AMC7812SPAPR datasheet(38/92 Pages) TI

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2N3904

D+

D-

2N3906

D+

D-

(b) PNP

(a) NPN

AMC7812

SBAS513E – JANUARY 2011 – REVISED SEPTEMBER 2013

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Filtering

Figure 88 shows the connection of recommended (a) NPN or (b) PNP transistors. Remote junction temperature

sensors are usually implemented in a noisy environment. Noise is most often created by fast digital signals, and

it can corrupt measurements. The AMC7812 has a built-in 65kHz filter on the inputs of D+ and D-, to minimize

the effects of noise. However, a bypass capacitor placed differentially across the inputs of the remote

temperature sensor can make the application more robust against unwanted coupled signals. If filtering is

required, the capacitance between D+ and D– should be limited to 330pF or less for optimum measurement

performance. This capacitance includes any cable capacitance between the remote temperature sensor and the

AMC7812.

Figure 88. Remote Temperature Sensor Using Transistor

Series Resistance Cancellation

Parasitic resistance (seen in series with the remote diode) to the D+ and D– inputs to the AMC7812 is caused by

a variety of factors, including printed circuit board (PCB) trace resistance and trace length. This series resistance

appears as a temperature offset in the remote sensor temperature measurement, and causes more than 0.45°C

error per ohm. The AMC7812 implements a technology to automatically cancel out the effect of this series

resistance, giving a more accurate result without the need for user characterization of this resistance. With this

technology, the AMC7812 is able to reduce the effects of series resistance to typically less than 0.0075°C per

ohm. The resistance cancellation is disabled when the RC bit in Temperature Configuration Register is cleared

('0').

Reading Temperature Data

The temperature is always read as 12-bit data. When the conversion finishes, the temperature is sent to the

corresponding Temp-Data Register. However, if a data transfer is in progress between the Temp-Data Register

and the AMC Shift Register, the Temp-Data Register is frozen until the data transfer is complete.

Conversion Time

The conversion time depends on the type of sensor and configuration, as shown in Table 4.

Table 4. Conversion Times

MONITORING

PROGRAMMABLE

TEMPERATURE SENSOR

CYCLE TIME (ms)

DELAY RANGE (s)

Local sensor is active, remote sensors are disabled or in power-down

0.48 to 3.84

One remote sensor is active and RC = '0', local sensor and one remote sensor are disabled

1.40 to 11.2

or in power-down

One remote sensor is active and RC = '1', local sensor and one remote sensor are disabled

2.97 to 23.8

or in power-down

One remote sensor and local sensor are active and RC = '0', one remote sensor is disabled

1.89 to 15.1

or in power-down

One remote sensor and local sensor are active and RC = '1', one remote sensor is disabled

108

3.45 to 27.65

or in power-down

Two remote sensors are active and RC = '0', local sensor is disabled or in power-down

2.81 to 22.5

Two remote sensors are active and RC = '1', local sensor is disabled or in power-down

186

5.95 to 47.6

All sensors are active and RC = '0'

103

3.92 to 26.38

All sensors are active and RC = '1'

201

6.43 to 51.45

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