you can! ISAQ 100 Secure Data Acquisition High precision measurements 18 bit resolution, 2 Mbit/s Potential free measurements Up to 3 km optical data transfer Absolute safety Galvanic Isolation > 1 MV On-desk operation up to 1000 V Wide input range ±250 V max. input range Outstanding portability 8000 hours battery power Light weight and compact design Simple integration OLE automation interface The perfect choice for EMC critical areas and high voltage applications! Find out more at: www.omicron-lab.com/daq www.edn-europe.com OCTOBER 2012 | EDN EUROPE 15 Smart Measurement Solutions http://www.edn.com/4392026 http://www.bit.ly/Peilok http://www.bit.ly/Nw4er2 http://www.bit.ly/NCsxHI http://www.omicron-lab.com/daq

Table of Contents for the Digital Edition of EDNE October 2012

Cover
Contents
International Rectifier
Microchip
RS Components
Masthead
Microchip
EDN Comment
Pulse
Analog Devices
Farnell
Baker's best
Omicron
Test & Measurement World
Agilent Technologies
Maxim Integrated Products
Maxim Integrated Products
Digi-Key
Mixed-signal SOC verification using analog behavioral models
Messe München
Bergquist
The future of connected-device security
Silicon Laboratories
Digi-Key
Coilcraft
Messe München
Image sensors evolve to meet emerging embedded vision needs
Digi-Key
Chip Expo
National Instruments
Supply chain
Introducing software-designed instrumentation to test-and-measurement
Digi-Key
Mechatronics in design
Design ideas
Product Roundup
Tales from the cube

EDNE October 2012