2

AVR401

0953C–AVR–02/03

The algorithm used for the conversion is as follows:

1.

Turn on the reference.

2.

Charge the capacitor until the reference voltage is reached. Measure the time

needed for this, T

3.

Turn off the reference and discharge the capacitor

4.

Charge the capacitor until the input voltage is reached. Measure the time needed

for this, T

The conversion cycle is shown in Figure 2.

The time measurement is performed by the Timer/Counter, which is expanded to nine

bits by using the Timer/Counter Overflow Interrupt.

Figure 2. Conversion Cycle

Calculation

Suppose that V

ence voltage is given by:

Equation 1

The ideal output from the conversion is an 8-bit number, where 0 volts corresponds to

zero and 5 volts is 255. The reference voltage V

equation can be re-written as:

Equation 2

However, with inaccuracy in the reference resistors, the reference voltage may vary

slightly. To compensate for this, a calibration can be performed by applying a known

voltage at the input, and compare this to the reference. If the applied calibration voltage

is exactly 2.5 volts, the reference voltage can be found by the equation:

Equation 3

V

in

V

ref

T

in

×

T

ref

---------------------------

=

V

in

T

in

128

×

T

ref

-------------------------

=

V

ref

T

ref

V

cal

×

T

cal

------------------------------

T

ref

128

×

T

cal

---------------------------

==