LM81 datasheet(14/32 Pages) NSC

14 / 32 page

Functional Description (Continued)

3.5 Reading Conversion Results

The conversion results are available in the Value RAM. Con-

versions can be read at any time and will provide the result

of the last conversion. Because the ADC stops, and starts a

new conversion whenever it is read, reads of any single

value should not be done more often than once every 56 ms.

When reading all values, allow at least 0.82 seconds be-

tween reading groups of values. Reading more frequently

than once every 0.82 seconds can also prevent complete

updates of Interrupt Status Registers and Interrupt Output’s.

A typical sequence of events upon power on of the LM81

would consist of:

Set WATCHDOG Limits

Set Interrupt Masks

Start the LM81 monitoring process

4.0 ANALOG INPUTS

All analog input voltages are digitized to 8-bits of resolution.

All analog inputs, except for Vccp1 and Vccp2, include inter-

nal resistor attenuators. The theoretical LSB size, theoretical

voltage input required for an ADC reading of 192 (3/4 scale)

and 255 (full scale) for each analog input is detailed in the

table below:

Input

LSB size

Vin for

192

Vin for

255

2.5 Vin

13 mV

2.5V

3.320V

3.3 Vin

17.2 mV

3.3V

4.383V

5 Vin

26 mV

6.641V

12 Vin

62.5 mV

12V

15.93V

Vccp1, Vccp2

14.1 mV

2.7V

3.586V

Thus monitoring power supplies within a system can be eas-

ily accomplished by tying the Vccp, +2.5 Vin, +3.3 Vin,

+5 Vin and +12 Vin analog inputs to the corresponding sys-

tem supply. A digital reading can be converted to a voltage

by simply multiplying the decimal value of the reading by the

LSB size.

For inputs with attenuators the input impedance is greater

than 90 k

Ω. Vccp inputs do not have resistor attenuators and

are are directly tied to the ADC, therefore having a much

larger input impedance.

A negative power supply voltage can be applied to a Vccp in-

put through a resistor divider referenced to a known positive

DC voltage as show in

Figure 6. The resistor values shown

in the table below for the circuit of

Figure 6 will provide ap-

proximately 1.25V at the Vccp analog inputs of the LM81 for

a nominal reading of 89.

Voltage

Measure-

ments

Voltage

Analog In-

puts

(ADC code

89)

−12V

40 k

Ω

141 k

Ω

+5V

+1.25V

−5V

40 k

Ω

66.7

Ω

+5V

+1.25V

The resistors were selected by setting R2 = 40 k

Ω and then

calculating R2 using the following equation:

R1 = [(1.25V − V

S) ÷ (V

+ − 1.25V)] x 40 k

Ω

The maximum R1 can be is restricted by the DC input cur-

rent of a Vccp input.

Inputs with internal resistor dividers (+2.5 Vin, +3.3 Vin or

+5 Vin, +12 Vin) can have voltage applied that exceeds the

power supply up to: 3.6V for +2.5 Vin, 4.6V for +3.3 Vin, 6.8V

for +5 Vin, and 15V for +12 Vin. The Vccp inputs have a

parasitic diode to the positive supply, so care should be

taken not to forward bias this diode. All analog inputs have

internal diodes that clamp the input voltage when going be-

low ground thus limiting the negative analog input voltage

range to −50 mV. Violating the analog input voltage range of

any analog input has no detrimental effect on the other ana-

log inputs. External resistors should be included to limit input

currents to the values given in the ABSOLUTE MAXIMUM

RATINGS for Input Current At Any Pin whenever exceeding

the analog input voltage range, even on an un-powered

LM81. Inputs with external attenuator networks will usually

meet these requirements. If it is possible for inputs without

attenuators (such as Vccp1 and Vccp2) to be turned on while

LM81 is powered off, additional resistors of about 10 k

Ω

should be added in series with the inputs to limit the input

current.

4.1 Analog Input Interrupts

A WATCHDOG window comparison on the analog inputs

can activate the INT interrupt output. A converted input volt-

age that is above its respective HIGH limit or less than or

equal to its LOW limit will cause a flag to be set in its Inter-

rupt Status Register. This flag will activate the INT output

when its mask bit is set low. Mask bits are found in the Inter-

rupt Mask Registers.

5.0 LAYOUT AND GROUNDING

A separate, low-impedance ground plane for analog ground,

which provides a ground point for both GND pins, voltage di-

viders and other analog components, will provide best per-

formance but is not mandatory. Analog components such as

voltage dividers should be located physically as close as

possible to the LM81.

DS100072-11

FIGURE 6. Input Examples. Resistor values shown in

table provide approximately 1.25V at the Vccp inputs.

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