Datasheet
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 15 0.48 to 3.84
One remote sensor is active and RC = '0', local sensor and one remote sensor are disabled
44 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
93 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
59 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 88 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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