AN-1148 Voltmeter Ammeter Autorange Control

This application note describes how to design a circuit to control autoranging of a volt/current meter. Using the GreenPAK4 SLG46621V’s onboard ADC, counters, comparators, and logic blocks, automatic adjustment of the measurement range (“autoranging”) is possible without MCU intervention, allowing for faster and more reliable signal scaling while freeing up MCU resources. This article focuses on two autoranging examples: low-side current sensing using an array of shunt resistors, and dynamically attenuating a voltage input to a simple voltmeter. However, other configurations are possible, such as varying the gain of a digitally-controlled instrumentation amplifier such as the AD8250. Autoranging is used when an application requires the accurate measurement of a voltage signal whose range spans several orders of magnitude, so the measurement process is more involved than simply routing the signal to an ADC. Using a high-resolution ADC (16 bits or more) may be prohibitively expensive or impractical for the application, and even then it may still be insufficient to span the breadth of the measurement range. And in a system where the microprocessor must attend to many tasks, it may be impractical to burden it with monitoring the signal, adjust the gain/attenuation to scale the signal, and dedicating more I/O pins. Timers and digital filtering may also be required to prevent overly frequent toggling between ranges. For these reasons, using a GreenPAK in the design becomes prudent.


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