In the Laboratory edited by
Cost-Effective Teacher
Harold H. Harris University of Missouri—St. Louis St. Louis, MO 63121
A Simple Supplementary Offset Device for Data Acquisition Systems
Thomas Kappes and Peter C. Hauser* Department of Chemistry, University of Basel, Spitalstrasse 51, 4056 Basel, Switzerland; *
[email protected] Chart recorders and digital data acquisition systems are widely used instruments for visualization of measured results in research as well as in teaching laboratories. For certain applications such as chromatography and flow injection analysis with potentiometric or conductometric detection, an offset function is required because a signal on top of a large background is to be measured within a small range and with a high resolution. In many cases this function is not sufficient or not available at all. We describe here an electronic device that is connected in front of the chart recorder. It is easy to construct, even for somebody who does not have much experience with electronics. The requirement for tools is limited and the cost is low. The device adds to the measured voltage a positive or negative offset voltage. As shown in the diagram in Figure 1, the circuitry comprises a voltage divider and two operational amplifiers. The offset voltage is adjusted manually by means of a 10-turn potentiometer (P1) (534 series, RS Components, Northants, UK), which allows a sensitive regulation and which may be provided with a scale. The ratio of R1 and R2 to P1 determines the offset range and sensitivity. We chose P1 = 10 kΩ and R1 = R2 = 33 kΩ to obtain a range of ± 1.6 V. The offset voltage is added to the measured voltage by the op amp IC1a, which works as voltage-summing amplifier (1). As the resulting voltage is inverted, it is returned to its original polarity by the amplifier IC1b. Operational amplifiers are available as dual package integrated circuits. We used a TL072ACP (Texas Instruments, Dallas, TX), but the type of op amp is not critical. The resistors R3-7 are precision resistors of equal values (e.g. 100 kΩ) for a gain of 1. If a higher gain is desired, the value of R6 is chosen lower. The gain is given by the ratio R7/R6. A low pass filter function can also be added by connecting a capacitor C1 (e.g., 0.047 µF metallized polyester) parallel to R7. Both input and output voltage may be indicated by two standard LCD-panel meters (OEM-33 5/9, Anders Electronics, London, UK). Any model will be suitable, and detailed instructions are provided by the supplier. As they can usually display voltages up to ±199.9 mV only, the two voltages have
C1 0.047 µF R5 100 kΩ
Input
R4 100 kΩ
IC1a +
R7 100 kΩ R6 100 kΩ
R3 100 kΩ
panel meter
- 15 V
+ 15 V
R1 33 kΩ
P1 10 kΩ
Output
IC1b +
R2 33 kΩ
panel meter
Figure 1. Electronic circuitry of the offset device.
to be reduced by a factor of 10 by a resistive divider. Precision resistor pairs are available for this purpose, but we used 10turn 10-kΩ trimmer potentiometers. A power supply capable of providing regulated voltages of ±15 V completes the circuitry. Alternatively, the device can be operated with two 9 V batteries. The panel meters usually require a supply voltage of + 5 V, which can be derived from the 15-V power supply by a voltage regulator integrated circuit such as an LM 78L05 AC (National Semiconductors, Santa Clara, CA). The assembly is integrated in an aluminium instrument box equipped with suitable connectors for input, output, and power supply. All components are standard items and should be available from any electronics components supplier (e.g., RS Components, Northants, UK). On request, we will provide detailed schematics. Literature Cited 1. Malmstadt, H. V.; Enke, C. G.; Crouch, S. R. Microcomputers and Electronic Instrumentation. Making the Right Connections; American Chemical Society: Washington, DC, 1994.
JChemEd.chem.wisc.edu • Vol. 76 No. 10 October 1999 • Journal of Chemical Education
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