This circuit allows an A/D converter to monitor a system supply voltage by connecting the supply directly to the ADC's reference input.
This design idea appeared in the April 28, 2005 edition of EDN magazine.
Systems in which an analog-digital converter (ADC) monitors the supply voltage must contend with the condition in which the ADC reference is usually lower than the supply voltage (Figure 1). An external resistor-divider can pull the supply voltage within the ADC's range, but even 0.1% resistors introduce an error that may be objectionable in certain applications.
One solution to the ADC reference voltage problem is simply to eliminate the divider (Figure 2). You can relate the reference to the supply voltage by connecting the supply voltage as a reference, and the reference (2.500V for the MAX6025A) to an input. As Figure 2 shows for the MAX1087, the ADC must be capable of accepting an external reference as high as the supply voltage. The other channels are now measured as a ratio to the supply voltage instead of the reference, but software can correct that problem.
Because the supply rail serves as a reference, any noise on the rail disturbs all channels. You may, therefore, need to add a local lowpass filter to quiet the supply voltage in noisy environments.