June 19, 1997

Low-power, low-voltage ADC is 0.05% linear

John Wettroth, Maxim Integrated Products, Sunnyvale, CA

The simple integrating 3V A/D converter in Figure 1a is small, requires no negative supply and no expensive precision components, and draws minimal supply current of 10 µA. A single conversion comprises 12,000 comparisons and takes about 300 msec.

Following each comparison, the µC closes the IC_2A comparator-high or the IC_2B comparator-low switch. The switches connect either V_REF (1.2V) or ground to their "B" terminals, producing a PWM signal. R₁ and C₁ filter and differentially integrate the signal against V_IN. IC₁ compares the result to the internally generated V_REF of 1.2V.

While this action integrates the error voltage up and down, the µC counts the number of comparisons for which the comparator output is high (IC_2A switch closed). This count (NH) divided by 12,000 equals the PWM duty cycle. The system is fully ratiometric, so the duty cycle equals NH/12,000=V_IN/V_REF. Rearranging and substituting V_REF=1.2V yields V_IN=NH/10,000.

An accompanying listing enables the LCD module to display voltage values directly, like a DPM. (Click here to download the file from DI-SIG, #2046.) The subroutine "DVM" produces the actual A/D-conversion values an embedded application requires. Setting the span constant (the number of comparisons) to 12,000 yields a 300-msec conversion with 41/2 digits of resolution and produces a full-scale display of 1.1999. You can speed the conversion to 30 msec by setting the span constant to 1200, which produces a 31/2-digit display that reads 1.199 at full scale.

IC₂'s nearly ideal switching characteristics account for the low 0.05% nonlinearity (Figure 1b). A high-performance, 3V-specified version of the industry standard 4066, IC₂ is a quad analog switch that features 35 ohms on-resistances and 0.1-nA maximum off leakages. You can save space by replacing IC₂ with a dual analog switch with specifications similar to those of the MAX4066.

V_CC is limited to the maximum the µC allows: 6V. IC₁, which operates with a V_CC as low as 2.8V over temperature, draws only 7 µA of supply current. The bandgap voltage reference connected to the negative comparator input in IC₁ is stable for capacitive loads smaller than 100 pF or larger than 0.05 µF. To ensure stability, the reference's external bypass capacitor, C₂, should be large. (DI #2046)

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