Rainbow-electronics DS1780 Manual do Utilizador descarregar pdf (Página 9)

DS1780

9 of 28

The worse such accident would be connecting -12V to +12V; a total of 24V difference, with the series

resistors this would draw a maximum of approximately 24 ma.

The internal scaling factor depends upon the particular input. The +12V

, +5V

, +3.3V

, and +2.5V

inputs are internally scaled such that the nominal value of the respective supply corresponds to 3/4 of full

range, or a decimal count of 192. The approximate resolution is thus equal to:

LSb (V

NOM

) ≅

256

V 4/3

NOM

This is depicted below in Table 5.

VOLTAGE/DATA RELATIONSHIPS FOR POSITIVE ONLY VOLTAGE INPUTS

(+12V

, +5V

, +3.3V

, and +2.5V

) Table 5

INPUT PIN +12V

+5V

+3.3V

+2.5V

LSb WEIGHTING

(mV)

62.5 26.0 17.2 13.0

ADC RESULT (BASE

10)

PIN VOLTAGE (V) PIN VOLTAGE (V) PIN VOLTAGE (V) PIN VOLTAGE (V)

00000

1 0.063 0.026 0.017 0.013

2 0.125 0.052 0.034 0.026

3 0.188 0.078 0.052 0.039

4 0.25 0.104 0.069 0.052

•

190 11.875 4.948 3.266 2.474

191 11.938 4.974 3.283 2.487

192 12.0 5.0 3.3 2.5

193 12.063 5.026 3.317 2.513

•

252 15.75 6.563 4.331 3.281

253 15.813 6.589 4.348 3.294

254 15.875 6.615 4.366 3.307

255 15.938 6.641 4.383 3.32

The other two voltage inputs use a slightly different scaling technique, due to the nature of the PC voltage

they are monitoring. Because processor voltage (V

CCP

) can vary to 3.6V, the +V

CCP1

and +2.5V

/+V

CCP1

inputs are internally scaled such that the ADC result is 0h for a 0V input and the maximum value of FFh

is returned for a voltage of 3.60V. This corresponds to an LSb weighting of 14.1 mV.

The inputs can also be used to monitor a negative supply, such as -12V. However, a resistor ladder and

positive reference voltage (V

REF

) must be used (see Figure 1) such that input voltage to the DS1780

swings between OV and +3.6V. Assuming the DS1780 +V

CCP1

and +2.5V

/+V

CCP2

pins have infinite

input impedance and the V

REF

is a perfect supply, then the resolution and range of -12V input are:

LSb (R

, R

) ≅

255

3.6

1 2 3 4 5 6 7 8 9 10 11 12 13 14 ... 27 28

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