Datasheet

LM87

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SNAS034J –APRIL 2000–REVISED MARCH 2013

ANALOG INPUTS

All analog input voltages are digitized to 8-bits of resolution. For safety purposes, and to provide maximum

accuracy, a 510 Ω resistor should be placed in series with all analog voltage inputs. The resistors will limit the

possible current drawn from the power supplies in the event that circuit board traces are bridged, or accidentally

shorted during test. All analog inputs, except for AIN1 and AIN2, include internal 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

2.5Vin 13 mV 2.5 V 3.320 V

3.3Vcc 17.2 mV 3.3 V 4.383 V

5Vin/Vcc 26 mV 5 V 6.641 V

12Vin 62.5 mV 12 V 15.93 V

Vccp1, Vccp2 14.1 mV 2.7 V 3.586 V

AIN1/AIN2 9.8 mV 1.875 V 2.49 V

Thus monitoring power supplies within a system can be easily accomplished by tying the Vccp, +2.5Vin, +5Vin

and +12Vin analog inputs to the corresponding system supply. Vcc of the LM87 will also be monitored. 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 kΩ. AIN inputs do not have resistor

attenuators and are directly tied to the ADC, therefore having a much larger input impedance.

A negative power supply voltage can be applied to a AIN input through a resistor divider referenced to a known

positive DC voltage as shown in Figure 9. The resistor values shown in the table below for the circuit of Figure 9

will provide approximately 1.25 V at the AIN analog inputs of the LM87 for a nominal reading of 128.

Table 2.

Voltage Measurements R2 R1 V

Voltage

) at

Analog Inputs

( ADC code 128)

−12V 20 kΩ 130 kΩ +3.3 V +1.25 V

−5V 20 kΩ 61.0 kΩ +3.3 V +1.25 V

Resistor values shown in Table 2 provide approximately 1.25V at the Vccp inputs.

Figure 9. Input Examples

The resistors were selected by setting R2 = 20 kΩ and then calculating R1 using the following equation, ( V

the maximum negative input voltage, V

is the positive pullup voltage):

R1 = [(1.25V − V

) ÷ (V

− 1.25V)] × 20 kΩ (1)

The maximum R1 can be is restricted by the DC input current of an AIN input.

Product Folder Links: LM87