AN420 datasheet(8/16 Pages) STMICROELECTRONICS

8 / 16 page

Example

AN420

8/15

Doc ID 2078 Rev 2

Example

Let us assume we have a voltage swing of 10 volts (0 to 10) that we want to convert with a

10-bit resolution. And let us assume we have a set of voltage sources VNj that we can

switch between 0 to 5 volts under software control, and each one independently from the

other.

Let us also assume we can 'cut' the 10 volt swing in 4 'pieces' of 2.5 volts each, and that

every 'piece' can be converted with 8-bit resolution. The overall resolution will therefore be:

28 (ST6 A/D resolution) * 2

(# of 'pieces') = 2

Let us call Vin the actual value of the source to be converted. For instance, if Vin ε [10, 7,5]

volts, we could supply the ST6 A/D with the voltage:

(Vin-7.5volt)x2 => ε[0,5]volt

or, for (10,7.5) volts:

(Vin-1.5xVN1)x2 = 2xVin-3xVN1

where VN1 is one of the VNj sources, either 0 or 5 volts. In similar fashion, for the other

intervals, we could obtain:

(7.5, 5) volts

(Vin-VN2)x2 = 2xVin-2xVN2

(5, 2.5) volts

(Vin-0.5xVN3)x2 = 2xVin-VN3

(2.5, 0) volts

(Vin-0xVN4)x2 = 2xVin

So, relation (6) becomes:

V0 = 2xVin-3xVN1-2xVN2-VN3 where Vin =VP1

The software driving the conversion will therefore verify if, given a certain status of the VNj

voltages, the conversion is far from being saturated. If so, another try will be performed with

a different status of the VNj voltages. Figure 3 gives the flow chart of such software.

AN420 Datasheet(PDF) 8 Page - STMicroelectronics