IC removes nonlinear temperature effects
Edited by Bill Travis
Kevin Schemansky, Maxim Integrated Products, Howell, MI -- EDN, September 25, 2003
A common technique for removing nonlinear temperature-related effects from a dc voltage is to incorporate a temperature sensor that a microprocessor samples via an A/D converter. The processor determines—by mathematical calculation or a temperature-indexed look-up table—the amount of adjustment is necessary at a given temperature and delivers the resulting value via a D/A converter. Though effective, that approach incurs design effort, cost, and multiple ICs. An alternative technique uses a single IC in an unorthodox fashion (Figure 1). The chip, intended as a low-cost interface to a Wheatstone bridge, normally provides precision analog-signal processing along with digitally programmable compensation of gain, offset, and temperature effects. Much of its capability remains unused in this application, but its size and pre-engineered internal circuitry can readily add nonlinear temperature compensation to a design.
IC1 contains a bandgap temperature sensor whose output is digitized by an 8-bit A/D converter, producing a digital representation of die temperature with 1.45°/bit resolution. An internal 768-byte EEPROM, read/write accessible via the one-wire asynchronous DIO pin, provides the user with two independent temperature-indexed look-up tables. (This design uses only one.) For the table in use, an 8-bit temperature register indexes which of 176 16-bit stored values is applied to an internal 16-bit D/A converter. Because the temperature-indexing boundaries of –69 to +184°C are outside the IC's operating range of –40 to +125°C, the IC can compensate for brief temperature excursions outside its normal operating range.
The Figure 1 circuit removes temperature-variable nonlinear offsets from the analog voltage, V1IN. First, you program IC1 for its minimum gain, 39V/V, so the R1/R2 attenuation provides an overall gain of unity. (The internal PGA provides a gain range of 39 to 264V/V.) Next, program the internal coarse-offset DAC with 0000 binary and the offset-TC DAC with 0000hex, to ensure that they have no effect on the V1 input voltage. To generate a compensated output (Pin 2), the output stage adds in the temperature-indexed value of a third DAC (the offset DAC), whose 16-bit resolution achieves adjustments as fine as 74 µV. After calculating coefficients for the offset DAC, you program them into the IC via the DIO pin. The equation for output voltage is V1OUT=39(V1IN/39)+VDD(offset-DAC value/65,536)(offset-DAC sign).
