LM80 datasheet(13/29 Pages) NSC

13 / 29 page

Functional Description (Continued)

A typical application, such as is shown in

Figure 6, might

select the input voltage divider to provide 1.9V at the analog

inputs of the LM80. This is sufficiently high for good resolu-

tion of the voltage, yet leaves headroom for upward excur-

sions from the supply of about 25%. To simplify the process

of resistor 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

provide 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

exceeding the power supply and ground. Exceeding any

analog input has no detrimental effect on other channels.

The input 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 requirements. If it is possible for inputs without attenu-

ators 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