Rearranged reference helps ADC measure its own supply voltage
Clever topology allows self-measurement of power supply rail.
Björn Starmark and Orville Buenaventura, Maxim Integrated Products Inc, Sunnyvale, CA, and Sören Käck, Audioväxlar, Sweden; Edited by Brad Thompson -- EDN, April 28, 2005
If you use an ADC to monitor a system's power-supply voltage, you may encounter situations in which the supply voltage exceeds the ADC's reference voltage (Figure 1). However, the ADC's input voltage cannot exceed its reference voltage. You can use an external resistive divider to bring the supply voltage within the ADC's input range, but even 0.1%-tolerance resistors may introduce an objectionable error. You can solve the problem by eliminating the divider, connecting the ADC's reference input to its power supply, and connecting one of the ADC's inputs to a precision voltage reference—in this case, a 2.5V MAX6025A (Figure 2).
In this configuration, the ADC measures its inputs with respect to the supply voltage. Using the digitized reference voltage as a standard, the system's software computes the ratio of the reference voltage with respect to the power-supply voltage and corrects the remaining inputs' measurements. The ADC must accommodate an external reference voltage that equals its power-supply voltage, and any noise on the supply rail disturbs measurements of all channels. Thus, to quiet the supply rail in electrically noisy environments, you may need to add a lowpass filter to provide extra decoupling at the ADC.
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Rearranged reference helps ADC measure its own supply voltage
Fig 1 & 2 swapped
Neveux Laurent - 2005-29-4 01:53:00 PDT -
I don't understand the reasoning behind this Idea, and it doesn't help that Figures 1 and 2 are swapped :^(
Why is a 0.2% error from a resistor divider unacceptable, but it's preferable to connect the ADC's reference to the supply which is both noisier _and_ less accurate than 0.2%, plus now the software has to rescale all of the _other_ readings???
Ron Bauerle - 2005-28-4 10:10:00 PDT -
Figure 1 and Figure 2 are reversed, at least based on what the text in the article says.
Howard Smith - 2005-28-4 10:02:00 PDT
