L9780 datasheet(21/56 Pages) STMICROELECTRONICS

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L9780

Main functionalities

4.11

Analog to digital conversion

In the device a 10bit A/D converter is present. The analog voltages to be converted are

multiplexed by the logic while the state machine is running and are: the output of RCT2

amplifier, the output of INRC amplifier and FV pin voltage (see block diagram in Figure 2).

Between the analog inputs and the A/D converter a scaling amplifier is present; this cell is

able to amplify the input signal according to the gain selected via SPI (using bit SAMP[2..0])

with the following relation:

OUTsa = REF2 + G * (Input_voltage – REF2)

where G is the gain selectable via SPI among the following values: 1 - 1.5 - 2 - 3 - 4 - 6 - 8 –

12, and OUTsa is the output of the scaling amplifier (and the input of A/D converter). When

the selected input of the scaling amplifier is the output of the INRC or RCT2 amplifier, G is

fixed (=1), while when the input is FV voltage the gain G can be modified by the user. The

multiplexing sequence at the input of the scaling amplifier when the state machine is running

is shown in Figure 8.

The absolute conversion accuracy compared to an ideal transfer curve is ±2 counts. All the

electrical parameters regarding the scaling amplifier and the A/D conversion are specified in

Table 11.

The nominal conversion of the scaling amplifier output is defined in the following formula:

count_SA

floor OUTsa

VCCA

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

1024







where:

count_SA is the result of the A/D conversion;

OUTsa is the output voltage of the scaling amplifier;

floor: a function that returns the greatest integer less than or equal to the argument.

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