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AN1812

APPLICATION NOTE

A High Precision, Low Cost, Single Supply ADC for

Positive and Negative Input Voltages

Introduction

In general the ADC embedded in the ST7 microcontroller is enough for most applications. But, in some

cases it is necessary to measure both positive and negative voltages. This requires an external ADC with

this particular capability. Most external ADCs require a dual supply to be able to do this. However,

microcontroller-based applications usually only have a positive supply available.

This application note describes a technique for implementing an ADC for measuring both positive and

negative input voltages while operating from a single (positive) supply. This converter is based on a

Voltage-to-time Conversion technique. Like other slope converters, this ADC also uses an Integrating

Capacitor, but the measured time is inversely proportional to the input voltage. An additional comparator

with a voltage reference is used to improve conversion accuracy.

As shown in the circuit diagram (Figure 1), the converter is implemented using an integrating capacitor,

resistor, external op-amp, comparators and some I/O pins (the ST72F264 microcontroller is used in this

example). The 16-bit timer of the microcontroller measures the time using its input capture pins (PB0 and

PB2). These pins are connected to the output of the Comp1 and Comp2 comparators. The I/O pins PB1

and PB3 are used to switch the M1 and M2 switches ON or OFF. The circuit could also work with a

microcontroller equipped with an 8-bit timer. Only a small modification to the software would be needed.

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