And +2.5v, Table 5 – Rainbow Electronics DS1780 User Manual
Page 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
IN
, +5V
IN
, +3.3V
IN
, and +2.5V
IN
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
IN
, +5V
IN
, +3.3V
IN
, and +2.5V
IN
) Table 5
INPUT PIN
+12V
IN
+5V
IN
+3.3V
IN
+2.5V
IN
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)
0
0
0
0
0
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
S
/+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
S
/+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
1
, R
2
)
≅
255
3.6
щ
кл
й
+
1
2
1
R
R
R