LM80 datasheet(13/29 Pages) NSC

13 / 29 page

Functional Description (Continued)

A typical application, such as is shown in

Figure 6, might se-

lect the input voltage divider to provide 1.9V at the analog in-

puts of the LM80. This is sufficiently high for good resolution

of the voltage, yet leaves headroom for upward excursions

from the supply of about 25%. To simplify the process of re-

sistor selection, set the value of R2 first. Select a value for

R2 or R4 between 10 k

Ω and 100 kΩ This is low enough to

avoid errors due to input leakage currents yet high enough to

both protect the inputs under overdrive conditions as well as

minimize loading of the source. Then select R1 or R3 to pro-

vide a 1.9V input as show in

Figure 6.

For positive input voltages the equation for calculating R1 is

as follows:

R1 = [(V

S −VIN)/VIN]R2

For negative input voltages the equation for Calculating R3

is as follows:

R3=[(V

S −VIN)/(VIN − 5V)] R4

The analog inputs have internal diodes that clamp inputs ex-

ceeding the power supply and ground. Exceeding any ana-

log input has no detrimental effect on other channels. The in-

put diodes will also clamp voltages appearing at the inputs of

an un-powered LM80. External resistors should be included

to limit input currents to the values given in the ABSOLUTE

MAXIMUM RATINGS for Input Current At Any Pin. Inputs

with the attenuator networks will usually meet these require-

ments. If it is possible for inputs without attenuators to be

turned on while LM80 is powered off, additional resistors of

about 10 k

Ω should be added in series with the inputs to limit

the input current.

Voltage

Measure-

ments

R1 or

R2 or

Voltage

Analog Inputs

( ADC code 190)

+2.5V

23.7 k

Ω

75 k

Ω

+1.9V

+3.3V

22.1 k

Ω

30 k

Ω

+1.9V

+5.0V

24 k

Ω

14.7 k

Ω

+1.9V

+12V

160 k

Ω

30.1 k

Ω

+1.9V

−12V

160 k

Ω

35.7 k

Ω

+1.9V

−5V

36 k

Ω

16.2 k

Ω

+1.9V

5.0 LAYOUT AND GROUNDING

Analog inputs will provide best accuracy when referred to the

AGND pin or a supply with low noise. A separate,

low-impedance ground plane for analog ground, which pro-

vides a ground point for the voltage dividers and analog

components, will provide best performance but is not man-

datory. Analog components such as voltage dividers should

be located physically as close as possible to the LM80.

The power supply bypass, the parallel combination of 10 µF

(electrolytic or tantalum) and 0.1 µF (ceramic) bypass ca-

pacitors connected between pin 9 and ground, should also

be located as close as possible to the LM80.

DS100040-11

FIGURE 6. Input Examples. Resistor values shown in

table provide approximately 1.9V at the analog inputs.

LM80 数据表(PDF) 13 Page - National Semiconductor (TI)

LM80 数据表(HTML) 13 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

数据表下载

链接网址

LM80 数据表(PDF) 13 Page - National Semiconductor (TI)

LM80 数据表(HTML) 13 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

数据表 下载

链接网址

数据表下载