Part # Application Notes Automotive ST72321-Auto ST72321AR6-Auto ST72321AR7-Auto ST72321BAR6-Auto ST72321BAR7-Auto ST72321BAR9-Auto ST7 datasheet

Part Manufacturer: ST Microelectronics

ST Microelectronics

Part Description: A high precision, low cost, single supply ADC for positive and negative input voltages


Part Details:

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. August 2007 Rev 1 1/37 www.st.com Contents AN2615 Contents 1 Circuit diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2 Theory of operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.1 Advantage of using two comparators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3 Timing diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 4 Circuit analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 5 Vout vs time diagram for different input voltages . . . . . . . . . . . . . . . . . 10 6 Characteristics of different slope converters . . . . . . . . . . . . . . . . . . . . 11 6.1 Single-slope converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 6.1.1 Single-slope converter timing diagram . . . . . . . . . . . . . . . . . . . . . . . . . . 11 6.2 Dual-slope converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 6.2.1 Dual-slope converter timing diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 6.3 Solution presented in this application note . . . . . . . . . . . . . . . . . . . . . . . . 13 7 Error analysis/constraints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 7.1 Input offset voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 7.2 Correction factor for the product of R*C . . . . . . . . . . . . . . . . . . . . . . . . . . 14 7.3 Value of charging resistance R . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 7.4 Charging capacitor C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 7.5 16-bit timer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 7.6 Effect of temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 7.7


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Application Notes Automotive ST72321-Auto ST72321AR6-Auto ST72321AR7-Auto ST72321BAR6-Auto ST72321BAR7-Auto ST72321BAR9-Auto ST7.pdf Datasheet