AN2615 datasheet(1/37 Pages) STMICROELECTRONICS

Part #	AN2615

Description	In general the ADC embedded in the ST7 microcontroller

PDF	37 Pages

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Manufacturer	STMICROELECTRONICS [STMicroelectronics]

Direct Link	http://www.st.com

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1 / 37 page

August 2007

Rev 1

1/37

AN2615

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 on page 6), the converter is implemented using an

integrating capacitor, resistor, external op-amp, comparators and some microcontroller I/O

pins. The ST72F264 microcontroller is used in this application note as an example, but the

implementation is feasible using any ST7 microcontroller. 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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