Test Ideas: Bias current modulation eliminates wiring errors
By modulating the excitation current applied to a temperature-sensing diode, you can eliminate errors caused by voltage drops along sensor wires.
By W. Stephen Woodward, Consultant, Chapel Hill, NC -- Test & Measurement World, 3/1/2009 2:00:00 AM
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Diodes make good temperature sensors because they are inexpensive, robust, stable, and sensitive, and they don’t need reference junctions. Diodes require an excitation current to produce a PN-junction voltage, which varies with temperature by –2 mV/°C. The excitation current causes voltage drops in measurement wires because of a wire’s resistance. You need to compensate for those voltage drops so that the measurement will accurately represent the diode voltage.
Many circuits remove the excitation current from the measurement using a four-wire Kelvin connection: Two wires carry the excitation current, while the other two wires connect the sensor to an ADC (analog-to-digital converter) or measurement instrument. But by using a modulated excitation current, you can use just two wires and still remove the losses in the wires.
The circuit in Figure 1 cancels the wiring-resistance error while needing only two conductors in the sensor cable. It relies on the fact that voltage drop in the wires is directly proportional to current (i.e., Ohm’s Law), but the sensor voltage is mostly constant. If you measure the total voltage drop across the wires and sensor at two current levels, you can remove the error. The circuit alternates the magnitude of the excitation current (Ib) between two values, Ib1 and Ib2, where Ib1 = 2Ib2. The AC component of the resulting signal is thus (approximately) IbRw where Rw = Rw1 + Rw2 plus a minor contribution from nonzero sensor impedance.
 Figure 1: Op amp A1 removes voltage drops across sensor wires. |
The internal oscillator of the LTC1043, set to ~500 Hz by connecting the external 0.01-µF capacitor to pin 16, becomes the clock for both Ib1 and Ib2 excitation modulation and synchronous demodulation of the resulting response. The resulting toggling of the excitation ballast resistance between 1 MΩ and 2 MΩ (1 MΩ + 1 MΩ) creates the 2:1 current modulation and an AC signal component proportional to wiring resistance Rw, which is IbRw.
The other side of the LTC1043 switch, with the Ib1Rw = Vc1 phase stored on C1 and the Ib2Rw = Vc2 phase on C2, synchronously rectifies the IbRwAC component. Op amp A2 buffers Vc1 from the resistor network where amplifier A1 subtracts it from the average sensor signal. The circuit’s output voltage is thus independent of error voltages in any cables.
One downside of the technique is that, due to nonzero sensor-impedance effects (on the order of 20 mV), you need to calibrate the sensor for accurate operation with this new circuit. You can calibrate the sensor in a variety of ways depending on the temperature span and precision requirements of your application. For example, you can perform a one-point calibration using a convenient temperature reference such as the boiling point of liquid nitrogen (77 K, –320.5°F, and –195.8°C).
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Earliest article I have on diode temperature sensing is "Solid-State Probe Thermometer" by Gordon Greg in Electronics World, March 1971 - pages 70-71. This article references: "Semiconductor Diodes and Transistors as Electrical Thermometers" by A.G McNamara in the "Review of Scientific Instruments", vol.33, pages 330-333, published in 1962. There is also a correction re. calibration techniques in Electronics World, June 1971, by Paul Galluzzi, P.E., Beverly. Mass. According to this article, the original research was done at the National Research Council of Canada. All the articles I've seen over the years using diodes and transistors seem to originate from this work.
anonymous - 2010-12-3 06:49:17 EST -
Oh Dear I think that is what you are describing.....read more carefully before firing off ...Sorry. But yes it works or did in 1966 for me.
Alan Melia - 2009-26-3 17:40:00 EDT -
It is many years since I played with internal diodes as temperature sensors. However I seem to remember that a "differential" technique was supposed to be better. I don't recall the detail now but I think it was dV/dI slope that was related to temperature rather than the steady voltage. Would this help??
Alan Melia - 2009-26-3 17:33:00 EDT
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