The supply rail normally powers a microcontroller’s voltage-reference source. In power-critical battery-operated applications, the constant drain, even of a few 10s of microamps, can be prohibitive. This situation requires adding a pin to turn the reference voltage on and off. By adding a 0.1-µF capacitor in parallel with the voltage reference and a simple bit of software that you can download from the online version of this Design Idea, you’ll need just one pin to both power and read the reference voltage.

When you connect the voltage reference as in Figure 1, the software configures the Microchip PIC chip’s V_REF (reference-voltage) pin as a switched-on output. After approximately 300 µsec, the voltage across the capacitor stabilizes at 1.225V.

There is an initial overshoot when the ZXRE4041 powers up. The pin is then reconfigured as an analog input for the ADC’s reference-voltage source. The reference voltage quickly drops by 20 mV in the next 50 µsec as the ZXRE4041 shuts down. With a 0.1-µF capacitor, the voltage then slowly drops 60 mV over 2 msec because of leakage. Although this delay is exponential, the rate is so slow that, for practical purposes, you can consider it linear for this short time window.

You must also consider that the ADC also draws current through the 10-kΩ resistor during conversion, causing voltage drop. Although Microchip doesn’t characterize this voltage drop in its documentation, tests consistently measured a drop of 80 mV for several devices, giving a calculated current of 6.67 µA. Using a conservative internal 4-MHz clock and allowing an ADC clock of frequency oscillation divided by 16 for operation at the minimum operating voltage, one conversion takes 45 µsec. This action slightly drains the capacitor, but this drainage appears to be only 2 or 3 mV. Calculations of initial watt-seconds minus watt-seconds used yield even lower values. Subtracting these fixed, repeatable losses from the initial steady-state 1.225V yields a new reference voltage of 1.225V_REF–0.020V shutdown drop–0.080 IR drop=1.145V.

Allowing 75 µsec to do the analog-to-digital conversion, store the value, and set up for the next conversion on another channel, 11 conversions will result in the last one’s reference voltage being lower by 22.5 mV—that is, 10 conversions×75 µsec×(60 mV/2000 µsec). This error is only 1.9% compared with the first conversion’s results.

If you just need an approximate voltage for a consumer product, for example, to warn of low battery voltage, you can use an LED instead of the ZXRE4041. Just change the value of R₁ to 300Ω to provide sufficient current to turn on the LED. Although LEDs lack the temperature stability of dedicated voltage-reference chips, the variation may be acceptable for the application because most consumer products find use within the comfort range of humans. If an LED is already part of the system, then the voltage-reference cost is only that of the software. Using this technique, an LED can now provide status-indicator, photodetector, and voltage-reference functions and enter a zero-power state using only software to reconfigure the changes.